[Haskell-cafe] Mystery of an Eq instance

[Bardur Arantsson]

On 2013-09-21 06:16, Mike Meyer wrote:
>  The single biggest gotcha is that two calculations
> we expect to be equal often aren't. As a result of this, we warn
> people not to do equality comparison on floats.

The Eq instance for Float violates at least one expected law of Eq:

  Prelude> let nan = 0/0
  Prelude> nan == nan
  False

There was a proposal to change this, but it didn't really go anywhere. See:

   http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/16218

(FWIW, even if the instances cannot be changed/removed, I'd love to see
some sort of explicit opt-in before these dangerous/suprising instances
become available.)

Regards,