COMPLETE pragmas

[Richard Eisenberg]

Hooray Sebastian!

Somehow, I knew cluing you into this conundrum would help find a solution. The approach you describe sounds quite plausible.

Yet: types *do* matter, of course. So, I suppose the trick is this: have the COMPLETE sets operate independent of types, but then use types in the PM-checker when determining impossible cases? And, about your idea for having pattern synonyms store pointers to their COMPLETE sets: I think data constructors can also participate. But maybe there is always at least one pattern synonym (which would be a reasonable restriction), so I guess you can look at the pattern-match as a whole and use the pattern synonym to find the relevant COMPLETE set(s).

Thanks for taking a look!
Richard

> On Aug 31, 2020, at 4:23 PM, Sebastian Graf <sgraf1337 at gmail.com> wrote:
> 
> Hi Richard,
> 
> Am Mo., 31. Aug. 2020 um 21:30 Uhr schrieb Richard Eisenberg <rae at richarde.dev <mailto:rae at richarde.dev>>:
> Hi Sebastian,
> 
> I enjoyed your presentation last week at ICFP!
> 
> Thank you :) I'm glad you liked it!
> 
> This thread (https://ghc-devs.haskell.narkive.com/NXBBDXg1/suppressing-false-incomplete-pattern-matching-warnings-for-polymorphic-pattern-synonyms <https://ghc-devs.haskell.narkive.com/NXBBDXg1/suppressing-false-incomplete-pattern-matching-warnings-for-polymorphic-pattern-synonyms>) played out before you became so interested in pattern-match coverage. I'd be curious for your thoughts there -- do you agree with the conclusions in the thread?
> 
> I vaguely remember reading this thread. As you write there <https://ghc-devs.haskell.narkive.com/NXBBDXg1/suppressing-false-incomplete-pattern-matching-warnings-for-polymorphic-pattern-synonyms#post9>
> 
> And, while I know it doesn't work today, what's wrong (in theory) with
> 
> {-# COMPLETE LL #-}
> 
> No types! (That's a rare thing for me to extol...)
> 
> I feel I must be missing something here.
> 
> Without reading the whole thread, I think that solution is very possible. The thread goes on to state that we currently attach COMPLETE sets to type constructors, but that is only an implementational thing. I asked Matt (who implemented it) somewhere and he said the only reason to attach it to type constructors was because it was the easiest way to implement serialisation to interface files.
> 
> The thread also mentions that type-directed works better for the pattern-match checker. In fact I disagree; we have to thin out COMPLETE sets all the time anyway when new type evidence comes up, for example. It's quite a hassle to find all the COMPLETE sets of the type constructors a given type can be "represented" (I mean equality modulo type family reductions here) as. I'm pretty sure it's broken in multiple ways, as #18276 <https://gitlab.haskell.org/ghc/ghc/-/issues/18276> points out.
> 
> Disregarding a bit of busy work for implementing serialisation to interface files, it's probably far simpler to give each COMPLETE set a Name/Unique and refer to them from the pattern synonyms that mention them (we'd have to get creative for orphans, though). The relation is quite like between a type class instance and the type in its head. A more worked example is here: https://gitlab.haskell.org/ghc/ghc/-/issues/18277#note_287827 <https://gitlab.haskell.org/ghc/ghc/-/issues/18277#note_287827>
> 
> So, it's on my longer term TODO list to fix this.
> 
> 
> My motivation for asking is https://github.com/conal/linalg/pull/54 <https://github.com/conal/linalg/pull/54> (you don't need to read the whole thing), which can be boiled down to a request for a COMPLETE pragma that works at a polymorphic result type. (Or a COMPLETE pragma written in a module that is not the defining module for a pattern synonym.) https://gitlab.haskell.org/ghc/ghc/-/issues/14422 <https://gitlab.haskell.org/ghc/ghc/-/issues/14422> describes a similar, but even more challenging scenario.
> 
> I'll answer in the thread. (Oh, you also found #14422.) I think the approach above will also fix #14422.
> 
> Do you see any ways forward here?
> . 
> 
> Thanks!
> Richard
> 
> Maybe I'll give it a try tomorrow. 

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