On this thread: * I'm strongly of the opinion that pattern signatures should start pattern P :: ...blah... Just like value signatures, the pattern name comes first, then a double colon. * I really don't like syntax that puts the pattern name in the middle. * It's vital that a GADT constructor should have a signature just like a GADT constructor. * I'm not wedded to ";". I chose it just because it seemed like a grouping operator that binds less tightly than ",". A vertical bar would be fine with me. But: I'm a little worried that "(|" is a useful bracketing lexeme, already used for arrow syntax, so you might have to put a space between the paren and the bar. * One other possibility would be two => thus pattern P :: (Eq b) => (Num a, Eq a) => ...blha... If you wanted something which had a "match-required" part but no "match-provided" part, you'd end up with patter P :: () => (Num a, Eq a) => ...blah... which is a little odd, but perhaps no odder than pattern P :: ( | Num a, Eq a ) => ...blah... This has the considerable merit of requiring no new syntax at all. There is the difficulty of remembering which way round the match-required and match-provided stuff is, but all versions have that problem. * We don't need to disturb the syntax for HsType; we can just have a parser for this particular form; in effect a modified HsForAll, just for pattern type signatures, and probably flattened out into the pattern-signature constructor form itself. Does that help? Simon | -----Original Message----- | From: Dr. ERDI Gergo [mailto:gergo@erdi.hu] | Sent: 09 November 2014 07:56 | To: Simon Peyton Jones | Cc: GHC Devs | Subject: RE: Concrete syntax for pattern synonym type signatures | | On Tue, 4 Nov 2014, Simon Peyton Jones wrote: | | > pattern P :: forall tvs. (match-provided ; match-required) => tau | > | > The "; match-required" part is optional, and the "match-provided" part | might be empty. So P1 and P2 would look like this: | > | > pattern P1 :: forall a. (; Num a) => b -> (a,b) | > pattern P2 :: forall a. (; Num a, Ord a) => a -> a | | Doesn't the ';' look a bit like something that could be incidentially | introduced by some layout-aware syntax rule? Wouldn't, e.g., '|' be more | explicit as a separator? | | example: | | pattern P :: forall tvs. (Eq b | Num a, Eq a) => b -> T a