# Reference card

From HaskellWiki

Revision as of 20:47, 7 February 2006 by JaredUpdike (talk | contribs) (Copied Neil Mitchell's Haskell Reference Card (with his Permission) http://www.nmitchell.co.uk/code/haskell.htm)

## Contents

## General Syntax

### Comments

{- A multiline comment which can continue for many lines -}

-- a single line comment

### Conditionals

#### if

if conditional then truePart else falsePart if a == 12 then 14 else 22

#### case

case exp of Pattern1 -> action1 Pattern2 -> action2 _ -> else_action

case x of [] -> 0 [x] -> 1 _ -> -1

#### Function pattern matching

f [ ] = 0 f [x] = 1 f _ = -1

#### Function conditionals (guards)

f x | x == [] = 1 | length x == 12 = 15 | otherwise = -1

### Saving work

#### where

f x = i * i where i = g x

#### let in

f x = let i = g x in i * i

### Declaring types

#### data

data List = Cons Int List | Nil deriving (Eq, Show, Ord)

#### type (type synonymns)

type String = [Char] type Name = TypeValue

#### class

class Check a where test :: a -> Bool force :: a -> a

#### instance

instance Show List where show x = "No show"

### Calling functions

#### Named functions

myFunc :: Int -> Int -> Int result = myFunc 1 2 result = 1 `myFunc` 2 result = (myFunc 1) 2 result = (`myFunc` 2) 1

#### Operators (and sections)

(+) :: Int -> Int -> Int result = 1 + 2 result = (+) 1 2 result = (1 +) 2 result = (+ 2) 1

#### Useful functions

myFunc 1 2 == (flip myFunc) 2 1 (f . g) x == f (g x) f (a+b) == f $ a+b

#### Lambda Expressions

myFunc = (\ a b -> a + b) result = map (\x -> head x) xs

### List Expressions

#### ..

[1..] = [1,2,3,4,5,6... [1..5] = [1,2,3,4,5] [1,3..5] = [1,3,5]

#### List Comprehensions

[ x*x | x <- [1..3] ] ==> [1,4,9] [ (x, y) | x < - [1..3], y <- "ab"] ==> [(1,'a'),(1,'b'),(2,'a'),(2,'b'),(3,'a'),(3,'b')] [ (x,y) | x <- [1..4], even x, y <- "ab" ] ==> [(2,'a'),(2,'b'),(4,'a'),(4,'b')] map f xs ==> [ f x | x <- xs ] filter p xs ==> [ x | x <- xs, p x ]

## Hello World

main = putStrLn "Hello World"

## Snippets

### foldm :: (x -> x -> x) -> [x] -> x

foldm f (x1:x2:xs) = foldm f (f x1 x2:xs) foldm f [x ] = x foldm f [ ] = error "Can't foldm on an empty list"

### fst3 snd3, thd3 :: (x, y, z) -> x

fst3 (x,_,_) = x snd3 (_,x,_) = x thd3 (_,_,x) = x

### ordPair :: Ord x => x -> x -> (x, x)

ordPair a b = if a < b then (a, b) else (b, a)

### lenXX# :: [x] -> Bool

lenEq0 x = null lenNe0 x = not . null

lenEq1 [x] = True lenEq1 _ = False

lenGt1 [x] = False lenGt1 [ ] = False lenGt1 _ = True

### sortUnique :: Ord a => [a] -> [a]

sortUnique [] = [] sortUnique [x] = [x] sortUnique xs = mergeUnique (sortUnique a) (sortUnique b) where (a,b) = split xs

### split :: [a] -> ([a], [a])

split [] = ([], []) split [a] = ([a], []) split (a:b:xs) = (a:as, b:bs) where (as,bs) = split xs

### mergeUnique :: Ord a => [a] -> [a] -> [a]

Precondition:

isUnique(#1) && isUnique(#2)

mergeUnique a [] = a mergeUnique [] b = b mergeUnique (a:as) (b:bs) = case compare a b of EQ -> a: mergeUnique as bs LT -> a: mergeUnique as (b:bs) GT -> b: mergeUnique (a:as) bs

fix :: (x -> x) -> x -> x

fix f x = if x == x' then x else fix f x' where x' = f x

## Command lines

### For hmake

hmake Test hmake -realclean Test

Where Test is the name of the executable you want to build, i.e. where Test.hs contains the main function.

### For ghc --make

ghc --make MainModule.hs -o ModuleExec ghc --make Module.hs

Where ModuleExec is the name of the output binary you want to make (if main is exported). Module.o will be output for Module.hs, if main is not exported.