Preparing for a review of pattern synonyms

[Dr. ERDI Gergo]

On Mon, 9 Dec 2013, Simon Marlow wrote:

>> Is Haddock a part of GHC? I don't know. If it's not, then whatever
>> parser it's using will have to be extended to support pattern synonyms.
>
> Haddock uses GHC's parser, but it will need to be taught about pattern 
> synonyms and how to render them in documentation.

OK, I'll add pattern synonym support for Haddock. We don't really have a 
surface syntax for pattern types (yet?), so for now I'll just use this 
format:

pattern P :: b -> a -> T a; requires (Num a); provides (Eq b)

> Right, I'm not suggesting that we should break the abstraction, but I think 
> it would be really nice if there was a way to say "the following 3 patterns 
> are exhaustive", perhaps in a pragma.
>
> One of the things that we ought to be able to do with this extension is to 
> provide abstract datatypes with pattern matching support.  The only thing 
> missing is support for exhaustive pattern matches.  F# active patterns has 
> it!

This should be something orthogonal to pattern synonyms that supports 
regular view patterns as well.

>> If there's no separate Trac ticket for ViewPatternsAlternative yet, I'm
>> happy to open one.
>
> That would be great!

I've added it as #8605.

Bye,
 	Gergo

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