Reference card

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

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 :: Eq x => (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.