[Haskell-cafe] Any precedent or plan for guaranteed-safe Eq and Ord instances?

Carter Schonwald carter.schonwald
Wed Oct 2 19:41:47 UTC 2013

So i think we can conclude the following

1) things are not perfect currently. But it requires some huge type class
changes to have a better story

2) certain types of data types will need to be newtyped to have instances
that play nice with Ryans concurrency work. Thats ok. Theres often good
reasons for those "illegal" instances. There is no sound and COMPLETE way
to enforce having "good" instances, and thats a good thing, though having
more libs work correctly is always a good thing

3) as always, people complain about floats, when the real issue is the
current numerical type classes, which are wrong in a number of ways. I hope
to experiment with my own ideas in that direction soon. Not worth a boring
no ideas worth taking seriously cafe thread

On Wed, Oct 2, 2013 at 2:39 PM, Mike Meyer <mwm at mired.org> wrote:

> On Wed, Oct 2, 2013 at 5:18 AM, Tom Ellis <
> tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> > Are there examples where application programmers would like there so be
> some
> > f, a and b such that a == b but f a /= f b (efficiency concerns aside)?
>  I
> > can't think of any obvious ones.
> Yes, and we already handle it properly:
>  Prelude> let f = (1.0 /)
> Prelude> let (z, negz) = (0.0, -0.0)
> Prelude> z == negz
> True
> Prelude> f z /= f negz
> True
> This is *not* an "IEEE Floats are weird" thing. Application
> programmers want 0.0 to equal -0.0, but -Infinity to not be equal to
> Infinity.
> Of course, given how many "IEEE Floats are weird" things there are,
> you can reasonably consider ignoring this example.
>    <mike
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