Why am I getting this error:

Couldn't match expected type `Integer' against inferred type `Int'
In the expression: foldl step 0 xs
In the definition of `asInt_foldAux':
asInt_foldAux xs
= foldl step 0 xs
where
step acc '.' = error "cannot handle decimal numbers"
step acc x = acc * 10 + digitToInt x

for this code?

import Data.Char (digitToInt)

asInt_fold :: String -> Integer
asInt_fold ('-':xs) = -(asInt_foldAux xs)
asInt_fold xs = asInt_foldAux xs

asInt_foldAux :: String -> Integer
asInt_foldAux xs = foldl step 0 xs
where step acc '.' = error "cannot handle decimal numbers"
step acc x = acc * 10 + digitToInt x

Note that I'm using Int, rather than Integer, to avoid silent overflow errors.