Introduction to Haskell Programming

Posted on 2022-03-09 Edited on 2022-07-29 Word count in article: 64k Reading time ≈ 59 mins.

A brief introduction to haskell programming.

1. Introduction

Haskell is a purely functional programming language.
- You cannot set a variable to something and then set it to something else later: No side-effects
- If a function is called twice with the same parameters, it's guaranteed to return the same result: Referential transparency
Haskell is lazy: unless specifically told otherwise, Haskell won't execute functions and calculate things until it's really forced to show you a result. Allow infinite data structures
Statically typed: Its type system that has type inference.
Most widely used Haskell compiler: GHC

2. Starting out

Some basic ideas

Haskell can be used as a calculator

Some infix function	Meaning
+, -, *, /	Arithmetic
==, /=, <, >, <=, >=	Equality
&&, \|\|	Logical
++	List concatenation
:	List cons
!!	List index access

Some prefix function	Meaning
not	Logical
succ	Increase by 1
max, min	Comparison
head	First element of a list
tail	The list without the head element
last	Last element of a list
init	The list without the last element
length	Get length of a list
null	Check if a list is empty
reverse	Reverse a list
take	Extracts a number of elements from a list
drop	Drop a number of elements from a list
minimum	Get min of a list
maximum	Get max of a list
sum	Get sum of a list
product	Get product of a list
elem	Check if an item is in a list
cycle	Cycles a list into an infinite list
repeat	Repeat an element into an infinite list
replicate	Repeat an element for a number of times
fst	Get the first component of a pair tuple
snd	Get the second component of a pair tuple
zip	Zips two lists to list of joining pairs
compare	return GT (greater than), LT (lesser than) or EQ
read	Takes a string and returns a type of Read class
fromIntegral	Takes an integral and turns it into a more general number

Change prefix function to infix function:

1	(div 42 5) == (42 `div` 5)

Change back:

1	((+) 42 5) == (42 + 5)

First function saved as the file doubleMe.hs:

1	doubleMe x = x + x

Used with:

ghci> :l doubleMe  
[1 of 1] Compiling Main             ( doubleMe.hs, interpreted )  
Ok, modules loaded: Main. 
ghci> doubleMe 9  
18

if statement is an expression which means that it returns a value thus the else part of the if statements in Haskell is mandatory

sumDoubledUs x y = doubleMe x + doubleMe y

doubleSmallNumber x = if x > 100
                        then x
                        else x*2  --The else part is mandatory.

conanO'Brien = "It's a-me, Conan O'Brien!"

Functions in Haskell don't have to be in any particular order. Thus the order of doubleMe and sumDoubleUs is not important

We usually use ' to either denote a strict version of a function (one that isn't lazy) or a slightly modified version of a function or a variable.

1	doubleSmallNumber' x = (if x > 100 then x else x*2) + 1

Intro to list

List is homogeneous data structure $\rightarrow$ stores elements with same type

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ghci> let lostNumbers = [4,8,15,16,23,42]
ghci> lostNumbers
[4,8,15,16,23,42]

To concatenate two lists:

ghci> [1,2,3,4] ++ [9,10,11,12]
[1,2,3,4,9,10,11,12]
ghci> "hello" ++ " " ++ "world"
"hello world"

To use ++ operator on long strings is however slow. ++ takes in two lists, so, even if you want to append one element to the end of a list with ++, you have to surround it with square brackets so it becomes a list

To append an element at the beginning of a list: : operator

ghci> 'A':" SMALL CAT"
"A SMALL CAT"
ghci> 5:[1,2,3,4]
[5,1,2,3,4]

[1,2,3] is actually just syntactic sugar for 1:2:3:[]

Strings are lists, "hello" is just syntactic sugar for ['h','e','l','l','o']. Thus, we can use list functions on them

To get an element out of a list by index: !! operator, the indices start at 0

ghci> "Steve Buscemi" !! 6
'B'
ghci> [1,2,3,4,5] !! 1
2

Lists can contain lists. The lists within a list can be of different lengths but they can't be of different types

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ghci> let b = [[1], [2,3], [4,5,6]]
ghci> b !! 2
[4,5,6]

Lists can be compared in lexicographical order

ghci> [3,2,1] > [2,10,100]
True
ghci> [3,4,2] > [3,4]
True

Texas ranges

A faster way to generate a list with a certain pattern

[1..20] == [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]  
['a'..'z'] == "abcdefghijklmnopqrstuvwxyz"  
['K'..'Z'] == "KLMNOPQRSTUVWXYZ"   
[2,4..20] == [2,4,6,8,10,12,14,16,18,20]
[3,6..20] == [3,6,9,12,15,18]
[20,19..1] == [20,19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1]
[0.5, 0.7 .. 1] == [0.5,0.7,0.8999999999999999,1.0999999999999999]
take 5 [13,26..] == [13,26,39,52,65]
take 10 (cycle [1,2,3]) == [1,2,3,1,2,3,1,2,3,1]  
take 10 (repeat 5) == [5,5,5,5,5,5,5,5,5,5]  
replicate 3 10 = [10, 10, 10]

List comprehension

Similar to set comprehensions. They are used for building more specific sets out of general sets

The part before the pipe is called the output function

[ x*2 | x <- [1..10], x*2 >= 12] == [12,14,16,18,20]  
[ x | x <- [50..100], x `mod` 7 == 3] == [52,59,66,73,80,87,94]
[ x | x <- [15..20], x /= 17, x /= 19] == [15,16,18,20]
[ x*y | x <- [2,5,10], y <- [8,10,11], x*y > 60] == [80,100,110] 
[ (a,b,c) | c <- [1..20], b <- [1..c], a <- [1..b], a^2 + b^2 == c^2, gcd a b == 1, gcd b c == 1] == [(3,4,5),(5,12,13),(8,15,17)]
boomBangs xs = [ if x < 10 then 2*x else x | x <- xs, odd x]   
boomBangs [7..13] == [14,18,11,13]

It can be used to combines a list of adjectives and a list of nouns:

ghci> let nouns = ["hobo", "frog", "pope"]
ghci> let adjectives = ["lazy", "grouchy", "scheming"]
ghci> [adjective ++ " " ++ noun | adjective <- adjectives, noun <- nouns]
["lazy hobo","lazy frog","lazy pope","grouchy hobo","grouchy frog","grouchy pope","scheming hobo","scheming frog","scheming pope"]

We can implement our own length:

1	length' xs = sum [1 \| _ <- xs]

Here _ means that we don't care what the output is

To work on strings:

1 2	removeNonUppercase st = [ c \| c <- st, c `elem` ['A'..'Z']] removeNonUppercase "Hahhhah! Ahahahha" == "HA"

Tuples

Tuples are similar to lists. The fundamental differences are:

Tuples have fixed size
Tuples is NOT homogeneous.
Tuples define its own type. [(1,2),(8,11,5),(4,5)] or [(1,2),("One",2)] are not valid syntactically because they have different types

Two useful functions that operate on pairs:

ghci> fst (8, 11)
8
ghci> snd (8, 11)
11

A cool function that produces a list of pairs: zip. It takes two lists and then zips them together into one list by joining the matching elements into pairs

ghci> zip [1 .. 5] ["one", "two", "three", "four", "five"]
[(1,"one"),(2,"two"),(3,"three"),(4,"four"),(5,"five")]
ghci> zip [1,2,3,4,5,6,7] ["im", "a", "turtle"]
[(1,"im"),(2,"a"),(3,"turtle")]
ghci> zip [1 ..] ["apple", "orange","cherry","mango"]
[(1,"apple"),(2,"orange"),(3,"cherry"),(4,"mango")]

A cool example:

1 2	ghci> let rightTriangles = [(a,b,c) \| c <- [1..20], b <- [1..c], a <- [1..b], a^2 + b^2 == c^2] [(3,4,5),(6,8,10),(5,12,13),(9,12,15),(8,15,17),(12,16,20)]

3. Types and Typeclasses

Some basic types

A type is a kind of label that every expression has. It tells us in which category of things that expression fits. A type is written in capital case

Type	Range
Int	2147483647 to -2147483648 on 32-bit machine
Integer	No boundary
Float	Single precision
Double	Double precision
Bool	True or False
Char	A character

Unlike Java or Pascal, Haskell has type inference. Haskell can infer some type based on context

ghci> :t 'a'
'a' :: Char
ghci> :t True
True :: Bool
ghci> :t "Hello"
"Hello" :: [Char]
ghci> :t (True, 'a')
(True, 'a') :: (Bool, Char)

:: is read as "has type of"

Types of functions

removeNonUppercase :: [Char] -> [Char]  
removeNonUppercase st = [ c | c <- st, c `elem` ['A'..'Z']]

addThree :: Int -> Int -> Int -> Int
addThree x y z = x + y + z

The parameters are separated with -> and there's no special distinction between the parameters and the return type

Type variables

For example:

1 2	ghci> : t head head :: [a] -> a

Here a is a type variable which means that a can be of any type. Functions that have type variables are called polymorphic functions. Another example:

1 2	ghci> :t fst fst :: (a, b) -> a

Intro to typeclasses

A typeclass is a sort of interface that defines some behaviour. If a type is a part of a typeclass, that means that it supports and implements the behaviour the typeclass describes

1 2	ghci> :t (==) (==) :: (Eq a) => a -> a -> Bool

Everything before the => symbol is called a class constraint. The type of those two values must be a member of the Eq class. This was the class constraint

The Eq typeclass provides an interface for testing for equality. Any type where it makes sense to test for equality between two values of that type should be a member of the Eq class

Some basic typeclasses:

Eq is used for types that support equality testing

ghci> 5 == 5
True
ghci> 'a' == 'a'
True
ghci> 3.432 == 3.432
True

Ord members have an ordering. The be a member of Ord, a type must first have membership in the prestigious and exclusive Eq club

ghci> :t (>)  
(>) :: (Ord a) => a -> a -> Bool
ghci> "Abrakadabra" < "Zebra"
True
ghci> "Abrakadabra" `compare` "Zebra"
LT
ghci> 5 `compare` 3
GT

