Haskell Intro Examples

Basics

Math

3+4
5*6
2^10
2^100
3/4
9 `mod` 5

Lists and Ranges

[]
[2]
[2,3,5]
[2,3,5] ++ [6,7]
'a'
['a']
['a','b']
"hello" ++ "world"
[1..20]

Functions

sqrt 5
even 5
even 6
head [1,2,3,4]
head "hello"
take 0 [1,2,3,4]
take 1 [1,2,3,4]
take 2 [1,2,3,4]
take 2 "hello"

Types

:t True
:t 'a'
:t "abc"
:t 42
:t [2,3,4]
:t 3.14
:t sqrt
:t even
:t head
:t take

First-Order Functions

Currying

take2 = take 2
:t take2
take2 [1,2,3,4]

:t (^)
:t (2^)
:t (^2)

(^2) 3
(2^) 3

square = (^2)
pow2 = (2^)

Filter

:t filter
filter odd [0..10]
filter even [0..10]
evens = filter even
evens [0.10]

Map

:t map
map even [0..10]
map square [0..10]
map square  (filter odd [0..10])
map square $ filter odd [0..10]

Recursive Definitions

Lists w/ Cons

2:[]
2:3:5:[]
4:[2,3,5]

List Functions

head [1,2,3,4]
tail [1,2,3,4]
length [1,2,3,4]
reverse [1,2,3,4]

List Patterns

In ghci, use :{ and :} to wrap multiline definitions.

len [] = 0
len (x:xs) = 1 + (len xs)

rev [] = []
rev (x:xs) = (rev xs)++[x]

Miscellaneous Haskell Novelties

Conditional Expressions

if (even 6) then "even" else "odd"
if (even 7) then "even" else "odd"

Let Clauses

nextOdd x = if odd (x+1) then x+1 else x+2
nextOdd x = let y=x+1 in if odd y then y else y+1

oddsGreaterThan x = let y = nextOdd x in y:(oddsGreaterThan y)
oddsGreaterThan 9

Lazy Evaluation

take 5  (oddsGreaterThan 9)
take 5 $ oddsGreaterThan 9

Quicksort

With filter:

qsort [] = []
qsort (x:xs) = (qsort $ filter (<=x) xs)
               ++ [x] ++
               (qsort $ filter (>x) xs)

With list comprehensions:

qsort [] = []
qsort (x:xs) = qsort [y | y <- xs, y <= x]
               ++ [x] ++
               qsort [y | y <- xs, y > x]