[Haskell-cafe] Solved but strange error in type inference
allbery.b at gmail.com
Tue Jan 3 12:23:36 CET 2012
On Tue, Jan 3, 2012 at 05:50, Yucheng Zhang <yczhang89 at gmail.com> wrote:
> On Tue, Jan 3, 2012 at 6:44 PM, Brandon Allbery <allbery.b at gmail.com>
> > On Tue, Jan 3, 2012 at 05:17, Yucheng Zhang <yczhang89 at gmail.com> 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 at gmail.com
wandering unix systems administrator (available) (412) 475-9364 vm/sms
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Haskell-Cafe