Show members can be presented as strings

ghci> show 3
"3"
ghci> show 5.334
"5.334"
ghci> show True
"True"

Read members values can be extract from a string
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ghci> read "True" || False
True
ghci> read "[1,2,3,4]" ++ [3]
[1,2,3,4,3]
- But we cannot run for example read "4", the GHCI will give an error because it doesn't know what we want in return. It knows we want some type that is part of the Read class, it doesn't know which one
- Thus we must use explicit type annotations, which is a way of explicitly saying what the type of expression should be. We do that by adding :: at the end of the expression and then specifying a type
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  ghci> read "5" :: Int
  5
  ghci> read "5" :: Float
  5.0
  ghci> read "[1,2,3,4]" :: [Int]
  [1,2,3,4]
  ghci> read "(3, 'a')" :: (Int, Char)
  (3, 'a')

Enum members are sequentially ordered types

ghci> ['a'..'e']
"abcde"
ghci> [LT .. GT]
[LT,EQ,GT]
ghci> succ 'B'
'C'

Bounded members have an upper and a lower bound

ghci> minBound :: Int
-9223372036854775808
ghci> maxBound :: Char
'\1114111'
ghci> maxBound :: (Bool, Int, Char)
(True,9223372036854775807,'\1114111')

Num members have the property of being able to act like numbers

ghci> 20 :: Int
20
ghci> 20 :: Integer
20
ghci> 20 :: Float
20.0
ghci> 20 :: Double
20.0
ghci> :t (*)
(*) :: Num a => a -> a -> a

Integral members represent integers. Num includes all numbers, including real numbers and integral numbers, Integral includes only integral (whole) numbers. In this typeclass are Int and Integer
Floating members includes only floating point numbers, so Float and Double

A very useful function for dealing with numbers is fromIntegral. It has a type declaration of fromIntegral :: (Num b, Integral a) => a -> b. That's useful when you want integral and floating point types to work together nicely. For example:

1	fromIntegral (length [1,2,3,4]) + 3.2 == 7.2

4. Syntax in Functions

Pattern matching

Pattern matching consists of specifying patterns to which some data should conform and then checking to see if it does and deconstructing the data according to those patterns

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lucky :: (Integral a) => a -> String
lucky 7 = "LUCKY NUMBER SEVEN!"
lucky x = "Sorry, you're out of luck, pal!"

When you call lucky, the patterns will be checked from top to bottom and when it conforms to a pattern, the corresponding function body will be used

Some examples:

factorial :: (Integral a) => a -> a  
factorial 0 = 1  
factorial n = n * factorial (n - 1)  

addVectors :: (Num a) => (a, a) -> (a, a) -> (a, a)  
addVectors (x1, y1) (x2, y2) = (x1 + x2, y1 + y2)  

first :: (a, b, c) -> a  
first (x, _, _) = x  
  
second :: (a, b, c) -> b  
second (_, y, _) = y  

third :: (a, b, c) -> c
third (_, _, z) = z

We can also pattern match in list comprehensions. Should a pattern match fail, it will just move on to the next element

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ghci> let xs = [(1,3), (4,5), (6,7), (10,12)]
ghci> [a+b | (a,b) <- xs]
[4,9,13,22]

The following examples show some pattern matches in list comprehensions

head' :: [a] -> a  
head' [] = error "Can't call head on an empty list, dummy!"  
head' (x:_) = x  

length' :: (Num b) => [a] -> b  
length' [] = 0  
length' (_:xs) = 1 + length' xs  

sum' :: (Num a) => [a] -> a
sum' [] = 0
sum' (x:xs) = x + sum' xs

There is also a thing called as patterns. Those are a handy way of breaking something up according to a pattern and binding it to names whilst still keeping a reference to the whole thing

You can do that by putting a name and an @ in front of a pattern. For instance, the pattern xs@(x:y:ys). This pattern will match exactly the same things as x:y:ys but you can easily get the whole list via xs

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capital :: String -> String  
capital "" = "Empty string, whoops!"  
capital all@(x:xs) = "The first letter of " ++ all ++ " is " ++ [x]

You can't use ++ in pattern matches. If you tried to pattern match against (xs ++ ys), Haskell will not be able to decide what would be in the first and what would be in the second list

Guards

Guards are a way of testing whether some property of a value (or several of them) are true or false. The guars are a lot more readable when you have several conditions

bmiTell :: (RealFloat a) => a -> a -> String  
bmiTell weight height  
    | bmi <= skinny = "You're underweight, you emo, you!"  
    | bmi <= normal = "You're supposedly normal. Pffft, I bet you're ugly!"  
    | bmi <= fat    = "You're fat! Lose some weight, fatty!"  
    | otherwise     = "You're a whale, congratulations!"  
    where bmi = weight / height ^ 2  
          skinny = 18.5  
          normal = 25.0  
          fat = 30.0  
          
max' :: (Ord a) => a -> a -> a  
max' a b 
	| a > b = a 
	| otherwise = b  

myCompare :: (Ord a) => a -> a -> Ordering  
a `myCompare` b  
    | a > b     = GT  
    | a == b    = EQ  
    | otherwise = LT

otherwise is defined simply as otherwise = True and catches everything. We put the keyword where after the guards and then we define several names or functions. These names are visible only across the guards. All the names in where should be aligned at a single column, otherwise Haskell will get confused

Let

let bindings let you bind to variables anywhere and are expressions themselves but are local. let bindings bind values to names, it can be used for pattern matching

cylinder :: (RealFloat a) => a -> a -> a  
cylinder r h = 
    let sideArea = 2 * pi * r * h  
        topArea = pi * r ^2  
    in  sideArea + 2 * topArea  

calcBmis :: (RealFloat a) => [(a, a)] -> [a]  
calcBmis xs = [bmi | (w, h) <- xs, let bmi = w / h ^ 2, bmi >= 25.0]

The names that you define in the let part are accessible to the expression after the in part

The difference between let and where is that let bindings are expressions themselves. where bindings are just syntactic constructs. We can do the following with let:

ghci> 4 * (if 10 > 5 then 10 else 0) + 2  
42  
ghci> 4 * (let a = 9 in a + 1) + 2  
42  
ghci> (let a = 100; b = 200; c = 300 in a*b*c, let foo="Hey "; bar="there!" in foo++bar)
(6000000,"Hey there!")
ghci> let zoot x y z = x * y + z  
ghci> zoot 3 9 2  
29  
ghci> let boot x y z = x * y + z in boot 3 4 2  
14  
ghci> boot  
<interactive>:1:0: Not in scope: `boot'

The in part can be omitted when defining functions and constants, then the names will be visible throughout the entire interactive session

Case expressions

Case expressions are expressions. We can evaluate expressions based on the possible cases of the value of a variable, we can also do pattern matching. We can rewrite the head' above in the following way

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head' :: [a] -> a
head' xs = case xs of [] -> error "No head for empty lists"
					  (x:_) -> x

The syntax for case expressions is pretty simple:

case expression of pattern -> result
				   pattern -> result
				   pattern -> result
				   ...

Whereas pattern matching on function parameters can only be done when defining functions, case expressions can be used pretty much anywhere. For instance:

describeList :: [a] -> String  
describeList xs = "The list is " ++ case xs of [] -> "empty."  
                                               [x] -> "a singleton list."   
                                               xs -> "a longer list."  

describeList' :: [a] -> String  
describeList' xs = "The list is " ++ what xs  
    where what [] = "empty."  
          what [x] = "a singleton list."  
          what xs = "a longer list."

5. Recursion

Some recursion examples:

maximum' :: (Ord a) => [a] -> a  
maximum' [] = error "maximum of empty list"  
maximum' [x] = x  
maximum' (x:xs) = max x (maximum' xs)  

replicate' :: (Num i, Ord i) => i -> a -> [a]  
replicate' n x  
    | n <= 0    = []  
    | otherwise = x:replicate' (n-1) x  

-- take' 3 [5,4,3,2,1] ==> [5,4,3]
take' :: (Num i, Ord i) => i -> [a] -> [a]  
take' n _  
    | n <= 0   = []  
take' _ []     = []  
take' n (x:xs) = x : take' (n-1) xs  

reverse' :: [a] -> [a]  
reverse' [] = []  
reverse' (x:xs) = reverse' xs ++ [x]  

zip' :: [a] -> [b] -> [(a,b)]  
zip' _ [] = []  
zip' [] _ = []  
zip' (x:xs) (y:ys) = (x,y):zip' xs ys  

elem' :: (Eq a) => a -> [a] -> Bool  
elem' a [] = False  
elem' a (x:xs)  
    | a == x    = True  
    | otherwise = a `elem'` xs   

quicksort :: (Ord a) => [a] -> [a]  
quicksort [] = []  
quicksort (x:xs) =   
    let smallerSorted = quicksort [a | a <- xs, a <= x]  
        biggerSorted = quicksort [a | a <- xs, a > x]  
    in  smallerSorted ++ [x] ++ biggerSorted

6. Higher order functions

Haskell functions can take functions as parameters and return functions as return values. A function that does either of those is called a higher order function

Curried functions

Every function in Haskell officially only takes one parameter. All the functions that accept several parameters have been curried functions. For example:

max :: (Ord a) => a -> a -> a is the same as max :: (Ord a) => a -> (a -> a)

The following two calls are equivalent:

ghci> max 4 5
5
ghci> (max 4) 5
5

If we call a function with too few parameters, we get back a partially applied function. Using partial application is a neat way to create functions on the fly so we can pass them to another function or to seed them with some data. For example:

