[Haskell-cafe] Wincomplete-uni-patterns and bidirectional patterns

[Tom Ellis]

On Mon, Jan 15, 2024 at 08:56:33PM +0100, Olaf Klinke wrote:
> > On Mon, Jan 15, 2024 at 12:00:16PM +0100, Olaf Klinke wrote:
> > > How can I teach GHC 9 that the pattern is indeed exhaustive? 
> > 
> > Have you tried COMPLETE pragmas?
> >  
> > https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/pragmas.html#complete-pragmas
> 
> Huh, of course that works. Thanks.

You're welcome.

> It never occurred to me to even consider the existence of manual
> completeness annotation, since completeness checks work so reliably for
> ordinary constructors. But maybe my types weren't esoteric enough so
> far. Are there pattern-free cases where the GHC completeness checker is
> wrong? Of what computational class is the problem? Can I simulate a
> Turing machine by checking completeness in the presence of patterns?

I think the completeness checker works fine in all
pattern-synonym-free cases, i.e. when you're using only real
constructors.  I don't actually understand why completeness
annotations are needed for pattern synonyms.

Tom