I've attached the code. The code does not make direct use of
unsafePerformIO. It uses QuickCheck, but I don't think, this is a
QuickCheck bug. The used Eq-instance is the one for Float.
I've only managed to reproduce this bug on 32-bit-linux with ghc-7.4.2
when compiling with -O2.
(The code might seem a bit odd, but this is the most boiled down version
I could come up with. Even removing the "module Main where" line changes
the behaviour.)
Cheers,
Sönke
On 07/10/2012 12:51 PM, malcolm.wallace wrote:
> Also, it is more likely to be a buggy instance of Eq, than a real loss
> of referential transparency.
>
> Regards,
> Malcolm
>
>
> On Jul 10, 2012, at 11:49 AM, Christopher Done <chrisd...@gmail.com> wrote:
>
>> Depends what the real offending code is. For example, if it contains
>> unsafePerformIO then it's not a bug.
>>
>> On 10 July 2012 12:42, Sönke Hahn <sh...@cs.tu-berlin.de
>> <mailto:sh...@cs.tu-berlin.de>> wrote:
>> > Hi!
>> >
>> > I've discovered a strange bug that violates simple equational reasoning.
>> > Basically, something similar to this:
>> >
>> > let a = f x
>> > in a == f x
>> >
>> > evaluates to False.
>> >
>> > I'd like to report this on ghc-trac, but I realised, that I don't know a
>> > good name for behaviour like this. Is there one? "Broken referential
>> > transparency", perhaps?
>> >
>> > Thanks,
>> > Sönke
>> >
>> >
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module Main where
import Test.QuickCheck
main :: IO ()
main = quickCheck $ prop 6.0 0.109998815
prop :: Float -> Float -> Property
prop m x =
let a = x * m
in printTestCase (show a ++ " foo")
(x * m == a)
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