#4259: Relax restrictions on type family instance overlap
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Reporter: lilac | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: 7.4.1
Component: Compiler (Type checker) | Version: 6.12.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by simonpj):
The trouble is that if both these instances were allowed, one could
readily construct (in FC) a coercion making STIOG equal to STIOR. And
that's unsound.
However I have been working with Dimitrios on an idea that would allow
overlapping type function definitions, provided they were all declared
together. So: would it be enough to allow overlapping type family
instances, but ''only if the overlapping ones were declared together'' (or
at least in the same module)?
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4259#comment:12>
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