FlexibleContexts and FlexibleInstances

[Claus Reinke]

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