Hi,
if you desugar the definition that's causing the type error,
perhaps it becomes a little bit clearer what's going on, i.e.,
from
problematic :: MyAnnotatedType Int
problematic = defaultVal{theInt=42,theAnnotation=10}
you expand this to
problematic' = case defaultVal of { MAT a b c -> MAT 42 b 10 }
Clearly, the type of 'c' is ambiguous here. Interestingly, if Haskell 98
had defined record update expr "e{fields}" as having the same type
as "e", this wouldn't have been an issue:
no_probs =
case defaultVal of { m@(MAT a b c) -> (MAT 42 b 10) `asTypeOf` m}
hth
--sigbjorn
----- Original Message -----
From: "Magnus Björk" <[EMAIL PROTECTED]>
To: <glasgow-haskell-bugs@haskell.org>; <hugs-bugs@haskell.org>
Sent: Monday, May 29, 2006 03:34
Subject: Strange behaviour with classes (both Hugs and GHC)
I just ran into a strange behaviour of both Hugs and GHC. I discussed it
with John Hughes who managed to find a workaround, and suggested that I
ask you whether this is really expected behaviour.
The attached file contains a small example that exhibits the problem.
--
/MpB
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