Haskell reminder

Haskell_reminder.md

Haskell reminder

A bunch of True (when relevant) haskell statements.

General
Flow control
Data structures
Conversions
Modules and types definition
Ghci
Usefull links

General

import Prelude hiding (foldl)
import qualified Data.List as L (nub, sort)
-- comment
print "hello world"
interact -- read stdin
(sqrt $ 3 + 4) == sqrt (3 + 4)
(negate . sum) [1,2] == negate (sum [1,2])

isAlphaNum’ = (`elem` ['A'..'z'])
compare100 = compare 100
reverse' = foldl (\acc x -> x : acc) [ ]

Flow control

if True then True else False
not (1 /= 1)
even :: Int -> Bool
even n
    | n `rem` 2 == 0 = True
    | otherwise         = False

cylinderArea r h =
    let sideArea = 2 * pi * r * h
        topArea = pi * r ^2
    in  sideArea + 2 * topArea

cylinderArea r h = sideArea + 2 * topArea
    where sideArea = 2 * pi * r * h
               topArea = pi * r ^2

Data structures

True, False
minbound :: int == -2147483648

tuple = (1,2)
fst (1,2) == 1
snd (1,2) == 2

list = [1, 2]
[1..3] == [1,2,3]
[x*2 | x <- [0..3], x > 0] == [2,4,6]
1:[2,3] == [1,2,3]
[1,2] ++ [3,4] == [1,2,3,4]
take 3 [1,2,3,4] == [1,2,3]
drop 2 [1,2,3] == [3]
length [1,2] == 2
head [1,2] == 1
last [1,2] == 2
tail [1,2,3] == [2,3]
init [1,2,3] == [1,2]
null [ ]
[4,5,6] !! 1 == 5
reverse [1,2] == [2,1]
maximum [1,2] == 2
product [1,2] == 2
sum [1,2] == 3
1 `elem` [1,2]
[0,5..13] == [0,5,10]
take 2 $ cycle [1] == [1,1]
replicate 2 1 == [1,1]
take 2 $ repeat 1 == [1,1]
zipWith (+) [1,2] [3,4] == [4,6]
takeWhile (/=3) [1,2,3,4] == [1,2]
all even [2,4]
any even [1,2]

Conversions

show 10 == "10"
(read "5" :: Float) == 5.0
(fromIntegral 1 :: Float) == 1.0
zip [1,2] ['a', 'b'] == [(1,'a'),(2,'b')]
words "a b" == ["a","b"]
unlines ["a","b"] == "a\nb\n"

Modules and types definition

In Gemometry/Shapes.hs :

module Shapes
( Point(..)
, Shape(..)
, surface) where
data Shape = Circle Point Float | Rectangle Point Point
data Point = Point { x :: Float
                   , y :: Float
                   } deriving (Show)
-- recurcive definition (with :-: operator defintion)
infixr 5 :-:
data List' a = Empty | a :-: (List' a) deriving (Show, Read, Eq, Ord)

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

-- type alias for [Char]
type String = [Char]

instance Eq Point where
    Red == Red = True
    Green == Green = True
    Yellow == Yellow = True
    _ == _ = False

class Empty a where
    empty :: a -> Bool
    filled x = not $ empty x

Ghci

:m + Data.List  
:l file.hs  
:t even

Usefull links

HTTP server library
Web services
Data structures and algorithms
Monadic parsing
Concurrent communication
Process monitoring
JSON