Possible instances problem with ghc-6.2.1

[Simon Peyton-Jones]

George

This "bug" is the same as the one I described in the enclosed message,
but it's a bit simpler this time.

The basic setup is:

newFormEntry :: HasButton value => Radio value -> Int
instance Show value => HasButton value

foogle :: (HasButton woggle) => Radio woggle -> Int
foogle lab = let
  		       form0 = newFormEntry lab 
		 in
		 undefined

The call to newFormEntry requires the constraint (HasButton woggle).
You point out that (HasButton woggle) is available in foogle's type. But
it's *also* available via
	instance Show value => HasButton value
So it depends on which of two (incompatible!) ways of deriving
(HasButton woggle) wins.  GHC makes no guarantees about this.

In fact, a recent change (don't ask) makes GHC just a bit keener to do
context simplification at monomorphic local definitions (like that for
form0).  So GHC tries to simplify the (HasButton woggle) constraint at
the "form0 = ..." point, and succeeds by using the instance declaration.



In short, you'd defined a non-confluent rewrite system, so you should
not be surprised if it unpredictably paints you into a corner, and does
so differently on different days of the week.

Simon


| -----Original Message-----
| From: glasgow-haskell-bugs-bounces at haskell.org
[mailto:glasgow-haskell-bugs-
| bounces at haskell.org] On Behalf Of George Russell
| Sent: 26 March 2004 10:21
| To: glasgow-haskell-bugs at haskell.org
| Subject: Possible instances problem with ghc-6.2.1
| 
| With ghc-6.2.1 compiled from source
| 
| tar -xzf instances2.tar.gz
| cd instances2
| gmake
| 
| produces the message:
| 
| AttributesType.hs:15:
|      Could not deduce (Show value)
| 	from the context (AttributeValue (Radio value),
| 			  HasConfigRadioButton value,
| 			  Bounded value,
| 			  Enum value,
| 			  FormLabel label)
|        arising from use of `newFormEntry' at AttributesType.hs:15
| 
| 
| 
| However:
| (1) this does not seem to happen with ghc-6.2.
| (2) why does newFormEntry need Show anyway?  It would only be
| relevant in deducing HasConfigRadioButton, but in fact
| it has that in the context anyway.
| 
| 
| Sorry I didn't report this earlier.
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