#6102: Subclass Specialization in Rewrite Rules
---------------------------------+------------------------------------------
Reporter: SamAnklesaria | Owner: SamAnklesaria
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
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:
Correct, and I see no way to avoid this. Consider
{{{
foo :: ClassOne a => a -> a
foo x = classOneOp (classOneOp a)
}}}
Imagine firing rule "wontcompile" on the source code. You'd get:
{{{
foo :: ClassOne a => a -> a
foo x = classTwoOp (classTwoOp a)
}}}
But that's ill-typed! `classTwoOp` needs `(ClassTwo a)` and that simply
isn't available in `foo`.
However THIS should work:
{{{
"wontcompile" forall i. classOneOp i = classTwoOp i :: ClassInstance
}}}
Note the signature, so that the rule fires only when specialised to
`ClassInstance`.
But it still doesn't:
{{{
T6102.hs:19:45:
Could not deduce (ClassTwo ClassInstance)
arising from a use of `classTwoOp'
from the context (ClassOne ClassInstance)
bound by the RULE "wontcompile" at T6102.hs:19:1-73
Possible fix:
add an instance declaration for (ClassTwo ClassInstance)
In the expression: classTwoOp i :: ClassInstance
When checking the transformation rule "wontcompile"
}}}
That's a bug. I'll look at it.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6102#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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