instance inference

[Simon Peyton-Jones]

Martin Sulzmann is the world expert on this stuff.  Maybe he'll know.

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Ross Paterson
| Sent: 19 December 2005 10:16
| To: Simon Peyton-Jones
| Cc: glasgow-haskell-users at haskell.org
| Subject: Re: instance inference
| 
| As a termination test, how about no restrictions on context and head
| except:
| 
|    Each assertion in the context must satisfy
|     * the variables of the assertion are a sub-multiset of those of
the
|       head (though they may be the same), and
|     * the assertion has fewer type constructors and variables (taken
|       together and counting repetitions) than the head.
| 
| This ensures that under any ground substitution the context assertion
has
| fewer type constructors than the head.
| 
| It would accept all instances accepted by the current test, plus
| 
| 	instance C a
| 	instance C (s a) => C (Sized s a)
| 	instance (C1 a, C2 b) => C a b
| 	instance C1 Int a => C2 Bool [a]
| 	instance C1 Int a => C2 [a] b
| 	instance C a a => C [a] [a]
| 
| But not things like
| 
| 	instance (C1 a, C2 a, C3 a) => C a
| 
| because that would need to check other instances as well.
| 
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