Re: How to describe this bug?

It also works (exposes the bug on x86) without Quickcheck and Doubles: main = prop 6.0 0.109998815 prop m x = do let a = x * m putStrLn (show a ++ " foo") print (x * m == a) 0.65999289 foo False The middle line seems to prevent CSE. C. Am 10.07.2012 13:06, schrieb Sönke Hahn:

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:

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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 wrote:

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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 mailto:shahn@cs.tu-berlin.de> wrote:

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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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