How to describe this bug?

[roconnor at theorem.ca]

On Tue, 10 Jul 2012, Sönke Hahn wrote:

> Hi!
>
> I've discovered a strange bug that violates simple equational reasoning.
> Basically, something similar to this:
>
> let a = f x
> in a == f x

While this code as it stands doesn't quite illustrate the referential 
transparency error, since == isn't guarenteed to return True on the same 
floating point value (see NaN), with a small tweek we can turn into an 
example that does illustrate the lack of referential transparency:

(let a = f x in a == f x) == (let a = f x in a == a)

or also perhaps

(let a = f x in a == f x) == (f x == f x)

If either of these return False than it is an error of referential 
transparency since equality on equivalent Bool expressions is always 
supposed to return True or diverge.

-- 
Russell O'Connor                                      <http://r6.ca/>
``All talk about `theft,''' the general counsel of the American Graphophone
Company wrote, ``is the merest claptrap, for there exists no property in
ideas musical, literary or artistic, except as defined by statute.''