Concrete syntax for pattern synonym type signatures

[Simon Peyton Jones]

Note though, it doesn't mean the same thing to say (Foo a, Bar a b) => ... as it does to say
Foo a => Bar a b => ...
The latter can use Foo a when working on Bar a b, but not Bar a b to discharge Foo a, which makes a difference when you have functional dependencies.

I disagree.  Can you offer a concrete example, and show that one typechecks when the other does not?

Simon

From: Edward Kmett [mailto:ekmett at gmail.com]
Sent: 10 November 2014 15:46
To: Richard Eisenberg
Cc: Simon Peyton Jones; GHC Devs
Subject: Re: Concrete syntax for pattern synonym type signatures

Note though, it doesn't mean the same thing to say (Foo a, Bar a b) => ... as it does to say

Foo a => Bar a b => ...

The latter can use Foo a when working on Bar a b, but not Bar a b to discharge Foo a, which makes a difference when you have functional dependencies.

So in some sense the 'pattern requires/supplies' split is just that.



That said, Richard's other option

pattern Foo a => P :: Bar a => a

has the benefit that it looks a bit like the old datatype contexts (but here applied to the constructor/pattern).

If we expect the left hand side or the right hand side to be most often trivial then that may be worth considering.

You'd occasionally have things like

pattern (Num a, Eq a) => Foo :: a

for

pattern Foo = 8

but most of the time they'd wind up just looking like a GADT constructor.

-Edward

On Sun, Nov 9, 2014 at 10:02 PM, Richard Eisenberg <eir at cis.upenn.edu<mailto:eir at cis.upenn.edu>> wrote:

On Nov 9, 2014, at 2:11 PM, Simon Peyton Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
>
> * One other possibility would be two => thus
>       pattern P :: (Eq b) => (Num a, Eq a) => ...blha...
>

I should note that I can say this in 7.8.3:

foo :: Show a => Eq a => a -> String
foo x = show x ++ show (x == x)

Note that I've separated the two constraints with a =>, not a comma. This syntax does what you might expect. (I actually believe that this is an improvement over the conventional syntax, but that's a story for another day.) For better or worse, this trick does not work for GADT constructors (which is a weird incongruence with function type signatures), so adding the extra arrow does not really steal syntax from GADT pattern synonyms.

Richard
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