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
Max Rabkin
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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