RE: incompatible signatur syntax within instance definition
| This is legal in Haskell 98, because the `a' in the inner declaration is | implicitly universally quantified. But if the scope of the outer type | variable `a' extends over the inner type declaration, then the inner `a' | will not be locally universally quantified, and the call to `bar' will | be a type error, because the `a' in the head of the instance declaration | will in general be different than `Int'. | ... | | P.S. I note that ghc 5.02.2 enables this extension always, regardless | of the setting of -fglasgow-exts. That seems like a bug to me. And indeed, GHC 6.0 fixes this bug: no scoped type variables without -fglagow-exts. Simon ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: incompatible signatur syntax within instance definition
Fergus Henderson wrote: Allowing repeated constraints would not be sufficient for that. Yes, you're right. A true (but senseless) constraint like Show Int is rejected as well. (The same applies to Show a if a is a scoped type variable, that is when a is monomorph.) So a function with type a - a in Haskell98 is always polymorphic, whereas it may be a monomorphic function in Haskell with glasgow-exts (iff a is a scoped type variable). The compatibility problem between Haskell98 and glasgow-exts is thus moved to the place where scoped typed variables can be introduced. Since type annotations of patterns are illegal in Haskell98 the only problem was the introduction of scoped type variables through the instance line. So why was this done (except to shorten notation)? Consider the following example: data MyType a = MkMyType a class Foo a where foo :: a - Int Since the type a usually occurs in every method it is no problem to introduce scoped type variables via patterns (that are illegal in Haskell98): instance Foo (MyType a) where foo (_ :: MyType a) = ... So, to summarize, this particular ghc extension is not a pure extension. It can change the legality or even the semantics of Haskell 98 code. Allowing repeated constraints won't change that. Given that allowing repeated constraints isn't sufficient to solve that problem, I don't think it is a good idea to allow them. I entirely agree with this. The first point would be to make the extension pure. Allowing or disallowing repeated constraints is then a mere (unimportant) design option. P.S. I note that ghc 5.02.2 enables this extension always, regardless of the setting of -fglasgow-exts. That seems like a bug to me. I would not worry about bugs in older versions. Cheers Christian ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: incompatible signatur syntax within instance definition
On 08-Dec-2003, Christian Maeder [EMAIL PROTECTED] wrote: Fergus Henderson wrote: I think the issue here is that in ghc (with -fglasgow-exts), the a here refers to the same type variable a in the top of the instance declaration, which has already been constained, and cannot be constrained again. Is that a bug or a feature? A feature. It's called scoped type variables. See http://www.haskell.org/ghc/docs/6.2/html/users_guide/type-extensions.html#SCOPED-TYPE-VARIABLES: The type variables in the head of a class or instance declaration scope over the methods defined in the where part.. Or were you referring to the fact that variables which are already constrained can't be constrained again? IMHO that is a feature too. It doesn't make sense to constrain a variable at any point other than the point where that variable is introduced. -- Fergus Henderson [EMAIL PROTECTED] | I have always known that the pursuit The University of Melbourne | of excellence is a lethal habit WWW: http://www.cs.mu.oz.au/~fjh | -- the last words of T. S. Garp. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: incompatible signatur syntax within instance definition
Fergus Henderson wrote: I think the issue here is that in ghc (with -fglasgow-exts), the a here refers to the same type variable a in the top of the instance declaration, which has already been constained, and cannot be constrained again. Is that a bug or a feature? With Haskell 98, it is a fresh type variable, for which the constraint is necessary. Ok, if I view the local function as a new/independent function, I accept to add the Constraint, but that should be acceptable for ghc -fglasgow-exts as well (as is for hugs with extensions). Try renaming the type variable as b in the inner declaration: the following should work both with and without -fglasgow-exts. showsl :: Show b = List b - ShowS Yes, this works, but I never thought that the choice of a or b would matter. Christian instance Show a = Show (List a) where showsPrec _ Nil = showString [] showsPrec _ l = showString [ . showsl l . showString ] where -- showsl :: List a - ShowS-- for ghc -- showsl :: Show a = List a - ShowS -- for hugs ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: incompatible signatur syntax within instance definition
Christian Maeder [EMAIL PROTECTED] writes: Fergus Henderson wrote: I think the issue here is that in ghc (with -fglasgow-exts), the a here refers to the same type variable a in the top of the instance declaration, which has already been constained, and cannot be constrained again. Is that a bug or a feature? It is a feature called scoped type variables. Regards, Malcolm ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
