#2356: GHC accepts multiple instances for the same type in different modules
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Reporter: claus | Owner:
Type: bug | Status: new
Priority: low | Milestone: _|_
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
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Changes (by simonpj):
* priority: high => low
* milestone: 6.10.1 => _|_
Comment:
Currently this is by design. What would the unique global symbol be for
{{{
instance C (T a) [Maybe b]
}}}
Oh, and `T` might be a type synonym.
We could encode the type as a string, I suppose, but the error message
(from the linker) would still be execrable.
For indexed-type instances, overlap is unsound, so there's an eager check.
On reflection, if you don't use `-fallow-overlapping-instances` GHC should
arguably do the same eager check for class instances; that would more
faithfully implement the Report. Even doing that is not trivial, because
GHC goes to some lengths to avoid looking at (literally) thousands of
interface files on every compilation, just on the off-chance that there'll
be an instance declaration lurking there.
With overlap allowed, it's clear that you can make different choices in
different parts of the program.
So I'm not sure what to do; and the only thing that comes to mind isn't
cheap. For now I'll put it as low-prio, unless people start yelling that
it's ruining their day.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2356#comment:4>
GHC <http://www.haskell.org/ghc/>
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