Polymorphic implicit parameters

[Simon Peyton-Jones]

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
|  
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