[Haskell-cafe] Re: Trying to write 'safeFromInteger'

[Peter Verswyvelen]

You must explicitly quantify the type parameters in the top level function
when you use this extension. This works:
{-# LANGUAGE ScopedTypeVariables #-}

safeFromInteger :: forall a . (Num a, Integral a, Bounded a) => Integer ->
Maybe a
safeFromInteger i =
   if i > (toInteger (maxBound :: a))
     then Nothing
     else Just (fromInteger i)

On Tue, Apr 7, 2009 at 11:42 PM, Kannan Goundan <kannan at cakoose.com> wrote:

> Max Rabkin <max.rabkin <at> gmail.com> writes:
>
> > The problem with your code is that the type of maxBound is
> > unspecified. You need (maxBound `asTypeOf` i), or enable
> > ScopedTypeVariables and use (maxBound :: a) (I think).
>
> I tried doing the (maxBound :: a) thing, but got another confusing error:
>
>  safeFromInteger :: (Num a, Integral a, Bounded a) => Integer -> Maybe a
>  safeFromInteger i =
>     if i > (toInteger (maxBound :: a))
>       then Nothing
>      else Just (fromInteger i)
>
> # ghci -XScopedTypeVariables Test.hs
>
>  Test.hs:3:20:
>    Could not deduce (Bounded a1) from the context ()
>      arising from a use of `maxBound' at TestIntegerBounds.hs:3:20-27
>    Possible fix:
>      add (Bounded a1) to the context of an expression type signature
>    In the first argument of `toInteger', namely `(maxBound :: a)'
>    In the second argument of `(>)', namely
>        `(toInteger (maxBound :: a))'
>    In the expression: i > (toInteger (maxBound :: a))
>
>
>
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