1 2	multThree :: (Num a) => a -> a -> a -> a multThree x y z = x * y * z

Then

ghci> let multTwoWithNine = multThree 9  
ghci> multTwoWithNine 2 3  
54  
ghci> let multWithEighteen = multTwoWithNine 2  
ghci> multWithEighteen 10  
180

Another way of writing it

1 2	compareWithHundred :: (Num a, Ord a) => a -> Ordering compareWithHundred = compare 100

Section an infix function

To section an infix function, simply surround it with parentheses and only supply a parameter on one side:

divideByTen :: (Floating a) => a -> a  
divideByTen = (/10)  

isUpperAlphanum :: Char -> Bool  
isUpperAlphanum = (`elem` ['A'..'Z'])

Some higher-orderism is in order

Functions can take functions as parameters and also return functions. Note that the parentheses in the following examples are mandatory. They indicate that the first parameter are functions

applyTwice :: (a -> a) -> a -> a  
applyTwice f x = f (f x)  

zipWith' :: (a -> b -> c) -> [a] -> [b] -> [c]  
zipWith' _ [] _ = []  
zipWith' _ _ [] = []  
zipWith' f (x:xs) (y:ys) = f x y : zipWith' f xs ys  

flip' :: (a -> b -> c) -> (b -> a -> c)  
flip' f = g  
    where g x y = f y x

Applying:

ghci> applyTwice (+3) 10  
16  
ghci> applyTwice (++ " HAHA") "HEY"  
"HEY HAHA HAHA"  
ghci> applyTwice ("HAHA " ++) "HEY"  
"HAHA HAHA HEY"  
ghci> applyTwice (3:) [1]  
[3,3,1]  

ghci> zipWith' (+) [4,2,5,6] [2,6,2,3]  
[6,8,7,9]  
ghci> zipWith' max [6,3,2,1] [7,3,1,5]  
[7,3,2,5]  
ghci> zipWith' (++) ["foo ", "bar ", "baz "] ["fighters", "hoppers", "aldrin"]  
["foo fighters","bar hoppers","baz aldrin"]  
ghci> zipWith' (*) (replicate 5 2) [1..]  
[2,4,6,8,10]

Maps and filters

map takes a function and a list and applies that function to every element in the list, producing a new list. Its definition:

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map :: (a -> b) -> [a] -> [b]  
map _ [] = []  
map f (x:xs) = f x : map f xs

filter is a function that takes a predicate (a predicate is a function that tells whether somethign is true or not) and a list and then returns the list of elements that satisfy the predicate. Its definition:

filter :: (a -> Bool) -> [a] -> [a]  
filter _ [] = []  
filter p (x:xs)   
    | p x       = x : filter p xs  
    | otherwise = filter p xs

With them, we can implement quicksort easily:

quicksort :: (Ord a) => [a] -> [a]    
quicksort [] = []    
quicksort (x:xs) =     
    let smallerSorted = quicksort (filter (<=x) xs)  
        biggerSorted = quicksort (filter (>x) xs)   
    in  smallerSorted ++ [x] ++ biggerSorted

filter doesn't work on infinite lists while takeWhile does. Eg. find the sum of all odd squares that are smaller than 10,000

ghci> sum (takeWhile (<10000) (filter odd (map (^2) [1..])))  
166650  
ghci> sum (takeWhile (<10000) [n^2 | n <- [1..], odd (n^2)])  
166650

Generate Collatz sequences

chain :: (Integral a) => a -> [a]  
chain 1 = [1]  
chain n  
    | even n =  n:chain (n `div` 2)  
    | odd n  =  n:chain (n*3 + 1)

Applying

1 2	ghci> chain 10 [10,5,16,8,4,2,1]

Another example is:

1 2	ghci> let listOfFuns = map (*) [0..] ghci> (listOfFuns !! 4) 5

What happens here is that the number in the list is applied to the function *, which has a type of (Num a) => a -> a -> a. Applying only one parameter to a function that takes two parameters returns a function that takes one parameter. If we map * over the list [0..], we get back a list of functions that only take one parameter. So this function produces a list like [(0*), (1*), (2*), (3*), (4*), ...]. Thus the fourth element is (4*) and it gives 20 when it applied to 5

Lambda

Lambdas are basically anonymous function. To make a lambda, we write a \ and then we write the parameters, separated by spaces. After that comes a -> and then the function body. We usually surround them by parentheses, because otherwise they extend all the way to the right

map (\(a,b) -> a + b) [(1,2),(3,5),(6,3),(2,6),(2,5)]  

addThree :: (Num a) => a -> a -> a -> a  
addThree = \x -> \y -> \z -> x + y + z  

flip' :: (a -> b -> c) -> b -> a -> c  
flip' f = \x y -> f y x

Foldl

foldl - fold a list from left. It needs a binary function. The binary function is applied between the starting value and the head of the list. That produces a new accumulator value and the binary function is called with that value and the next element, etc. For example:

sum' :: (Num a) => [a] -> a
sum' xs = foldl (\acc x -> acc + x) 0 xs

sum'' :: (Num a) => [a] -> a  
sum'' = foldl (+) 0  

elem' :: (Eq a) => a -> [a] -> Bool  
elem' y ys = foldl (\acc x -> if x == y then True else acc) False ys

foldr - fold a list from right. The starting value and the list swapped places. To generate a list as the result, right fold append is faster. Also only right fold work on infinite list.

The accumulator value (and hence, the result) of a fold can be of any type. It can be a number, a boolean or even a new list. For example:

1 2	map' :: (a -> b) -> [a] -> [b] map' f xs = foldr (\x acc -> f x : acc) [] xs

Folds can be used to implement any function where you traverse a list once, element by element, and then return something based on that

foldl1 and foldr1 is similar but do not require starting value and assume that the starting value equal with the first inputted element from the list. Examples:

maximum' :: (Ord a) => [a] -> a  
maximum' = foldr1 (\x acc -> if x > acc then x else acc)  
  
reverse' :: [a] -> [a]  
reverse' = foldl (\acc x -> x : acc) []  
  
product' :: (Num a) => [a] -> a  
product' = foldr1 (*)  
  
filter' :: (a -> Bool) -> [a] -> [a]  
filter' p = foldr (\x acc -> if p x then x : acc else acc) []  
  
head' :: [a] -> a  
head' = foldr1 (\x _ -> x)  
  
last' :: [a] -> a  
last' = foldl1 (\_ x -> x)

scanl and scanr are similar but they report all the intermediate accumulated states in the form of a list. scanl1 and scanr1 are analogous.

Function application with `$`

The function application operator $ has the lowest precedence and it is right-associative. It can help us by saving lots of parenthesis

1 2	($) :: (a -> b) -> a -> b f $ x = f x

$ means that function application can be treated just like another function. That way, we can for instance, map function application over a list of functions

1 2	ghci> map ($ 3) [(4+), (10*), (^2), sqrt] [7.0,30.0,9.0,1.7320508075688772]

Function composition

The . function define function composition $(f\circ g)(x)=f(g(x))$

1 2	(.) :: (b -> c) -> (a -> b) -> a -> c f . g = \x -> f (g x)

Examples

ghci> map (negate . abs) [5,-3,-6,7,-3,2,-19,24]  
[-5,-3,-6,-7,-3,-2,-19,-24]  
ghci> map (negate . sum . tail) [[1..5],[3..6],[1..7]]  
[-14,-15,-27]  
ghci> sum (replicate 5 (max 6.7 8.9)) == sum . replicate 5 . max 6.7 $ 8.9

If you want to rewrite an expression with a lot of parentheses by using function composition, you can start by putting the last parameter of the innermost function after a $ and then just composing all the other function calls, writing them without their last parameter and putting dots between them

Thus replicate 100 (product (map (*3) (zipWith max [1,2,3,4,5] [4,5,6,7,8]))) becomes replicate 100 . product . map (*3) . zipWith max [1,2,3,4,5] $ [4,5,6,7,8]

7. Modules

import Data.List  
  
numUniques :: (Eq a) => [a] -> Int  
numUniques = length . nub

In ghci:

1	ghci> :m + Data.List Data.Map Data.Set

Import only some functions

1	import Data.List (nub, sort)

Import everything but some functions

1	import Data.List hiding (nub)

Qualified import

1 2	import qualified Data.Map as M -- M.filter

`Data.List`

foldl' and foldr' are stricter versions of their respective lazy incarnations. When using lazy folds on really big lists, you might often get a stack overflow error. The stricter versions will not.

length, take, drop, splitAt, !! and replicate all take an Int as one of their parameters. To use the more general type Num, Data.list has genericLength, genericTake, genericDrop, genericSplitAt, genericIndex and genericReplicate.

The nub, delete, union, intersect and group functions all have their more general counterparts called nubBy, deleteBy, unionBy, intersectBy and groupBy. They take in a function for comparison the inputted elements.

