Would you say this the same as the one described in the user guide?
http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs.html#bugs-ghc
GHC's inliner can be persuaded into non-termination using the
standard way to encode recursion via a data type:
data U = MkU (U - Bool)
russel :: U - Bool
russel u@(MkU p) = not $ p u
x :: Bool
x = russel (MkU russel)
We have never found another class of programs, other than this
contrived one, that makes GHC diverge, and fixing the problem
would impose an extra overhead on every compilation. So the bug
remains un-fixed. There is more background in Secrets of the GHC
inliner.
Duncan
On Sun, 2007-02-04 at 13:49 +, Wouter Swierstra wrote:
When I try to compile to the following program, GHC seems to loop:
data Evil = Evil (Evil - Evil)
instance Show Evil where
show _ = t
apply :: Evil - Evil - Evil
apply (Evil f) x = f x
delta :: Evil
delta = Evil (\x - x `apply` x)
omega :: Evil
omega = delta `apply` delta
main = print omega
The program codes up a non-terminating term omega - very similar to
(\x - x x) (\x - x x) - without using recursion. The trick is to
define an Evil data type which has a negative occurrence of Evil.
I realize it's a contrived example, but I wasn't sure if it's a known
issue. In case it matters, I'm using GHC 6.6 on a PowerPC Mac.
All the best,
Wouter
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