#7543: Constraint synonym instances
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Reporter: monoidal | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by simonpj):
* difficulty: => Unknown
Comment:
I can see your point; and I agree it's odd that it's accepted when no
methods are given. But it's not easy to fix in the way you want. Here's
why.
Suppose we have
{{{
class C a where
reverse :: a -> a
instance C Int where
reverse x = x
}}}
When GHC encounters the binding `reverse x = x`, it has to figure out
which `reverse` you mean -- there are two in scope. Ah! Since this is in
an instnace declaraction, it must be the one that's method of class `C`,
not the one from `Prelude`.
This scope resolution is done by the '''renamer'''. The renamer does not
understand type synonyms (they are interpreted by the subsequent '''type
checker'''), so it can't figure out that `Ring` really means `Num` in your
example.
I can't see an easy way round this in GHC's current structure. The only
think that comes to mind is to postpone all scope resolution for instance
bindings until the type checker. But that would mean a bit of an upheaval
of datatypes etc. Quite do-able, but not very simple.
As things stand it might be more consistent to reject an instance
declaration if the "class" turns out to be a synonym.
I'm rather inclined to do nothing!
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7543#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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