#6067: regression: cgrun051(ghci) failing in HEAD
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Reporter: simonmar | Owner:
Type: bug | Status: new
Priority: highest | Milestone: 7.6.1
Component: Compiler | Version: 7.5
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by simonpj):
I can see what is happening. We have
{{{
case error "OK" of { x::T1 -> blah }
}}}
where `T1` is a data type with no constructors.
* With some level of optimisation GHC sees that `error` returns bottom,
and simplifies to `error "OK"`.
* But in GHCi, GHC doesn't see the bottom, but instead sees that the case
is scrutinising a data type with no constructors, so it replaces the whole
case with `error "Impossible case alternative"`.
The latter transformation is really wrong, even with our liberal exception
semantics. Really it should only be done if the scrutinee is guaranteed
not to raise an exception.
This links to a converstaion I had with Max. The idea is:
* In Core, allow an empty list of alternatives in `case`.
* If the scrutinee provably is bottom, discard all the alternatives
(since they are unreachable)
Things to bear in mind:
* This avoid the need to add "unsafe" coercions.
* Case expressions would need to carry their type (as they do at present)
* The code generator should treat `(case e of [])` just like `e`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6067#comment:1>
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