Re: Spurious [-Woverlapping-patterns] -- [cont] ghc should be able to deduce correct use of partial functions

> On Sun, 28 Apr 2024 at 07:03, Olaf Klinke <[email protected]> wrote:

>  I think what we see here is an instance of the fact that all checks
> must break down in presence of view patterns.

Sadly, yes. I'd originally tried that example with a view pattern
`fromJust`, to keep closer to last week's o.p. Switching to explicit
constructor didn't help.
To contrast with other pattern-related styles:

* with guards, GHC can see the coverage, doesn't complain, works fine.

* with Pattern Synonyms (at least simple bidir ones) GHC complains about
non-exhaustiveness, but in fact works fine.

* with PattSyns defined explicitly bidir using a View Pattern, again checks
break down.


As well as the 'Boolean blindness' mentioned on that thread, there's 'Maybe
blindness'
or even 'algebraic blindness'
https://github.com/quchen/articles/blob/master/algebraic-blindness.md
(and more articles/threads via Google).
Walk a data structure (could be a JSON), come back with a bunch of Maybes,
now too easy to confuse which means what.
The 'audit trail' has gone cold.

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