Two solutions using immutable and mutable arrays and no unsafe operations:
----
module Main where
import Control.Monad.ST
import Data.Ix
import Data.Array
import Data.Array.MArray
import Data.Array.ST
-- using immutable arrays
hist1 :: String -> Array Char Int
hist1 str = accumArray (+) 0 ('\0','\255') [(c,1) | c<-str]
freq1 :: String -> [(Char,Int)]
freq1 = assocs . hist1
-- using mutable ST arrays
hist2 :: String -> STArray s Char Int -> ST s ()
hist2 str arr = sequence_ [do { i<-readArray arr c; writeArray arr c (1+i) }
| c<-str]
freq2 :: String -> [(Char,Int)]
freq2 str = runST (do { arr<-newArray ('\0', '\255') 0
; hist2 str arr
; getAssocs arr
})
--
Cheers,
Pedro
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