Hello!
I think NHC does not properly handle exports and imports of field labels.
Problem 1
--------
The following two modules seem to be legal according to the Haskell 98 report
(Sec.5.2, p. 66):
==================================================================
module See1 ( D( Undef, Def, value ) ) where
data D = Undef | Def{ value :: Int }
------------------------------------------------------------------
module See2 ( foo ) where
import See1 ( D( Undef, Def, value ) )
foo :: D -> Int
foo d = case d
of
Undef -> error "no value"
Def{} -> value d
==================================================================
Both GHC and Hugs accept this, but NHC reports an error:
hmake -d../nhc See2.hs
nhc98 -c -d ../nhc See1.hs
====================================
In file: ./See1.hs:
1:16 Found ( but expected one of ) ,
If one changes the heading of See1 to
module See1 ( D( Undef, Def ), value ) where
then one gets
hmake -d../nhc See2.hs
nhc98 -c -d ../nhc See2.hs
====================================
In file: ./See2.hs:
3:13 Found ( but expected a {-end-of-definition-or-EOF-}
Problem 2
---------
There are no syntactic problems when one uses another, equally legal form:
==================================================================
module See1 ( D( Undef, Def ), value ) where
data D = Undef | Def{ value :: Int }
------------------------------------------------------------------
module See2 ( foo ) where
import See1 ( D( Undef, Def ), value )
foo :: D -> Int
foo d = case d
of
Undef -> error "no value"
Def{} -> value d
==================================================================
Again, this is accepted both by GHC and Hugs. But now NHC has another bone to
pick:
hmake -d../nhc See2.hs
nhc98 -c -d ../nhc See2.hs
====================================
Error when renaming::
Identifier value defined 2 times.
To make this go away, one has to change the importation clause to
import See1 ( D( Undef, Def ) )
Now everything is OK, and NHC does not complain about the occurrence of "value"
in "foo"!
Regards,
-- Feliks
