qualified instance declarations

[Ralf Hinze]

Hi,

GHC (5.03.20020410) wrongly accepts the following:

::
C.lhs
::
 module C
 where

 class A a where
 a :: a - Int
::
X.lhs
::
 module X
 where
 import qualified C

 instance C.A Int where
   C.a = id

Note that the class method is qualified on the LHS which is
not legal H98.

---

Incidentally, I also noticed that GHC and Hugs behave differently
for the following variant of X.lhs.

::
X.lhs
::
 module X
 where
 import qualified C

 instance C.A Int where
   a = const X.a

 a = 4

GHC happily accepts the file while Hugs complains that

ERROR X.lhs:6 - Type error in instance member binding
*** Term   : a
*** Type   : a - Integer
*** Does not match : Int - Int

There seems to be a difference in defaulting. [Don't ask me
which is the correct behaviour.] The problem disappears if I
supply an explicit type signature for `a'.

Cheers, Ralf

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Re: qualified instance declarations

[Alastair Reid]

For those puzzled by the number of things called 'a' in Ralf's
program, I'm attaching an alpha-renamed version.  Removing the type
signature (or changing the type signature to Integer) for a results in
Ralf's type error in Hugs.  

It does indeed look as though Hugs applies defaulting to 'a' and comes
up with the type Integer.  Off the top of my head, I'd say this was
correct since the default default is (Integer,Double).

--
Alastair Reid

  module C where
  
  class B a where f :: a - Int


  
  module X where
  
  import qualified C
  
  instance C.B Int where
f = const X.a
  
  a :: Int
  a = 4
  
  
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RE: qualified instance declarations

[Simon Marlow]

 GHC (5.03.20020410) wrongly accepts the following:
 
 ::
 C.lhs
 ::
  module C
  where
 
  class A a where
  a :: a - Int
 ::
 X.lhs
 ::
  module X
  where
  import qualified C
 
  instance C.A Int where
C.a = id

This has been fixed, and GHC 5.04 will behave correctly.

 Incidentally, I also noticed that GHC and Hugs behave differently
 for the following variant of X.lhs.
 
 ::
 X.lhs
 ::
  module X
  where
  import qualified C
 
  instance C.A Int where
a = const X.a
 
  a = 4
 
 GHC happily accepts the file while Hugs complains that
 
 ERROR X.lhs:6 - Type error in instance member binding
 *** Term   : a
 *** Type   : a - Integer
 *** Does not match : Int - Int
 
 There seems to be a difference in defaulting. [Don't ask me
 which is the correct behaviour.] The problem disappears if I
 supply an explicit type signature for `a'.

Hugs's defaulting behaviour differs from Haskell 98 (and always has
done).  See for example this discussion on the Haskell mailing list:

   http://www.haskell.org/pipermail/haskell/2002-May/009616.html

(plus other messages with the same subject).

Could someone from the Hugs camp please document this?

Cheers,
Simon
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