Similarly, the sort, insert, maximum and minimum have their more general equivalents sortBy, insertBy, maximumBy and minimumBy. They take in function for comparison as well.

ghci> intersperse '.' "MONKEY"  
"M.O.N.K.E.Y"  
ghci> intersperse 0 [1,2,3,4,5,6]  
[1,0,2,0,3,0,4,0,5,0,6]  

ghci> intercalate " " ["hey","there","guys"]  
"hey there guys"  
ghci> intercalate [0,0,0] [[1,2,3],[4,5,6],[7,8,9]]  
[1,2,3,0,0,0,4,5,6,0,0,0,7,8,9]  

ghci> transpose [[1,2,3],[4,5,6],[7,8,9]]  
[[1,4,7],[2,5,8],[3,6,9]]  
ghci> transpose ["hey","there","guys"]  
["htg","ehu","yey","rs","e"]  

ghci> concat ["foo","bar","car"]  
"foobarcar"  
ghci> concat [[3,4,5],[2,3,4],[2,1,1]]  
[3,4,5,2,3,4,2,1,1]  

ghci> concatMap (replicate 4) [1..3]  
[1,1,1,1,2,2,2,2,3,3,3,3]  

ghci> and $ map (>4) [5,6,7,8]  
True  
ghci> and $ map (==4) [4,4,4,3,4]  
False  

ghci> or $ map (==4) [2,3,4,5,6,1]  
True  
ghci> or $ map (>4) [1,2,3]  
False 

ghci> any (==4) [2,3,5,6,1,4]  
True  
ghci> all (>4) [6,9,10]  
True  
ghci> all (`elem` ['A'..'Z']) "HEYGUYSwhatsup"  
False  
ghci> any (`elem` ['A'..'Z']) "HEYGUYSwhatsup"  
True  

ghci> take 10 $ iterate (*2) 1  
[1,2,4,8,16,32,64,128,256,512]  
ghci> take 3 $ iterate (++ "haha") "haha"  
["haha","hahahaha","hahahahahaha"]  

ghci> splitAt 3 "heyman"  
("hey","man")  
ghci> splitAt 100 "heyman"  
("heyman","")  
ghci> splitAt (-3) "heyman"  
("","heyman")  
ghci> let (a,b) = splitAt 3 "foobar" in b ++ a  
"barfoo"  

ghci> takeWhile (>3) [6,5,4,3,2,1,2,3,4,5,4,3,2,1]  
[6,5,4]  
ghci> takeWhile (/=' ') "This is a sentence"  
"This"  
ghci> sum $ takeWhile (<10000) $ map (^3) [1..]  
53361  

ghci> dropWhile (/=' ') "This is a sentence"  
" is a sentence"  
ghci> dropWhile (<3) [1,2,2,2,3,4,5,4,3,2,1]  
[3,4,5,4,3,2,1]  

ghci> let stock = [(994.4,2008,9,1),(995.2,2008,9,2),(999.2,2008,9,3),(1001.4,2008,9,4),(998.3,2008,9,5)]  
ghci> head (dropWhile (\(val,y,m,d) -> val < 1000) stock)  
(1001.4,2008,9,4)  

ghci> let (fw, rest) = span (/=' ') "This is a sentence" in "First word:" ++ fw ++ ", the rest:" ++ rest  
"First word: This, the rest: is a sentence"  

ghci> break (==4) [1,2,3,4,5,6,7]  
([1,2,3],[4,5,6,7])  
ghci> span (/=4) [1,2,3,4,5,6,7]  
([1,2,3],[4,5,6,7])  

ghci> sort [8,5,3,2,1,6,4,2]  
[1,2,2,3,4,5,6,8]  
ghci> sort "This will be sorted soon"  
"    Tbdeehiillnooorssstw"  

ghci> group [1,1,1,1,2,2,2,2,3,3,2,2,2,5,6,7]  
[[1,1,1,1],[2,2,2,2],[3,3],[2,2,2],[5],[6],[7]]  

ghci> map (\l@(x:xs) -> (x,length l)) . group . sort $ [1,1,1,1,2,2,2,2,3,3,2,2,2,5,6,7]  
[(1,4),(2,7),(3,2),(5,1),(6,1),(7,1)]  

ghci> inits "w00t"  
["","w","w0","w00","w00t"]  
ghci> tails "w00t"  
["w00t","00t","0t","t",""]  
ghci> let w = "w00t" in zip (inits w) (tails w)  
[("","w00t"),("w","00t"),("w0","0t"),("w00","t"),("w00t","")]  

search :: (Eq a) => [a] -> [a] -> Bool  
search needle haystack =   
    let nlen = length needle  
    in  foldl (\acc x -> if take nlen x == needle then True else acc) False (tails haystack)  

ghci> "cat" `isInfixOf` "im a cat burglar"  
True  
ghci> "Cat" `isInfixOf` "im a cat burglar"  
False  

ghci> "hey" `isPrefixOf` "hey there!"  
True  
ghci> "hey" `isPrefixOf` "oh hey there!"  
False  
ghci> "there!" `isSuffixOf` "oh hey there!"  
True  
ghci> "there!" `isSuffixOf` "oh hey there"  
False  

ghci> partition (`elem` ['A'..'Z']) "BOBsidneyMORGANeddy"  
("BOBMORGAN","sidneyeddy")  
ghci> partition (>3) [1,3,5,6,3,2,1,0,3,7]  
([5,6,7],[1,3,3,2,1,0,3])  

ghci> span (`elem` ['A'..'Z']) "BOBsidneyMORGANeddy"  
("BOB","sidneyMORGANeddy")  

ghci> find (>4) [1,2,3,4,5,6]  
Just 5  
ghci> find (>9) [1,2,3,4,5,6]  
Nothing  
ghci> :t find  
find :: (a -> Bool) -> [a] -> Maybe a  

ghci> :t elemIndex  
elemIndex :: (Eq a) => a -> [a] -> Maybe Int  
ghci> 4 `elemIndex` [1,2,3,4,5,6]  
Just 3  
ghci> 10 `elemIndex` [1,2,3,4,5,6]  
Nothing  

ghci> ' ' `elemIndices` "Where are the spaces?"  
[5,9,13]  

ghci> zipWith3 (\x y z -> x + y + z) [1,2,3] [4,5,2,2] [2,2,3]  
[7,9,8]  
ghci> zip4 [2,3,3] [2,2,2] [5,5,3] [2,2,2]  
[(2,2,5,2),(3,2,5,2),(3,2,3,2)]  

ghci> lines "first line\nsecond line\nthird line"  
["first line","second line","third line"]  

ghci> unlines ["first line", "second line", "third line"]  
"first line\nsecond line\nthird line\n"  

ghci> words "hey these are the words in this sentence"  
["hey","these","are","the","words","in","this","sentence"]  
ghci> words "hey these           are    the words in this\nsentence"  
["hey","these","are","the","words","in","this","sentence"]  
ghci> unwords ["hey","there","mate"]  
"hey there mate"  

ghci> nub [1,2,3,4,3,2,1,2,3,4,3,2,1]  
[1,2,3,4]  
ghci> nub "Lots of words and stuff"  
"Lots fwrdanu"  

ghci> delete 'h' "hey there ghang!"  
"ey there ghang!"  
ghci> delete 'h' . delete 'h' $ "hey there ghang!"  
"ey tere ghang!"  

ghci> [1..10] \\ [2,5,9]  
[1,3,4,6,7,8,10]  
ghci> "Im a big baby" \\ "big"  
"Im a  baby"  

ghci> "hey man" `union` "man what's up"  
"hey manwt'sup"  
ghci> [1..7] `union` [5..10]  
[1,2,3,4,5,6,7,8,9,10]  

ghci> [1..7] `intersect` [5..10]  
[5,6,7]  

ghci> insert 4 [3,5,1,2,8,2]  
[3,4,5,1,2,8,2]  
ghci> insert 4 [1,3,4,4,1]  
[1,3,4,4,4,1]  

ghci> let values = [-4.3, -2.4, -1.2, 0.4, 2.3, 5.9, 10.5, 29.1, 5.3, -2.4, -14.5, 2.9, 2.3]  
ghci> groupBy (\x y -> (x > 0) == (y > 0)) values  
[[-4.3,-2.4,-1.2],[0.4,2.3,5.9,10.5,29.1,5.3],[-2.4,-14.5],[2.9,2.3]]  
ghci> groupBy ((==) `on` (> 0)) values  
[[-4.3,-2.4,-1.2],[0.4,2.3,5.9,10.5,29.1,5.3],[-2.4,-14.5],[2.9,2.3]]  

ghci> let xs = [[5,4,5,4,4],[1,2,3],[3,5,4,3],[],[2],[2,2]]  
ghci> sortBy (compare `on` length) xs  
[[],[2],[2,2],[1,2,3],[3,5,4,3],[5,4,5,4,4]]

`Data.Char`

Data.Char has the following trivial functions. isControl, isSpace, isLower, isUpper, isAlpha, isAlphaNum, isPrint, isDigit, isOctDigit, isHexDigit, isLetter, isMark, isNumber, isPunctuation, isSymbol, isSeparator, isAscii, isAsciiUpper, isAsciiLower, toUpper, toLower, toTitle, digitToInt, intToDigit, ord, chr.

ghci> generalCategory ' '  
Space  
ghci> generalCategory 'A'  
UppercaseLetter  
ghci> generalCategory 'a'  
LowercaseLetter  
ghci> generalCategory '.'  
OtherPunctuation  
ghci> generalCategory '9'  
DecimalNumber  
ghci> map generalCategory " \t\nA9?|"  
[Space,Control,Control,UppercaseLetter,DecimalNumber,OtherPunctuation,MathSymbol]  

ghci> map digitToInt "34538"  
[3,4,5,3,8]  
ghci> map digitToInt "FF85AB"  
[15,15,8,5,10,11]  

ghci> intToDigit 15  
'f'  
ghci> intToDigit 5  
'5'  

ghci> ord 'a'  
97  
ghci> chr 97  
'a'  
ghci> map ord "abcdefgh"  
[97,98,99,100,101,102,103,104]

`Data.Map`

Trivial list map implementation

findKey :: (Eq k) => k -> [(k,v)] -> v  
findKey key xs = snd . head . filter (\(k,v) -> key == k) $ xs  

findKey :: (Eq k) => k -> [(k,v)] -> Maybe v  
findKey key = foldr (\(k,v) acc -> if key == k then Just v else acc) Nothing  

phoneBook =   
    [("betty","555-2938")  
    ,("bonnie","452-2928")  
    ,("patsy","493-2928")  
    ,("lucille","205-2928")  
    ,("wendy","939-8282")  
    ,("penny","853-2492")  
    ]  
    
findKey "penny" phoneBook == Just "853-2492" 
findKey "wilma" phoneBook == Nothing

But Data.Map is a lot faster. It can be imported by

1	import qualified Data.Map as Map

Usage:

ghci> Map.fromList [("betty","555-2938"),("bonnie","452-2928"),("lucille","205-2928")] 
fromList [("betty","555-2938"),("bonnie","452-2928"),("lucille","205-2928")]  
ghci> Map.fromList [(1,2),(3,4),(3,2),(5,5)]  
fromList [(1,2),(3,2),(5,5)]  

ghci> Map.empty  
fromList []  
ghci> Map.insert 3 100 Map.empty  
fromList [(3,100)]  
ghci> Map.insert 5 600 (Map.insert 4 200 ( Map.insert 3 100  Map.empty))  
fromList [(3,100),(4,200),(5,600)]  
ghci> Map.insert 5 600 . Map.insert 4 200 . Map.insert 3 100 $ Map.empty  
fromList [(3,100),(4,200),(5,600)]  

ghci> Map.null Map.empty  
True  
ghci> Map.null $ Map.fromList [(2,3),(5,5)]  
False  

ghci> Map.size Map.empty  
0  
ghci> Map.size $ Map.fromList [(2,4),(3,3),(4,2),(5,4),(6,4)]  
5  

ghci> Map.singleton 3 9  
fromList [(3,9)]  
ghci> Map.insert 5 9 $ Map.singleton 3 9  
fromList [(3,9),(5,9)]  

ghci> Map.member 3 $ Map.fromList [(3,6),(4,3),(6,9)]  
True  
ghci> Map.member 3 $ Map.fromList [(2,5),(4,5)]  
False  

ghci> Map.map (*100) $ Map.fromList [(1,1),(2,4),(3,9)]  
fromList [(1,100),(2,400),(3,900)]  
ghci> Map.filter isUpper $ Map.fromList [(1,'a'),(2,'A'),(3,'b'),(4,'B')]  
fromList [(2,'A'),(4,'B')]  
ghci> Map.toList . Map.insert 9 2 $ Map.singleton 4 3  
[(4,3),(9,2)]  

ghci> Map.fromListWith max [(2,3),(2,5),(2,100),(3,29),(3,22),(3,11),(4,22),(4,15)]  
fromList [(2,100),(3,29),(4,22)]  
ghci> Map.fromListWith (+) [(2,3),(2,5),(2,100),(3,29),(3,22),(3,11),(4,22),(4,15)]  
fromList [(2,108),(3,62),(4,37)]  
ghci> Map.insertWith (+) 3 100 $ Map.fromList [(3,4),(5,103),(6,339)]  
fromList [(3,104),(5,103),(6,339)]

`Data.Set`

Imported

1	import qualified Data.Set as Set

Usage

ghci> Set.null Set.empty  
True  
ghci> Set.null $ Set.fromList [3,4,5,5,4,3]  
False  
ghci> Set.size $ Set.fromList [3,4,5,3,4,5]  
3  
ghci> Set.singleton 9  
fromList [9]  
ghci> Set.insert 4 $ Set.fromList [9,3,8,1]  
fromList [1,3,4,8,9]  
ghci> Set.insert 8 $ Set.fromList [5..10]  
fromList [5,6,7,8,9,10]  
ghci> Set.delete 4 $ Set.fromList [3,4,5,4,3,4,5]  
fromList [3,5]  

ghci> Set.fromList [2,3,4] `Set.isSubsetOf` Set.fromList [1,2,3,4,5]  
True  
ghci> Set.fromList [1,2,3,4,5] `Set.isSubsetOf` Set.fromList [1,2,3,4,5]  
True  
ghci> Set.fromList [1,2,3,4,5] `Set.isProperSubsetOf` Set.fromList [1,2,3,4,5]  
False  
ghci> Set.fromList [2,3,4,8] `Set.isSubsetOf` Set.fromList [1,2,3,4,5]  
False  

ghci> Set.filter odd $ Set.fromList [3,4,5,6,7,2,3,4]  
fromList [3,5,7]  
ghci> Set.map (+1) $ Set.fromList [3,4,5,6,7,2,3,4]  
fromList [3,4,5,6,7,8]

Your own modules

Example:

module Geometry  
( sphereVolume  
, sphereArea  
, cubeVolume  
, cubeArea  
, cuboidArea  
, cuboidVolume  
) where  
  
sphereVolume :: Float -> Float  
sphereVolume radius = (4.0 / 3.0) * pi * (radius ^ 3)  
  
sphereArea :: Float -> Float  
sphereArea radius = 4 * pi * (radius ^ 2)  
  
cubeVolume :: Float -> Float  
cubeVolume side = cuboidVolume side side side  
  
cubeArea :: Float -> Float  
cubeArea side = cuboidArea side side side  
  
cuboidVolume :: Float -> Float -> Float -> Float  
cuboidVolume a b c = rectangleArea a b * c  
  
cuboidArea :: Float -> Float -> Float -> Float  
cuboidArea a b c = rectangleArea a b * 2 + rectangleArea a c * 2 + rectangleArea c b * 2  
  
rectangleArea :: Float -> Float -> Float  
rectangleArea a b = a * b

Or organized by files and directories. For example if we have a folder called Geometry. There are three files in the folder - Sphere.hs, Cuboid.hs, and Cube.hs.

In Sphere.hs:

module Geometry.Sphere  
( volume  
, area  
) where  
  
volume :: Float -> Float  
volume radius = (4.0 / 3.0) * pi * (radius ^ 3)  
  
area :: Float -> Float  
area radius = 4 * pi * (radius ^ 2)

In Cuboid.hs:

module Geometry.Cuboid  
( volume  
, area  
) where  
  
volume :: Float -> Float -> Float -> Float  
volume a b c = rectangleArea a b * c  
  
area :: Float -> Float -> Float -> Float  
area a b c = rectangleArea a b * 2 + rectangleArea a c * 2 + rectangleArea c b * 2  
  
rectangleArea :: Float -> Float -> Float  
rectangleArea a b = a * b

In Cube.hs:

module Geometry.Cube  
( volume  
, area  
) where  
  
import qualified Geometry.Cuboid as Cuboid  
  
volume :: Float -> Float  
volume side = Cuboid.volume side side side  
  
area :: Float -> Float  
area side = Cuboid.area side side side

Usage:

1
2
3

import qualified Geometry.Sphere as Sphere  
import qualified Geometry.Cuboid as Cuboid  
import qualified Geometry.Cube as Cube

8. Make your own types and typeclasses

Algebraic data types

How boolean type is defined in the standard library:

1	data Bool = False \| True

Another example:

data Shape = Circle Float Float Float | Rectangle Float Float Float Float   

surface :: Shape -> Float  
surface (Circle _ _ r) = pi * r ^ 2  
surface (Rectangle x1 y1 x2 y2) = (abs $ x2 - x1) * (abs $ y2 - y1)  

surface $ Circle 10 20 10 == 314.15927 
surface $ Rectangle 0 0 100 100 == 10000.0

To improve the previous example, let's make an intermediate data type that defines a point in two-dimensional space and add deriving (Show) at the end a data declaration. Haskell will automatically make the type part of the Show typeclass

data Point = Point Float Float deriving (Show)  
data Shape = Circle Point Float | Rectangle Point Point deriving (Show)  

surface :: Shape -> Float  
surface (Circle _ r) = pi * r ^ 2  
surface (Rectangle (Point x1 y1) (Point x2 y2)) = (abs $ x2 - x1) * (abs $ y2 - y1)  

surface (Rectangle (Point 0 0) (Point 100 100)) == 10000.0  

map (Circle 10 20) [4,5,6,6] == [Circle 10.0 20.0 4.0,Circle 10.0 20.0 5.0,Circle 10.0 20.0 6.0,Circle 10.0 20.0 6.0]

How about a function that nudges a shape? It takes a shape, the amount to move it on the x axis and the amount to move it on the y axis and then returns a new shape that has the same dimensions, only it's located somewhere else

nudge :: Shape -> Float -> Float -> Shape  
nudge (Circle (Point x y) r) a b = Circle (Point (x+a) (y+b)) r  
nudge (Rectangle (Point x1 y1) (Point x2 y2)) a b = Rectangle (Point (x1+a) (y1+b)) (Point (x2+a) (y2+b))  

nudge (Circle (Point 34 34) 10) 5 10 == Circle (Point 39.0 44.0) 10.0

If we don't want to deal directly with points, we can make some auxiliary functions that create shapes of some size at the zero coordinates and then nudge those:

baseCircle :: Float -> Shape
baseCircle r = Circle (Point 0 0) r

baseRect :: Float -> Float -> Shape
baseRect width height = Rectangle (Point 0 0) (Point width height)

nudge (baseRect 40 100) 60 23 == Rectangle (Point 60.0 23.0) (Point 100.0 123.0)

If we wanted to export the functions and types that we defined here in a module, we could start it off like this:

module Shapes 
( Point(..)
, Shape(..)
, surface
, nudge
, baseCircle
, baseRect
) where

By doing Shape(..), we exported all the value constructors for shape, so that means that whoever imports our module can make shapes by using the Rectangle and Circle value constructors. It's the same as writing Shape (Rectangle, Circle)

We could also opt not to export any value constructors for Shape by just writing Shape in the export statement. That way, someone importing our module could only make shapes by using the auxiliary function functions baseCircle and baseRect

Data.Map uses that approach. You cannot create a map by doing Map.Map [(1,2), (3,4)] because it doesn't export that value constructor. However, you can make a mapping by using one of the auxiliary functions like Map.fromList. Not exporting the value constructors of a data types makes them more abstract in such a way that we hide their implementation

Record syntax

If the data type we want to create is complicated, then to create it with the previous method will be unreadable. For example: data Person = Person String string Int Float String String deriving (Show)

