MArray and runST

[Dean Herington]

Simon Peyton-Jones wrote:

> Interesting example!
>
> | Coincidentally, I tripped over this subtlety myself just last night.
> (I,
> | too, often use '$' to avoid nested parentheses.)  I concluded it was
> an
> | instance of the "partial-application restriction" that I found
> described in
> | section 7.11.4 of the GHC 5.02 User's Guide (still accessible at the
> GHC web
> | site if you look carefully enough).
>
> No, that's not the reason.  That restriction has been lifted.
>
> Consider this simpler program;
>
>         foo = id runST
>
> Now we have
>         id :: forall a. a -> a
>
> So to apply 'id' to 'runST' we'd have to instantiate the type variable
> 'a' to runST's type
>         'forall b. (forall s. ST s b) -> b'
>
> But GHC only implements predicative polymorphism; it does not allow you
> to instantiate a type variable with a for-all type.  It would be nice to
> lift this restriction, but I don't know how do to so without losing type
> inference.
>
> Now, the error message you get is deeply obscure, I grant you.  Trouble
> is, this variant IS ok:
>
>         foo = id wibble
>
>         wibble :: ((forall s. ST s b) -> b) -> b
>
> Why?  For the same reason that this is OK
>
>         foo = id reverse
>
> That, is we can instantiate the type of the argument to a monotype (a
> type without for-alls).

Sorry, I don't understand yet.  Why isn't the "forall s" in wibble's type a
problem?

> So it's not easy to see how to spot the bad case and give a better
> message.  If this comes up again I'll think about it again.  Meanwhile,
> I'm going to put it under "higher-rank polymorphism gives strange error
> message" file.
>
> Simon

-- Dean