#3877: Require XOverlappingInstances for the most specific instance only
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Reporter: traz161616 | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: 7.2.1
Component: Compiler (Type checker) | Version: 6.12.1
Resolution: | Keywords: XOverlappingInstances
Testcase: | Blockedby:
Difficulty: | Os: Unknown/Multiple
Blocking: | Architecture: Unknown/Multiple
Failure: GHC rejects valid program |
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Comment(by simonpj):
OK so the current rule is this:
* If an instance decl is compiled with `-XOverlappingInstances` then it
can be overlapped by a more-specific instance. The (later) more specific
instance does not need to be compiled with `-XOverlappingInstances`
You want precisely the reverse of this. The less-specific instance is in
some library compiled without `-XOverlappingInstances`, and you want to
write an instance that deliberately overlaps it.
To make this change would break other people's code, because a library
author could not write an instance declaration (such as a "default
instance" `instance Foo a where...`) that he ''expects'' to be overlapped.
So to meet your goals, we either need a new flag altogether (which seems a
bit heavyweight), or to say
* Overlap between two instance decls is allowed if ''either'' (or both)
of them was compiled with `-XOverlappingInstances`.
This seems reasonable to me... one party or the other has to agree to an
overlap. It's a bit more liberal than the current policy.
Ok with everyone? It's a one-line change to the code; a bit more in the
documentation.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3877#comment:11>
GHC <http://www.haskell.org/ghc/>
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