[Haskell-cafe] Breaking change to Eq accepted by Core Libraries Committee

Sven Panne svenpanne at gmail.com
Sun Nov 14 19:24:14 UTC 2021

Am So., 14. Nov. 2021 um 20:05 Uhr schrieb Olaf Klinke <
olf at aatal-apotheke.de>:

> That module defines two intervals to be equal if their bounds are
> equal, but to be unequal only if they are disjoint.

Well, this is simply a broken implementation. Quick question: Is the Ord
instance lawful or not? Hard to tell... (at least without pencil & paper)

> In my eyes this speaks for the proposal, because the library author would
> have been
> unable to write the broken instance. [...]

But this is an extremely weak argument for the proposal: One can easily
write broken instances even with a single (==) method, e.g. violating
symmetry etc.  As another example: Given some e.g. Monad instance, can one
quickly see if it is lawful? I very much doubt so, unless one tries to
prove it. Laws of type classes are very much "outside of the Haskell
language", anyway, so I see nothing special for Eq.
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