Polymorphic implicit parameters

[Roman Cheplyaka]

That makes sense, thank you.

Roman

* Simon Peyton-Jones <simonpj at microsoft.com> [2013-03-21 08:25:03+0000]
> Generally speaking ALL constraints (class constraints, equality constraints, implicit parameters) range only over monotypes.  
> 
> The apparatus should extend to polymorphic types, be it's somewhat uncharted territory.  I doubt there'd be much problem in the case of implicit parameters.
> 
> In short, in principle it might be possible, but it would take a little careful thought and I have too few careful-thought electrons available right now.   
> 
> Simon
> 
> |  -----Original Message-----
> |  From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
> |  bounces at haskell.org] On Behalf Of Roman Cheplyaka
> |  Sent: 20 March 2013 08:58
> |  To: glasgow-haskell-users at haskell.org
> |  Subject: Polymorphic implicit parameters
> |  
> |  I'm curious as to why GHC doesn't accept the following (contrived)
> |  module:
> |  
> |    {-# LANGUAGE ImplicitParams, RankNTypes #-}
> |  
> |    f :: (?g :: (forall a . a -> a)) => a -> a
> |    f = ?g
> |  
> |  The error message is:
> |  
> |    Illegal polymorphic or qualified type: forall a. a -> a
> |    In the type signature for `f':
> |      f :: ?g :: (forall a. a -> a) => a -> a
> |  
> |  It's not a big deal since one can wrap the polymorphism in a newtype,
> |  but it's somewhat unexpected.
> |  
> |  Roman
> |  
> |  _______________________________________________
> |  Glasgow-haskell-users mailing list
> |  Glasgow-haskell-users at haskell.org
> |  http://www.haskell.org/mailman/listinfo/glasgow-haskell-users