[Haskell-cafe] (names for) invariants for Eq and Ord?

[Simon Jakobi]

Hi Johannes,

https://ghc.haskell.org/trac/ghc/ticket/15078 might be interesting to you.

Cheers,
Simon

2018-05-28 16:47 GMT+02:00 Johannes Waldmann <
johannes.waldmann at htwk-leipzig.de>:

> Dear Cafe,
>
> I am somewhat surprised that the Haskell Standard (*)
> https://www.haskell.org/onlinereport/haskell2010/
> haskellch6.html#x13-1270006.3
> does not contain any
> notation for (intended) properties of Eq and Ord instances,
> and their relation. For Java (standard library),
> the API spec uses wording like "consistent with equals"
> https://docs.oracle.com/javase/10/docs/api/java/lang/Comparable.html
>
> I am well aware that semantics (transitivity, etc.)
> cannot be enforced statically, in either language.
> But both standards still speak of "total order".
> Do we (Haskell) need something similar to "consistent with equals"?
>
> It depends. E.g., looking at a (random) source in containers
> (Data.Map.Internal), it seems that it will always use the result of
> compare, never of (==), on keys. That's not obvious from the docs.
> I just checked -- I can make a type where (==) = undefined,
> but write a proper Ord instance, and use it as key type.
>
> I am asking this because I will be teaching type classes,
> with Eq and Ord as examples. I detected the funny situation
> that the Java specification looks "more mathematical"
> than the corresponding Haskell one. What will the students think ...
>
> - J.W.
>
> (*) The Haskell standard is what you find when you scroll
> down, down, down to the very bottom of
> https://www.haskell.org/documentation . So, probably not that important...
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