{-# 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@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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