strToInt and StrToDouble

fold_excersizes.hs

import Data.Char

strToInt :: String -> Int
strToInt ('+':xs) = strToInt xs
strToInt ('-':xs) = - strToInt xs
strToInt xs =
    foldl step 0 xs
    where
    step acc x = acc * 10 + (digitToInt x)
strToInt _ = undefined

strToDouble :: String -> Double
strToDouble ('-':xs) = -strToDouble xs
strToDouble ('+':xs) = strToDouble xs
strToDouble xs =
    whole + frac
    where
    (l, r)          = break (\x -> x == '.') xs
    whole           = fromIntegral . strToInt $ l
    frac            = fraction . tail $ r
    fraction (ys)   = foldr step 0 . map (fromIntegral . digitToInt) $ ys
    step acc n         = acc * 0.1 + n * 0.1
strToDouble _ = undefined