Pattern Synonym Signature Confusion

[Andres Loeh]

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
<matthewtpickering at 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
> <simonpj at 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-bounces at 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 at 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 at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs