Functional Programming with Haskell - Strings are [Char]

하스켈에서 문자열은 단순히 문자의 리스트이다. 

 
Prelude> "testing"
"testing"
it :: [Char]
 
Prelude> ['a'..'z']
"abcdefghijklmnopqrstuvwxyz"
it :: [Char]
 
Prelude> ["ust","a","test"]
["ust","a","test"]
it :: [[Char]]
 
Prelude> ["just","a","test"]
["just","a","test"]
it :: [[Char]]
 

리스트에서 사용가능한 함수는 모두 문자열에서도 사용할 수 있다.

 
Prelude> let asciiLets = ['A'..'Z'] ++ ['a'..'z']
asciiLets :: [Char]
 
Prelude> length asciiLets
52
it :: Int
 
Prelude> reverse (drop 26 asciiLets)
"zyxwvutsrqponmlkjihgfedcba"
it :: [Char]
 

 
Prelude> :type elem
elem :: (Eq a, Foldable t) => a -> t a -> Bool
 
Prelude> let isAsciiLet c = c `elem` asciiLets
isAsciiLet :: Char -> Bool
 

Prelude 에서는 문자열을 [Char] (a type synonym)로 정의한다. 아래를 보자.

 
Prelude> :info String
type String = [Char]    -- Defined in ‘GHC.Base’
 

문자열에는 다수의 함수를 적용할 수 있는 대표적인 2가지 함수 words 와 putStr 을 보자.

 
Prelude> let z = "Just some words!"
z :: [Char]
 
Prelude> words z
["Just","some","words!"]
it :: [String]
 
Prelude> tail(words z)
["some","words!"]
it :: [String]
 
Prelude> unwords (tail (words z))
"some words!"
it :: String
 
Prelude> putStr (unwords (tail (words z)))
some words!it :: ()
 

• words : words breaks a string up into a list of words, which were delimited by white space.
• unwords : unwords is an inverse operation to words. It joins words with separating spaces.
• putStr : Write a string to the standard output device (same as hPutStr stdout).