#5400: GHC loops on compiling with optimizations
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Reporter: noschinl | Owner:
Type: bug | Status: new
Priority: normal | Milestone: _|_
Component: Compiler | Version: 7.0.4
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Linux | Blocking:
Architecture: x86_64 (amd64) | Failure: Compile-time crash
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Changes (by simonpj):
* milestone: => _|_
Comment:
Interesting example! This program should diverge, of course, but it
should not do so at runtime. Yet it turns out to be a disguised case of
GHC's well-known compile-time divergence bug
([http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/bugs.html
#bugs-ghc User manual 12.2.1, bullet 3]).
Here is the desugaring of the program after dictionaries have become
explicit:
{{{
data C a = MkC (C a -> Int)
fH :: C Int -> Int
fH x = case x of { MkC g -> g x }
fCInt :: C Int -- The instance decl for (C Int) gives this
fCInt = MkC fH
}}}
Notice that
* None of these definitions is recursive
* But the data type `C a` has a `(C a)` on the left of an arrow.
Now consider evaluating
{{{
fH fCInt
--> (inline fH)
case fCInt of MkC g -> g fCInt
--> (inline fCInt and eliminate the case)
fH fCInt
and so on forever...
}}}
It's a classic example of the Russell-paradox loop. The manual claims
this "never happens in practice" and I guess this bug report is a counter-
example! However, it's not easy to fix this particular loop; and as you
found, it's not a program you want to write anyway.
See also #3872
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5400#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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