Tillmann Rendel wrote:
Brian Hulley wrote:
class Foo a where
f :: a -> b -> (a, b)
Here there is no capitalization distinction in the type of (f) but the implementation can still insert the "forall b." since (a) is already in scope. Therefore similarly, if type constructors like "List" were instead written using lowercase, since they would already be in scope it would be clear even without explicit quantifiers that (a) and (b), but not (list), were to be quantified in the type of (map) (assuming of course that there was no top level data decl for (a) or (b)).
So adding b to the export list of an imported module would change the type of f?
Yes, if the module in which the above class decl were defined imported a data type (b) from some other module (or defined it elsewhere in the same module) then (f)'s type would change. (Of course at the moment this can't happen because (b) is lowercase so it can't be introduced by data/type decls or imported/exported.) The type of (f) would also change if Haskell were extended to support nested classes: class Bar b where class Foo a where f :: a -> b -> (a, b) But as Jonathan pointed out this behaviour is not really all that ideal since even in Haskell at the moment a simple spelling mistake could cause silent quantification instead of an error (which is perhaps the main justification for why constructors introduced by data/type decls must currently be capitalized -- unfortunately there is no corresponding protection when writing types inside class decls): class Zap aaaaa where g :: aaaaa -> aaaa ^ oops! Cheers - Brian.