Then, instead, we can do the following:

data Person = Person { firstName :: String  
                     , lastName :: String  
                     , age :: Int  
                     , height :: Float  
                     , phoneNumber :: String  
                     , flavor :: String  
                     } deriving (Show)   

let guy = Person "Buddy" "Finklestein" 43 184.2 "526-2928" "Chocolate"  

firstName guy == "Buddy"  
height guy == 184.2 
flavor guy == "Chocolate"

data Car = Car {company :: String, model :: String, year :: Int} deriving (Show)  

Car {company="Ford", model="Mustang", year=1967}

Type parameters

Don't put type constraints into data declarations even if it seems to make sense, because you'll have to put them into the function type declarations either way.

data Maybe a = Nothing | Just a  

data Car a b c = Car { company :: a  
                     , model :: b  
                     , year :: c   
                     } deriving (Show)  

tellCar :: Car -> String  
tellCar (Car {company = c, model = m, year = y}) = "This " ++ c ++ " " ++ m ++ " was made in " ++ show y  

let stang = Car {company="Ford", model="Mustang", year=1967}  

tellCar stang == "This Ford Mustang was made in 1967"  


data Vector a = Vector a a a deriving (Show)  
  
vplus :: (Num t) => Vector t -> Vector t -> Vector t  
(Vector i j k) `vplus` (Vector l m n) = Vector (i+l) (j+m) (k+n)  
  
vectMult :: (Num t) => Vector t -> t -> Vector t  
(Vector i j k) `vectMult` m = Vector (i*m) (j*m) (k*m)  
  
scalarMult :: (Num t) => Vector t -> Vector t -> t  
(Vector i j k) `scalarMult` (Vector l m n) = i*l + j*m + k*n

Derived instances

data Person = Person { firstName :: String  
                     , lastName :: String  
                     , age :: Int  
                     } deriving (Eq, Show, Read)  

let mikeD = Person {firstName = "Michael", lastName = "Diamond", age = 43}  
let adRock = Person {firstName = "Adam", lastName = "Horovitz", age = 41}  
let mca = Person {firstName = "Adam", lastName = "Yauch", age = 44}  

let beastieBoys = [mca, adRock, mikeD]  
mikeD `elem` beastieBoys == True  

"mikeD is: " ++ show mikeD == "mikeD is: Person {firstName = \"Michael\", lastName = \"Diamond\", age = 43}"  

read "Person {firstName =\"Michael\", lastName =\"Diamond\", age = 43}" == mikeD  


data Day = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday   
           deriving (Eq, Ord, Show, Read, Bounded, Enum)  

Monday `compare` Wednesday == LT  
minBound :: Day == Monday  
maxBound :: Day == Sunday  
succ Monday == Tuesday  
pred Saturday == Friday  
[Thursday .. Sunday] == [Thursday,Friday,Saturday,Sunday]  
[minBound .. maxBound] :: [Day] ==[Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday]

Type synonyms

type String = [Char]  

type PhoneNumber = String  
type Name = String  
type PhoneBook = [(Name,PhoneNumber)]  

inPhoneBook :: Name -> PhoneNumber -> PhoneBook -> Bool  
inPhoneBook name pnumber pbook = (name,pnumber) `elem` pbook  

type AssocList k v = [(k, v)]

type IntMap = Map Int

The names of the types and values are independent of each other. We only use a type constructor in a type declaration or a type signature. We only use a value constructor in actual code. So they can be the same.

Another useful data type is

1	data Either a b = Left a \| Right b deriving (Eq, Ord, Read, Show)

Usage:

ghci> :t Right 'a'
Right 'a' :: Either a Char
ghci> :t Left True
Left True :: Either Bool b

This type can be used instead of Maybe to get real information about the returned value.

For example a locker simulator:

import qualified Data.Map as Map

data LockerState = Taken | Free deriving (Show, Eq)

type Code = String

type LockerMap = Map.Map Int (LockerState, Code)

lockerLookup :: Int -> LockerMap -> Either String Code
lockerLookup lockerNumber map =
  case Map.lookup lockerNumber map of
    Nothing -> Left $ "Locker number " ++ show lockerNumber ++ " doesn't exist!"
    Just (state, code) -> if state /= Taken
                          then Right code
                          else Left $ "Locker " ++ show lockerNumber ++ " is already taken!"

Recursive data structures

For example

1	data List a = Empty \| Cons a (List a) deriving (Show, Read, Eq, Ord)

Which can be written in the record syntax as:

1	data List a = Empty \| Cons { listHead :: a, listTail :: List a} deriving (Show, Read, Eq, Ord)

Here Cons is another word for :. : is actually a constructor that takes a value and another list and returns a list.

We can define functions to be automatically infix by making them comprised of only special characters. We can also do the same with constructors

1 2	infixr 5 :-: data List a = Empty \| a :-: (List a) deriving (Show, Read, Eq, Ord)

When we define functions as operators, we can use that to give them a fixity. A fixity states how tightly the operator binds and whether it's left-associative infixl or right-associative infixr.

When deriving Show for our type, Haskell will still display it as if the constructor was a prefix function.

Binary search tree

An example of implementation of the binary search tree

data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show, Read, Eq)

singleton :: a -> Tree a
singleton x = Node x EmptyTree EmptyTree

treeInsert :: (Ord a) => a -> Tree a -> Tree a
treeInsert x EmptyTree = singleton x
treeInsert x (Node a left right)
	| x == a = Node x left right
	| x <  a = Node a (treeInsert x left) right
	| x >  a = Node a left (treeInsert x right)
	
treeElem :: (Ord a) => a -> Tree a -> Bool
treeElem x EmptyTree = False
treeElem x (Node a left right)
	| x == a = True
	| x <  a = treeElem x left
	| x >  a = treeElem x right
	
let numsTree = foldr treeInsert EmptyTree [8,6,5,1,3,7,4]

Typeclasses

For example:

class Eq a where
	(==) :: a -> a -> Bool
	(/=) :: a -> a -> Bool
	x == y = not (x /= y)
	x /= y = not (x == y)
	
data TrafficLight = Red | Yellow | Green

instance Eq TrafficLight where
	Red == Red = True
	Green == Green = True
	Yellow == Yellow = True
	_ == _ = False
	
instance Show TrafficLight where
	show Red = "Red light"
	show Yellow = "Yellow light"
	show Green = "Green light"
	
instance (Eq m) => Eq (Maybe m) where
	Just x == Just y = x == y
	Nothing == Nothing = True
	_ == _ = False

The Functor typeclass

Types with the Functor typeclass is basically those things that can be mapped over, i.e. the list type is part of the Functor typeclass.

For example:

class Functor f where
	fmap :: (a -> b) -> f a -> f b

instance Functor [] where
	fmap = map

instance Functor Maybe where
	fmap f (Just x) = Just (f x)
	fmap f Nothing = Nothing

instance Functor Tree where
	fmap f EmptyTree = EmptyTree
	fmap f (Node x leftsub rightsub) = Node (f x) (fmap f leftsub) (fmap f rightsub)
	
instance Functor (Either a) where
	fmap f (Right x) = Right (f x)
	fmap f (Left x) = Left x

9. Input and Output

Hello, world!

In file helloworld.hs

1	main = putStrLn "Hello, world"

To compile and run:

$ ghc --make helloworld
[1 of 1] Compiling Main             ( helloworld.hs, helloworld.o )  
Linking helloworld ...  
$ ./helloworld
Hello, world

The type of the function putStrLn:

1 2	ghci> :t putStrLn putStrLn :: String -> IO ()

A more useful example:

main = do
	putStrLn "Hello, what's your name?"
	name <- getLine
	putStrLn ("Hey " ++ name ++ ", you rock!")

Each of these steps is an I/O action. By putting them together with do syntax, we glued them into one I/O action. The action that we got has a type of IO (), because that's the type of the last I/O action inside. The last action cannot be bound to a name like the first two.

The type of getLine

1 2	ghci> :t getLine getLine :: IO String

The getLine perform the I/O action and then bind its result value to name. getLine has a type of IO String, so name will have a type of String.

Another example:

main = do
	line <- getLine
	if null line
		then return ()
		else do 
			putStrLn $ reverseWords line
			main

reverseWords :: String -> String
reverseWords = unwords . map reverse . words

Using return doesn't cause the I/O do block to end in execution or anything like that. The return makes I/O actions that don't really do anything except have an encapsulated result and that result is thrown away because it isn't bound to a name. For example:

main = do 
	a <- return "hell"
	b <- return "yeah!"
	putStrLn $ a ++ " " ++ b

return is sort of the opposite to <-. While return takes a value and wraps it up in a box, <- takes a box (and performs it) and takes the value out of it, binding it to a name.

