全角文字列を半角文字列に変換する。

import Char (isDigit)

toHanDigit :: String -> String
toHanDigit = toDigit.zenToHan

-- 数字以外は無視します。
toDigit :: String -> String
toDigit  []    = []
toDigit (x:xs) = if isDigit x then x:toDigit xs else toDigit xs

zenToHan :: String -> String
zenToHan s =  tr  "０１２３４５６７８９" "0123456789" s

-- http://d.hatena.ne.jp/takatoh/20060424/tr
tr :: String -> String -> String -> String
tr from to = map f
  where 
    f c = case lookup c tbl of
            Nothing -> c
            Just c' -> c'
    tbl = zip from to

> toHanDigit "abc０２dd３４123ghｈｇｓ" --=> "0234123"

import Data.Map


tr :: String -> String -> String -> String
tr from to = Prelude.map f
  where 
    f c = case Data.Map.lookup c charMap of
            Nothing -> c
            Just c' -> c'
    charMap = fromList $ zip from to

han = han0 ++ han1 ++ han2 ++ han3
zen = zen0 ++ zen1 ++ zen2 ++ zen3
han0 =" !\"#$%&'()*+,-./"
zen0 ="　！”＃＄％＆’（）＊＋，ー．／"
 
han1 ="0123456789:;<=>?@"
zen1 ="０１２３４５６７８９：；＜＝＞？＠"
han2 ="ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`"
zen2 ="ＡＢＣＤＥＦＧＨＩＪＫＬＭＮＯＰＱＲＳＴＵＶＷＸＹＺ［＼］＾＿‘"
han3 ="abcdefghijklmnopqrstuvwxyz{|}~"
zen3 ="ａｂｃｄｅｆｇｈｉｊｋｌｍｎｏｐｑｒｓｔｕｖｗｘｙｚ｛｜｝〜"

> putStrLn $  tr zen han  "ＡＢＣＤＥ読んだことが"
--=> ABCDE読んだことが

> putStrLn $ tr han zen ['A'..'Z']
--=> ＡＢＣＤＥＦＧＨＩＪＫＬＭＮＯＰＱＲＳＴＵＶＷＸＹＺ