[Haskell-cafe] an evil type hangs GHC

[Ryan Ingram]

>From http://www.haskell.org/ghc/docs/6.12.2/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)


On Fri, Nov 12, 2010 at 10:52 AM, Petr Pudlak <deb at pudlak.name> wrote:
> Hi, I was playing with the following example I found in D.A.Turner's paper
> Total Functional Programming:
>
>> data Bad a = C (Bad a -> a)
>>
>> bad1 :: Bad a -> a
>> bad1 b@(C f) = f b
>>
>> bad2 :: a
>> bad2 = bad1 (C bad1)
>
> To my surprise, instead of creating a bottom valued function (an infinite
> loop), I managed to send the GHC compiler (ver. 6.12.1) to an infinite loop.
> Could anybody suggest an explanation? Is this a GHC bug? Or is this "Bad"
> data type so evil that type checking fails?
>
>    Thanks,
>    Petr
>
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