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Haskell

Nov 05, 20232 min read

List

  • ++ and : for concatenation
  • head, tail, init, last
  • reverse
  • take num, drop num
  • null
  • elem
    • elem 'x' ['a', 'b', 'c']
  • concat
    • concat [[3, 1], [2], [], [5, 10]]
  • Linear time function
    • length
    • !! index
  • range
    • [1, 3 .. 20]
      • [1,3,5,7,9,11,13,15,17,19]
    • [10, 9 .. 1]
      • [10,9,8,7,6,5,4,3,2,1]
      • [0 .. ] infinite lists

String

list of characters

  • word
  • unword

Pakages

  • module — a collection of functions and custom data types

  • package — a collection of modules + metadata

  • Hackage — a central repository of Haskell packages

  • base — standard Haskell library

  • Prelude — the module from base imported by default

  • import Data.List

Control Flow

  • if then else
    • price = if product == "milk" then 1 else 2
  • guards
  • case of
describe :: Integer -> String
describe n = case n of 0 -> "zero"
                       1 -> "one"
                       2 -> "an even prime"
                       n -> "the number " ++ show n
					   
-- parse country code into country name, returns Nothing if code not recognized
parseCountry :: String -> Maybe String
parseCountry "FI" = Just "Finland"
parseCountry "SE" = Just "Sweden"
parseCountry _ = Nothing
 
flyTo :: String -> String
flyTo countryCode = case parseCountry countryCode of Just country -> "You're flying to " ++ country
                                                     Nothing -> "You're not flying anywhere"

Misc

  • let in
  • where

Maybe

TypeValues
Maybe BoolNothing, Just False, Just True
Maybe IntNothing, Just 0, Just 1, …
Maybe [Int]Nothing, Just [], Just [1,1337], …
Prelude> :t Nothing
Nothing :: Maybe a
Prelude> Just "a camel"
Just "a camel"
Prelude> :t Just "a camel"
Just "a camel" :: Maybe [Char]   -- the same as Maybe String
Prelude> Just True
Just True
Prelude> :t Just True
Just True :: Maybe Bool

Either

-- right means correct
readInt :: String -> Either String Int
readInt "0" = Right 0
readInt "1" = Right 1
readInt s = Left ("Unsupported string: " ++ s)

Constructor

Start with capital letter.

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