I made a ticket[1] to track the progress of this issue. I agree that the whole situation is confusing but I can't see anything much better.
[1]: https://ghc.haskell.org/trac/ghc/ticket/10928#ticket On Sat, Oct 3, 2015 at 11:04 AM, Andres Loeh <m...@andres-loeh.de> wrote: > Hi. > > Matthew, I'm in favour of your proposed change (probably slightly in > favour of option 2 compared to option 1). I must say that the whole > "double constraint" syntax is very confusing no matter what. If we > could somehow find a better / more explicit way to specify provided > constraints, that would be even better. > > Cheers, > Andres > > On Fri, Oct 2, 2015 at 1:58 PM, Matthew Pickering > <matthewtpicker...@gmail.com> wrote: >> I have grepped the whole of hackage. There are only 10 pattern synonym >> signatures in total in three separate packages (one of them being my >> own). None of them specify provided constraints, three of them specify >> required constraints. Of course this is a very small sample size but >> it means that there wouldn't be any widespread breakage either way >> with this proposal. >> >> ``` >> ../hackage-packages//ghc-exactprint-0.4.1.0/tests/examples/Sigs.hs:pattern >> Single :: () => (Show a) => a -> [a] >> ../hackage-packages//structs-0/src/Data/Struct/Internal.hs:pattern >> Struct :: () => Struct t => SmallMutableArray# s Any -> t s >> ../hackage-packages//structs-0/src/Data/Struct/Internal.hs:pattern Nil >> :: forall t s. () => Struct t => t s >> ../hackage-packages//transformations-0.2.0.0/examples/MultiRec.hs:pattern >> Ref' :: Path phi ix top -> HWithRef phi top ix >> ../hackage-packages//transformations-0.2.0.0/examples/MultiRec.hs:pattern >> Const' :: forall top. Integer -> HWithRef AST top AExpr >> ../hackage-packages//transformations-0.2.0.0/examples/MultiRec.hs:pattern >> BConst' :: forall top. Bool -> HWithRef AST top BExpr >> ../hackage-packages//transformations-0.2.0.0/examples/MultiRec.hs:pattern >> And' :: forall top. HWithRef AST top BExpr -> HWithRef AST top BExpr >> ../hackage-packages//transformations-0.2.0.0/examples/MultiRec.hs:pattern >> GT' :: forall top. HWithRef AST top AExpr -> HWithRef AST top AExpr >> ../hackage-packages//transformations-0.2.0.0/examples/MultiRec.hs:pattern >> GT_0 :: Path AST top AExpr -> Path AST top BExpr >> ../hackage-packages//transformations-0.2.0.0/examples/MultiRec.hs:pattern >> Neg_0 :: Path AST top AExpr -> Path AST top AExpr >> ``` >> >> On Thu, Oct 1, 2015 at 10:48 PM, Simon Peyton Jones >> <simo...@microsoft.com> wrote: >>> Matthew >>> >>> What you say sounds reasonable to me. I certainly don't have a strong >>> opinion that the current design is the "right" one. >>> >>> Simon >>> >>> | -----Original Message----- >>> | From: ghc-devs [mailto:ghc-devs-boun...@haskell.org] On Behalf Of Matthew >>> | Pickering >>> | Sent: 01 October 2015 12:23 >>> | To: GHC developers >>> | Subject: Pattern Synonym Signature Confusion >>> | >>> | I think that the current state of pattern synonym signatures is quite >>> | confusing, especially regarding the constraints. For those unfamiliar, >>> | a signature looks like the following, >>> | >>> | pattern ExNumPat :: (Show b) => (Num a, Eq a) => b -> T a >>> | >>> | The first constraint being the "provided constraints" and the second >>> | the "required constraints". >>> | >>> | My main concern is when only a single constraint is specified then >>> | these are treated as the provided constraints. The manual gives the >>> | reason that this is the "more common" choice. It seems that this >>> | motivation is driven by the original ticket which had a lengthy >>> | discussion about GADTs. In my experience, the opposite is true, it is >>> | more common to have required constraints. >>> | >>> | This is true especially in a few common cases such as "pattern Foo = >>> | 27", where users define pattern synonyms which have (overloaded) >>> | literals on the RHS. The most general signature for such a pattern is >>> | "pattern :: () => (Eq a, Num a) => a". >>> | >>> | For this reason, I think it would be better if either >>> | >>> | 1. Only specifying one constraint corresponded to the required constraints >>> | 2. We required users to specify both sets of constraints, even if the >>> | provided constraints are empty. >>> | >>> | In terms of breakage, I don't think that pattern synonym signatures >>> | are widely used yet. In both schemes it is possible to write backwards >>> | compatible code by writing both sets of constraints. >>> | >>> | I think a lot of the confusion also arises from the unusual form of >>> | the signature, it is unusual to specify two sets of constraints and I >>> | suspect users tends to assume that a single set of constraints is >>> | either provided or required depending on what they want it to mean. >>> | Forcing the specification of both, forces the user to make the >>> | distinction earlier rather than trying to decipher error messages. >>> | >>> | Thoughts? >>> | >>> | Matt >>> | _______________________________________________ >>> | ghc-devs mailing list >>> | ghc-devs@haskell.org >>> | https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.haske >>> | ll.org%2fcgi-bin%2fmailman%2flistinfo%2fghc- >>> | devs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c403e493d2a54438d264 >>> | 408d2ca52b5b9%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=sjc2n0Gm1A%2ffe >>> | OKEpntmEYqTfbYaLvk2sb%2b2vUqIqLU%3d >> _______________________________________________ >> ghc-devs mailing list >> ghc-devs@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
