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
[EMAIL PROTECTED]
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
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