[Haskell-cafe] Re: -fno-monomorphism-restriction makes type-inference ambiguous?

[Ben Rudiak-Gould]

Eike Scholz wrote:
>> mylength = synAttr listLength
> 
> $ *Main> :type synAttr
> $ synAttr :: (Data b) => ((?stack::[Dyn]) => b -> a) -> Attr a
> 
> $ *Main> :type listLength
> $ listLength :: (?stack::[Dyn]) => List -> Float
> 
> $ *Main> :type (synAttr listLength)
> $ (synAttr listLength) :: Attr Float
> 
> $ *Main> :type mylength
> $ mylength :: (?stack::[Dyn]) => Dyn -> Dyn -> [Dyn] -> Maybe Float
> 
> where
> 
>> type Attr a = Dyn -> Dyn -> [Dyn]-> Maybe a

This may be a bug. But note that both interpretations are legitimate. One 
way of applying synAttr to listLength is first to apply ?stack to 
listLength, obtaining listLength' :: List -> Float and creating a 
(?stack::[Dyn]) constraint on the application node, then specializing 
listLength' to the type (?stack::a) => List -> Float, then passing that to 
synAttr.

Again, I recommend that you not try to use implicit parameters.

-- Ben