[Redirecting to ghc-bugs]
| module A where
| data A = A { label :: Int }
|
| module B where
| data B = B { label :: Int }
|
| b :: B
| b = B { label = 0 }
|
| x :: B -> ()
| x B { label = 0 } = ()
|
| GHC reports an ambiguity for both usages of label.
Are you sure? Module B doesn't even import A!
And if you mean that B does import A, then Haskell 98 indeed says it's
ambiguous.
Now, it's true that in the case of both
a) pattern matching
b) record construction
there's a lot less ambiguity, because you know which *data constructor* is
involved. (Even better than knowing which module is involved. But for
c) record update (x { label = 0 })
d) field selection (label x)
you have no such clues.
Haskell could be defined to take advantage of (a) and (b), but if it was then
it'd *also* work if data A and data B were defined in the same module, and
Section 3.15 of the Haskell Report clearly states otherwise.
I conclude that Hugs is not implementing Haskell 98 here. In fact there's
clearly a bug in Hugs:
module A where
data A = A { foo :: Int }
module B where
import A
data B = B { foo :: Bool }
a = A {foo=3}
yields:
ERROR "B.hs":8 - Constructor "A" does not have selected fields in A{foo
= 3}
Commenting out the declaration of data B makes the definition of 'a' work. I'm
ccing hugs-bugs.
Simon
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