Re: [Haskell-cafe] Solved but strange error in type inference

On Tue, Jan 3, 2012 at 05:50, Yucheng Zhang wrote:

On Tue, Jan 3, 2012 at 6:44 PM, Brandon Allbery wrote:

...

On Tue, Jan 3, 2012 at 05:17, Yucheng Zhang wrote:

...

subsome :: [RRule nt t s] -> Either String ([t], s)

It seems to me that the compiler is not sure the two 'nt' are equal. The ScopedTypeVariables can make the compiler believe they are equal.

But ScopedTypeVariables is enabled already.

Sorry, I meant actually using ScopedTypeVariables as in the first function, which compiles well:

legSome :: LegGram nt t s -> nt -> Either String ([t], s)

Except he's not; an explicit "forall" is needed to indicate the type variables to be brought into scope. It would need to be

legSome :: forall nt t s. LegGram nt t s -> nt -> Either String ([t], s)

-- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms