[Haskell-cafe] Mystery of an Eq instance

Bardur Arantsson spam at scientician.net
Sat Sep 21 09:21:37 CEST 2013

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

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


(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.)


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