There are more I/O functions:

putStr
putChar
print == putStrLn . show
getChar
when: it looks like a control flow statement, but it's actually a normal function. It takes a boolean value and an I/O action. If that boolean value is True, it returns the same I/O action that we supplied to it. If it is False, it returns the return ().
1
2
3
4
5
6
7
import Control.Monad

main = do
c <- getChar
when (c /= ' ') $ do
putChar c
main
sequence takes a list of I/O actions and returns an I/O actions that will perform those actions one after the other. The result contained in that I/O action will be a list of the results of all the I/O actions that were performed.
1
2
3
main = do
rs <- sequence [map print [1,2,3,4,5]]
print rs
mapM: takes a function and a list, maps the function over the list and then sequences it
- mapM_ does the same, only it throws away the result later. We usually use mapM_ when we don't care what result our sequenced I/O actions have.
- forM: like mapM, only that it has its parameters switched around.

forever: takes an I/O action and returns an I/O action that just repeats the I/O action it got forever.

import Control.Monad
import Data.Char

main = forever $ do
	putStr "Give me some input: "
	l <- getLine
	putStrLn $ map toUpper l

Files and streams

getContents is an I/O action that reads everything from the standard input until it encounters an end-of-file character. Its type is getContents :: IO String. getContents goes a lazy I/O, it doesn't read all of the input at once. For example:

import Data.Char

main = do
	contents <- getContents
	putStr (map toUpper contexts)

There is a function called interact that takes a function of type String -> String as a parameter and returns an I/O action that will take some input, run that function on it and then print out the function's result. For example to filter only lines shorter than 10 characters:

main = interact shortLinesOnly

shortLinesOnly :: String -> String
shortLinesOnly input = 
	let allLines = lines input
		shortLiens = filter (\line -> length line < 10) allLines
		result = unlines shortLines
	in result

Which can be reduced to one line:

1	main = interact $ unlines . filter ((< 10) . length) . lines

For file read, there is a function called hGetContents. It takes a Handle, so it knows which file to get the contents from and returns an IO String - an I/O action that holds as its result the contents of the file. hGetContents won't attempt to read the file at once and store it in memory, but it will read it as needed. For example:

import System.IO

main = do
	handle <- openFile "file_to_read.txt" ReadMode
	contents <- hGetContents handle
	putStr contents
	hClose handle

Another way of doing this is with withFile:

import System.IO

main = do
	withFile "file_to_read.txt" ReadMode (\handle -> do
		contents <- hGetContents handle
		putStr contents)

The withFile can be implemented in the following way:

withFile' :: FilePath -> IOMode -> (Handle -> IO a) -> IO a
withFile' path mode f = do
	handle <- openFile path mode
	result <- f handle
	hClose handle
	return result

Other functions to read and write file: * readFile has a type signature of readFile :: FilePath -> IO String. readFile takes a path to a file and return an I/O action that will read that file (lazily) and bind its contents to something as a string. * writeFile has a type of writeFile :: FilePath -> String -> IO (). It takes a path to a file and a string to write to that file and returns an I/O action that will do the writing. * appendFile has a type signature that's just like writeFile, only appendFile doesn't truncate the file to zero length if it already exists but it appends stuff to it.

You can control how exactly buffering is done by using the hSetBuffering function. It takes a handle and a BufferMode and returns an I/O action that sets the buffering. BufferMode is a simple enumeration data type and the possible values it can hold are: NoBuffering, LineBuffering or BlockBuffering (Maybe Int). The Maybe Int is for how big the chunk should be, in bytes. If it is Nothing, then the operating system determines the chunk size. NoBuffering means that it will be read one character at a time. For example:

main = do
	withFile "something.txt" ReadMode (\handle -> do
		hSetBuffering handle $ BlockBuffering (Just 2048)
		contents <- hGetContents handle
		putStr contents)

We can also use hFlush, which is a function that takes a handle and returns an I/O action that will flush the buffer of the file associated with the handle.

Command line arguments

The System.Environment module has two cool I/O actions. One is getArgs, which has a type of getArgs :: IO [String] and is an I/O action that will get the arguments that the program was run with and have as its contained result a list with the arguments. getProgName has a type of getProgName :: IO String and is an I/O action that contains the program name:

import System.Environment
import Data.List

main = do
	args <- getArgs
	progName <- getProgName
	putStrLn "The arguments are: "
	mapM putStrLn args
	putStrLn "The program name is: "
	putStrLn progName

A simple todo list apps

import System.Environment
import System.Directory
import System.IO
import Data.List

dispatch :: [(String, [String] -> IO ())]
dispatch = [
	("add", add),
	("view", view),
	("remove", remove)
	]
	
main = do
	(command:args) <- getArgs
	let (Just action) = lookup command dispatch
	action args

add :: [String] -> IO ()
add [fileName, todoItem] = appendFile fileName (todoItem ++ "\n")

view :: [String] -> IO ()
view [fileName] = do 
	contents <- readFile fileName
	let todoTasks = lines contents
		numberedTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks
	putStr $ unlines numberedTasks

remove :: [String] -> IO ()
remove [fileName, numberString] = do 
	handle <- openFile fileName ReadMode
	(tempName, tempHandle) <- openTempFile "." "temp"
	contents <- hGetContents handle
	let number = read numberString 
		todoTasks = lines contents
		newTodoItems = delete (todoTasks !! number) todoTasks
	hPutStr tempHandle $ unlines newTodoItems
	hClose handle
	hClose tempHandle
	removeFile fileName
	renameFile tempName fileName

Bytestrings

Bytestrings are sort of like lists, only each element is one byte (or 8 bits) in size. Bytestrings come in two flavors: strict and lazy ones. Strict bytestrings reside in Data.ByteString. A strict bytestring represents a series of bytes in an array. The upside is that there's less overhead because there are no thunks involved. The downside is that they're likely to fill your memory up faster.

The other variety of bytestrings resides in Data.ByteString.Lazy. If you evaluatea byte in a lazy bytestring, the first 64K will be evalated. After that, it's just a promise for the rest of the chunks.

For example:

import qualified Data.ByteString.Lazy as B 
import qualified Data.ByteString as S 

ghci> B.pack [99,97,110] 
Chunk "can" Empty 
ghci> B.pack [98..120] 
Chunk "bcdefghijklmnopqrstuvwx" Empty 
ghci> B.fromChunks [S.pack [40,41,42], S.pack [43,44,45], S.pack [46,47,48]] 
Chunk "()*" (Chunk "+,-" (Chunk "./0" Empty)) 
ghci> B.cons 85 $ B.pack [80,81,82,84] 
Chunk "U" (Chunk "PQRT" Empty) 
ghci> B.cons' 85 $ B.pack [80,81,82,84] 
Chunk "UPQRT" Empty 
ghci> foldr B.cons B.empty [50..60] 
Chunk "2" (Chunk "3" (Chunk "4" (Chunk "5" (Chunk "6" (Chunk "7" (Chunk "8" (Chunk "9" (Chunk ":" (Chunk ";" (Chunk "<" 
Empty)))))))))) 
ghci> foldr B.cons' B.empty [50..60] 
Chunk "23456789:;<" Empty

Exceptions

Pure code can throw exceptions, but they can only be caught in the I/O part of our code. But for a good practice, don't mix exceptions and pure code. Take advatange of Haskell's powerful type system and use types like Either and Maybe to represent results that may have failed.

An example:

import System.Environment
import System.IO
import System.Directory

main = do (fileName:_) <- getArgs
	fileExists <- doesFileExist fileName
	if fileExists
		then do contents <- readFile fileName
			putStrLn $ "The file has " ++ show (length (lines contents)) ++ " lines!"
		else do putStrLn "The file doesn't exist!"

With exception:

import System.Environment
import System.IO
import System.IO.Error

main = toTry `catch` handler

toTry :: IO ()
toTry = do (fileName:_) <- getArgs
	contents <- readFile fileName
	putStrLn $ "The file has " ++ show (length (lines contents)) ++ " lines"

handler :: IOError -> IO ()
handler e
	| isDoesNotExistError e = putStrLn "The file doesn't exist"
	| otherwise = ioError e

The predicates that act on IOError are: * isAlreadyExistError * isDoesNotExistError * isAlreadyInUseError * isFullError * isEOFError * isIllegalOperation * isPermissionError * isUserError

10. Functors, Applicative, Functors and Monoids

Functors redux

Functors are things that can be mapped over, like lists, Maybes, trees, and such. In Haskell, they are described by the typeclass Functor, which has only one typeclass method, namely fmap, which has a type of fmap :: (a -> b) -> f a -> f b.

Let's see how IO is an instance of Functor. When we fmap a function over an I/O action, we want to get back an I/O action that does the same thing, but has our function applied over its result value.

instance Functor IO where 
	fmap f action = do 
		result <- action
		return (f result)

main = do line <- fmap reverse getLine
	putStrLn $ "You said " ++ line ++ " backwards!"

The result of mapping something over an I/O action will be an I/O action, so right off the bat we use do syntax to glue two actions and make a new one.

Another instance of Functor that we've been dealing with all along without knowing was (->) r.

1 2	instance Functor ((->) r) where fmap f g = (\x -> f (g x))

Its type is then fmap :: (a -> b) -> (r -> a) -> (r -> b) which reminds us about function compositions. Thus it can be written as:

1 2	instance Functor ((->) r) where fmap = (.)

Law of Functor

If we map the id function over a functor, the functor that we get back should be the same as the original functor.
Composing two functions and then mapping the resulting function over a functor should be the same as first mapping one function over the functor and then mapping the other one.

Applicative Functors

Applicative functors are represented in Haskell by the Applicative typeclass, found in the Control.Applicative module.

1
2
3

class (Functor f) => Applicative f where 
	pure :: a -> f a
	(<*>) :: f (a -> b) -> f a -> f b

It starts the definition of the Applicative class and it also introduces a class constraint. It says that if we want to make a type onstructor part of the Applicative typeclass, it has to be in Functor first. The first method it defines is called pure. pure should take a value of any type and return an applicative functor with that value inside it. The <*> function has a type declaration of f (a -> b) -> f a -> f b. It's a sort of a beefed up fmap. <*> takes a fnctor that has a function in it and another functor and sort of extracts that function from the first functor and then maps it over the second one.

For example, Applicative instance implementation for Maybe:

instance Applicative Maybe where 
	pure = Just
	Nothing <*> _ = Nothing
	(Just f) <*> something = fmap f something

ghci> pure (+) <*> Just 3 <*> Just 5 
Just 8 
ghci> pure (+) <*> Just 3 <*> Nothing 
Nothing 
ghci> pure (+) <*> Nothing <*> Just 5 
Nothing

Applicative instance implementaton for lists:

instance Applicative [] where
	pure x = [x]
	fs <*> xs = [f x | f <- fs, x <- xs]

ghci> [(+),(*)] <*> [1,2] <*> [3,4] 
[4,5,5,6,3,4,6,8]

Control.Applicative also exports a function called <$>, which is just fmap as an infix operator:

(<$>) :: (Functor f) => (a -> b) -> f a -> f b
f <$> x = fmap f x

(*) <$> [2,5,10] <*> [8,10,11] 
[16,20,22,40,50,55,80,100,110]

Monoids

A monoid is when you have an associative binary function and a value which acts as an identity with respect to that function. When something acts as an identity with respect to a function, it means hat when called with that function and some other value, the result is always equal to that other value.

