COMPLETE pragmas

Sebastian Graf sgraf1337 at
Mon Aug 31 20:23:16 UTC 2020

Hi Richard,

Am Mo., 31. Aug. 2020 um 21:30 Uhr schrieb Richard Eisenberg <
rae at>:

> Hi Sebastian,
> I enjoyed your presentation last week at ICFP!

Thank you :) I'm glad you liked it!

This thread (
> 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

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
<> 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

So, it's on my longer term TODO list to fix this.

> My motivation for asking is (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.)
> 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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