[Haskell-cafe] Re: QuickCheck: test passes in GHCi, fails when compiled

Lauri Pesonen lauri.pesonen at iki.fi
Wed Jan 13 05:07:29 EST 2010

2010/1/13 Lauri Pesonen <lauri.pesonen at iki.fi>:
> I provided a Java solution to a problem of returning the first digit
> of an integer on StackOverflow and someone wondered if there were any
> floating point point problems with the solution, so I though I'd
> implement the algorithm in Haskell and run QuickCheck on it.
> Everything works fine on GHCi, but if I compile the code and run the
> test, it fails with -1000, 1000, -1000000. Any ideas why?
> In any case it seems that the commenter was right and there are some
> subtle problems with my solution. I'd just like to know more
> details...

Ok, I've figured out why the compiled version fails the check:

main = putStrLn $ show $ logBase 10 1000

when compiled returns 2.9999999999999996 on my system (WinXp 32-bit)
which then gets truncated to 2.

So it seems that there's a precision difference between the compiled
and the interpreted code.

'logBase 10 999' on the other hand produces exactly the same value in
both cases.

I expect this to be a known issue with floats. Sorry for the noise.

  ! Lauri

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