It is defined as:

class Monoid m where
	mempty :: m
	mappend :: m -> m -> m
	mconcat :: [m] -> m
	mconcat = foldr mappend mempty

The Monoid type class is defined in Data.Monoid. List, product, and sum are all monoids. For example:

instance Num a => Monoid (Product a) where 
	mempty = Product 1
	Product x `mappend` Product y = Product (x * y)

ghci> getProduct $ Product 3 `mappend` Product 9 
27 
ghci> getProduct $ Product 3 `mappend` mempty 
3 
ghci> getProduct . mconcat . map Product $ [3,4,2] 
24

Monoid instance for `Ordering`

instance Monoid Ordering where
	mempty = EQ 
	LT `mappend` _ = LT
	EQ `mappend` y = y
	GT `mappend` _ = GT

`Maybe` the Monoid

instance Monoid a => Monoid (Maybe a) where  
    mempty = Nothing  
    Nothing `mappend` m = m  
    m `mappend` Nothing = m  
    Just m1 `mappend` Just m2 = Just (m1 `mappend` m2)

11. Monads

Monads are just beefed up applicative functors, much like applicative functors are only beefed up functors.

An applicative value can be seeen as a value with an added context. The Applicative type class allowed us tp use normal functions on these values with context and the context was preserved. For example, Maybe a values represent computations that might have failed, [a] values represent computations that have several results (non-deterministic computations), IO a values represent values that have side-effects, etc.

Monads are a natural extension of applicative functors. If you have a value with a context, m a, how do you apply it to a function that takes a normal a and returns a value with a context? So essentially, we will want this function:

1	(>>=) :: (Monad m) => m a -> (a -> m b) -> m b

Monads are just applicative functors that support >>=. The >>= function is pronounced as bind.

`Maybe` Monad

Maybe is a monad. >>= would take a Maybe a value and a function of type a -> Maybe b and somehow apply the function to the Maybe a. For example

1 2	ghci> (\x -> Just (x+1)) 1 Just 2

Instead of calling it >>=, let's call it applyMaybe for now. It takes a Maybe a and a function that returns a Maybe b and manages to apply that function to the Maybe a:

applyMaybe :: Maybe a -> (a -> Maybe b) -> Maybe b
applyMaybe Nothing f  = Nothing
applyMaybe (Just x) f = f x

ghci> Just 3 `applyMaybe` \x -> Just (x+1) 
Just 4 
ghci> Just "smile" `applyMaybe` \x -> Just (x ++ " :)") 
Just "smile :)"

The monad type class

class Monad m where 
	return :: a -> m a
	
	(>>=) :: m a -> (a -> m b) -> m b
	
	(>>) :: m a -> m b -> m b
	x >> y = x >>= \_ -> y
	
	fail :: String -> m a
	fail msg = error msg

We don't need to write class (Applicative m) => Monad m where because every monad is an applicative functor by default.
return is the same as pure, only with a different name. It takes a value and puts it in a minimal default context that still holds that value. In other words, it takes something and wraps it in a monad.
The next function is >>=, or bind. It's like tunction application, only instead of taking a normal value and feeding it to a normal function, it takes a monadic value and feeds it to a function that takes a normal value but returns monadic value.
The >> function comes with a default implementation and we pretty much never implement it when making Monad instances. >> is used when we want to pass some value to a function that ignores its parameter and always just returns some predetermined value.
fail function will not be used explicitly in our code. Instead, it's used by Haskell to enable failure in a special syntactic construct for monads that we'll meet later.

Maybe monad:

instance Monad Maybe where
	return x  = Just x
	Nothing >>= f = Nothing
	Just x  >>= f x
	fail _    = Nothing

ghci> return "wait" :: Maybe String
Just "wait"
ghci> Just 9 >>= \x -> return (x * 10)
Just 90
ghci> Nothing >>= \x -> return (x * 10)
Nothing

do notation

The do notation isn't just for IO, but can be used for any monad. Its principle is the same, gluing together monadic values in sequence.

The following example:

1 2	ghci> Just 3 >>= (\x -> Just "!" >>= (\y -> Just (show x ++ y))) Just "3!"

Can be written as:

foo :: Maybe String
foo = do
	x <- Just 3
	y <- Just "!"
	Just (show x ++ y)

In a do expression, every line is a monadic value.

routine :: Maybe Soup
routine = do
	start <- return (0,0)      -- means (water, salt)
	first <- addSalt 4 start   -- becomes (0, 4)
	Nothing                    -- means too salty
	second <- addWater 2 first -- too late to add water
	addWater 2 second          -- final result

The list monad

The Monad instance for lists:

instance Monad [] where 
	return x = [x]
	xs >>= f = concat (map f xs)
	fail _ = []

ghci> [3, 4, 5] >>= \x -> [x, -x]
[3,-3,4,-4,5,-5]

Another example of propagation of the non-determinism:

1 2	ghci> [1,2] >>= \n -> ['a', 'b'] >>= \ch -> return (n, ch) [(1,'a'),(1,'b'),(2,'a'),(2,'b')]

It is much clearer with do notation:

listOfTuples :: [(Int, Char)]
listOfTuples = do
	n <- [1, 2]
	ch <- ['a', 'b']
	return (n, ch)

This is similar to list comprehensions. In fact, list comprehensions are just syntactic sugar for using lists as monads.

1 2	ghci> [ (n, ch) \| n <- [1,2], ch <- ['a', 'b']] [(1,'a'),(1,'b'),(2,'a'),(2,'b')]

Monad laws

Left identity: If we take a value, put it in a default context with return and then feed it to a function by using >>=, it's the same as just taking the value and applying the function to it. Formally:
- return x >>= f is the same thing as f x
Right identity: If we have a monadic value and we use >>= to feed it to return, the result is our original monadic value. Formally:
- m >>= return is no different than just m
Associativity: When we have a chain of monadic function applications with >>=, it shouldn't matter how they're nested. Formally:
- Doing (m >>= f) >>= g is just like doing m >>= (\x -> f x >>= g)

Add log to code with the Writer type

newtype Writer w a = Writer { runWriter :: (a, w) }

instance (Monoid w) => Monad (Writer w) where 
	return x = Writer (x, mempty)
	(Writer (x, v)) >>= f = let (Writer (y, v')) = f x in Writer (y, v `mappend` v')

-- --------------------------------------------------------
import Control.Monad.Writer

logNumber :: Int -> Writer [String] Int
logNumber x = Writer (x, ["Got number: " ++ show x])

multWithLog :: Writer [String] Int 
multWithLog = do
	a <- logNumber 3
	b <- logNumber 5
	return ( a * b )

gcd' :: Int -> Int -> Writer [String] Int
gcd' a b
	| b == 0 = do 
		tell ["Finished with " ++ show a]
		return a
	| otherwise = do 
		tell [show a ++ " mod " ++ show b ++ " = " ++ show (a `mod` b)]
		gcd' b (a `mod` b)

ghci> runWriter multWithLog 
(15,["Got number: 3","Got number: 5"]) 
ghci> mapM_ putStrLn $ snd $ runWriter (gcd' 8 3) 
8 mod 3 = 2 
3 mod 2 = 1 
2 mod 1 = 0 
Finished with 1

The apply monad

instance Monad ((->) r) where 
	return x = \_ -> x
	h >>= f = \w -> f (h w) w

addStuff :: Int -> Int
addStuff = do 
	a <- (*2)
	b <- (+10)
	reutrn (a + b)

addStuff 3 == 19

Stateful computations

newtype State s a = State { runState :: s -> (a, s) }

instance Monad (State s) where 
	return x = State $ \s -> (x, s)
	(State h) >>= f = State $ \s -> let (a, newState) = h s
		                                (State g) = f a
									in g newState

-- -------------------------------------------------------- 
import Control.Monad.State 

type Stack = [Int]
pop :: State Stack Int
pop = State $ \(x:xs) -> (x, xs)

push :: Int -> State Stack ()
push a = State $ \xs -> ((), a:xs)

stackManip :: State Stack Int
stackManip = do 
	a <- pop
	if a == 5
		then push 5
		else do
			push 3
			push 8

ghci> runState stackStuff [9,0,2,1,0]
((),[8,3,0,2,1,0])

1. Introduction

2. Starting out

Some basic ideas

Intro to list

Texas ranges

List comprehension

Tuples

3. Types and Typeclasses

Some basic types

Types of functions

Type variables

Intro to typeclasses

Some basic typeclasses:

4. Syntax in Functions

Pattern matching

Guards

Let

Case expressions

5. Recursion

Some recursion examples:

6. Higher order functions

Curried functions

Section an infix function

Some higher-orderism is in order

Maps and filters

Lambda

Foldl

Function application with $

Function composition

7. Modules

Data.List

Data.Char

Data.Map

Data.Set

Your own modules

8. Make your own types and typeclasses

Algebraic data types

Record syntax

Type parameters

Derived instances

Type synonyms

Recursive data structures

Binary search tree

Typeclasses

The Functor typeclass

9. Input and Output

Hello, world!

Files and streams

Command line arguments

A simple todo list apps

Bytestrings

Exceptions

10. Functors, Applicative, Functors and Monoids

Functors redux

Law of Functor

Applicative Functors

Monoids

Monoid instance for Ordering

Maybe the Monoid

11. Monads

Maybe Monad

The monad type class

do notation

The list monad

Monad laws

Add log to code with the Writer type

The apply monad

Stateful computations

Function application with `$`

`Data.List`

`Data.Char`

`Data.Map`

`Data.Set`

Monoid instance for `Ordering`

`Maybe` the Monoid

`Maybe` Monad