question re hugs and func dependencies

Simon Peyton-Jones
Mon, 5 Mar 2001 00:42:59 -0800

I replied last week:

-----Original Message-----
From: Simon Peyton-Jones 
Sent: 28 February 2001 16:51
To: 'Dominic Duggan';
Subject: RE: please confirm that this is a feature

GHC infers the type

  g :: (Foo [[a]] [b], Foo [[b]] [a]) => [a] -> [b] -> Int

As you imply, if GHC used the instance decl to simplify
the first constraint to Foo [a] b, for example, then the functional
dependency would kick in, and there would be an infinite loop.
But GHC's constraint simplification is (now) lazy, unless forced
to be eager by supplying a type signature.

I think Hugs is the same, and I think that's probably why it

Interesting example!


| -----Original Message-----
| From: Dominic Duggan []
| Sent: 02 March 2001 21:55
| To:
| Subject: question re hugs and func dependencies
| Dear all, I haven't got a reply from hugs-bugs,
| so I'll see if anyone here can answer the question.
| class foo a b | a |-> b where foo :: a -> b -> Int
| instance foo Int Float where foo x y = 0
| instance foo [a] [b] where foo [x] [y] = foo x y
| g x y = (foo [x] y) + (foo [y] x)
| It is my conjecture that Hugs terminates on type-checking
| g because of a depth bound in the type-checker
| (although there is no indication from the type-checker
| that this is what is going on).
| Can anyone confirm or refute this conjecture?
| FWIW this is as part of the related work section
| for a paper that has been accepted to JFP:
| "Type-Checking Multi-Parameter Type Classes,"
| by D. Duggan and J. Ophel.
| I'd appreciate cc-ing any replies to me, I'm not
| sure that I'm still on this mailing list.
| Cheers
| --dd