#7066: isInstance does not work for compound types
---------------------------------+------------------------------------------
Reporter: edsko | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone:
Component: Template Haskell | Version: 7.4.2
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by edsko):
Yes, I guess that's the tricky case -- but this problem already exists.
isInstance just checks if reifyInstances doesn't return the empty list; so
reifyInstances for Show (a, Int) could return
{{{
Instance (Show a) => Instance (a, Int)
}}}
just like it returns
{{{
Instance (Show a, Show b) => Instance (a, b)
}}}
now for that exact same argument. If 'isInstance' is not modified, then an
answer of True would mean: ''this could be satisfied'' (if there are
instances for the type variables), and for monomorphic types it would mean
''yes, this is satisfied''.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7066#comment:6>
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