just a few comments from a user (who would really, really, like to be able to define pragma collections, so that he doesn't have to switch on half a dozen separate extensions every time;-).
The following toy program requires MultiParamTypeClasses OR FlexibleContexts in order to be accepted by GHC(i):
f :: (T a b) => a -> Int f _ = 0
This of course assumes that we import the definition of T, we *must* have MultiParamTypeClasses enabled if we want to declare T. Both extensions thus enable classes with more than one argument to appear in contexts.
Only MultiParamTypeClasses does (and neither extension is needed in the module defining 'f', if 'T' is imported, which suggests that MultiParamTypeClasses is propagated to importers - this isn't true for most other extensions). The documentation still points to -fglasgow-exts, so it doesn't seem to answer these questions..
f :: (T a ()) => a -> Int f _ = 0
i.e. changing the second argument to T to () instead, means we now *must* have FlexibleInstances, in order to allow the non-tyvar argument. This is nothing surprising, this is what FlexibleInstances are supposed to do.
You mean FlexibleContexts.
But the question is, is this a syntactic issue or a typing issue?
FlexibleContexts has both syntax and static semantics implications.
Now, FlexibleInstances *also* lifts the restriction on contexts, just like FlexibleContexts - but *only* for the contexts of instance declarations.
No. FlexibleInstances is about instance *heads*, FlexibleContexts is about contexts everywhere (in practice, there are some bugs;-). class T a b -- requires MultiParamTypeClasses instance T a a -- requires FlexibleInstances instance Eq () => T a [b] -- requires FlexibleContexts instance Eq [a] => T a b -- requires UndecidableInstances (if you actually wanted to use 'T's methods, you'd then need OverlappingInstances, but that, UndecidableInstances, and the static semantics aspects of FlexibleContexts are beyond parsing) I also seem to recall more dependencies between flags than I can find documented. Hth, Claus