How to describe this bug?

[Aleksey Khudyakov]

On Tue, Jul 10, 2012 at 3:06 PM, Sönke Hahn <shahn at cs.tu-berlin.de> wrote:
> 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.
>
It's expected behaviour with floats. Calculations in FPU are done in
maximul precision available.  If one evaluation result is kept in registers
and another has been moved to memory and rounded and move back to registers
number will be not the same indeed.

In short. Never compare floating point number for equality unless you
really know
what are you doing.