Hi Simon (PJ), cc Simon (DF), I rather reckon we are facing a bug here. The attached minimalised Foo.hs shows the offending code pattern. With GHC 6.2 we get "*** Exception: <<loop>> With GHC 6.4 we get " (still waiting for the rest of the string) The scenario is about class/instance-head-level recursion through superclassing and instance constraints. Nothing too weird. There are no _explicit_ recursive dictionaries. An observations though. The relevant class head does not just mention a recursive superclass, but also an innocent superclass ClassB. If we move this innocent superclass constraint to the instance level (see Bar.hs), then we get termination with both 6,2 and 6.4. Another issue. This feature seems to need multi-parameter classes really! Ralf Simon Peyton-Jones wrote:
Simon
You've found an interesting case.
First, you are skating on thin ice here. GHC's ability to build recursive dictionaries is quite experimental, and you are relying on it completely.
But you're right: it "should" work. I can see why it isn't but I have not got it clear enough in my head to know the best way to fix it. Still less do I have a formal story about what should type-check (without loops) and what should not.
So this is going to continue to fail in 6.4, but it's on my list to look at.
Simon
-- Ralf Lammel ralfla@microsoft.com Microsoft Corp., Redmond, Webdata/XML http://www.cs.vu.nl/~ralf/ {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} {- Try: (sat::Int -> String -> String) undefined "hello" -} module Foo where class (Sat (a -> b -> String), ClassB b) => ClassA a b class ClassB a where fun :: a -> String class Sat x where sat :: x instance ClassA a b => Sat (a -> b -> String) where sat = const fun instance ClassA a String instance ClassB String where fun = id {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} {- Try: (sat::Int -> String -> String) undefined "hello" -} module Foo where class Sat (a -> b -> String) => ClassA a b class ClassB a where fun :: a -> String class Sat x where sat :: x instance (ClassA a b, ClassB b) => Sat (a -> b -> String) where sat = const fun instance ClassA a String instance ClassB String where fun = id