[Haskell-cafe] Couldn't match expected type `Integer' against inferred type `Int'

[Paul Reiners]

On Aug 8, 2011, at 7:59 PM, Ivan Lazar Miljenovic wrote:

> On 9 August 2011 10:49, Paul Reiners <paul.reiners at gmail.com> wrote:
>> 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
> 
> digitToInt returns an Int; as such the result of asInt_foldAux is an
> Int, but you've specified in its type signature that it should be
> returning an Integer.

So is there something like digitToInteger?  Or can I somehow cast the Int to an Integer?

> 
>> Note that I'm using Int, rather than Integer, to avoid silent overflow
>> errors.
> 
> Should that be the other way round?

Yes, it should be the other way round.