id	summary	reporter	owner	description	type	status	milestone	resolution	keywords	cc	section	state
81	scoping of type variables in class instances	guest	none	"The scoping of type variables in class instances is underspecified. GHC scopes them for the entire instance, Hugs does not.

GHC's approach is more expressive, it is sometimes needed - especially in combination with multi-parameter classes.
Example:


{{{
class Ping a b where ping:: a->b
newtype A a = A a
instance (Show b,Ping a b) => Show (A a) where
   show (A x) = show (ping x)
}}}


This is ambiguous and thus rejected, but we could resolve the ambiguity by annotating 'ping' with its expected type a->b, but that only works if the scoping of the class instance extends to the member definition.


== pros ==
: extends expressiveness, implemented in GHC anyway, was underspecified in Haskell anyway


== cons ==
: very minor: it could break some existing programs as it makes some local functions non-polymorphic, though these would have been badly written

See also: [http://hackage.haskell.org/trac/haskell-prime/wiki/ScopedTypeVariables]"	enhancement	new			scoping type variables