Stupid Curry Tricks

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Revision as of 17:00, 11 October 2009 by Eric Gesell (talk | contribs) (Added code for frequency.hs)

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This is a useless piece of code that counts and prints the number of times each character in the English alphabet occurs in a string. This version is hard-coded to read from the file "hi.txt", but it can easily be modified to interact with standard IO or read a file passed on the command line. The most interesting part is the use of currying to generate the counting functions.

--system to show character frequencies in a set of strings
import System.IO
import Data.Char(toLower)

--letters to scan for
letters = ['a'..'z']

--counts the ocurences of a particualr letter
countLetters :: Char -> [Char] -> Int
countLetters target str = length (filter (\c -> c == target) str)

--creates a curryed version of countletters for a particular letter
makeCounter :: Char -> ([Char] -> Int)
makeCounter c = countLetters c

--creates a countLetters funciton for each letter in the alphabet
counters :: [([Char] -> Int)]
counters = map makeCounter letters

frequencies :: [([Char] -> Int)] -> [Char] -> [Int]
frequencies [] _ = []
frequencies (f:fs) cs = (f cs) : frequencies fs cs

freqs :: [Char] -> [(Char, Int)]
freqs cs = zip letters (frequencies counters cs)

--make pretty output
prettyPrint :: (Char, Int) -> String
prettyPrint (c, n) = [c] ++ ":" ++ (show n) ++ "\n"

printList :: [(Char, Int)] -> String
printList [] = []
printList ((c, i):frs) = ((c:[]) ++ ": " ++ (show i) ++ "\n") ++ printList frs

--io and main program
main = do
       str <- readFile "hi.txt"
       let out = (freqs (map toLower str))
       putStrLn $ printList out