Comparing floating point numbers (Floats or Doubles, using (==)) that are constructed by equal terms sometimes evaluates to False. A term that produces normal floating point values (as opposed to NaN or Infinity) should always be equal to itself. This bug effectively breaks referential transparency.

This bug occurs with ghc-7.4.2 when compiling with "-O2" on 32-bit linux ubuntu-11.10. (I guess, it happens on all 32-bit platforms.)

The attached file demonstrates the bug.

Simon Marlow expressed his doubts, this bug will be fixed. See also: http://www.haskell.org/pipermail/glasgow-haskell-users/2012-July/thread.html#22565

Possible workarounds:

• compile with "-msse2" if available
• use approximate equivalence