#8582: Record syntax for pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: cactus Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: 5144 | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by blamario): I just ran into this issue. I'm trying to generalize a legacy record type like {{{ data Foo = Foo{bar :: String} }}} to {{{ data GenFoo s = GenFoo{bar :: s} }}} Unfortunately there is no way to provide a backward-compatible interface after this change. I'm hoping with this feature the solution might be as simple as {{{ type Foo = GenFoo String pattern Foo{bar} = GenFoo bar }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:2> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#8582: Record syntax for pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: cactus Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: Resolution: | Keywords: Operating System: Unknown/Multiple | PatternSynonyms Type of failure: None/Unknown | Architecture: Blocked By: 5144 | Unknown/Multiple Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by mpickering): I was also thinking this would be nice and would be interested to implement it before 7.12. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:4> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#8582: Record syntax for pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: mpickering Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: 5144 | Blocking: Related Tickets: | Differential Revisions: Phab:D1152 -------------------------------------+------------------------------------- Changes (by mpickering): * differential: => Phab:D1152 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:6> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#8582: Record syntax for pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: mpickering Type: feature request | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: 5144 | Blocking: Related Tickets: | Differential Revisions: Phab:D1152 -------------------------------------+------------------------------------- Comment (by goldfire): Some idle thought has spurred on some questions about the design of this feature. See [https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms#Trickybits a new section on the wiki page]. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:9> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#8582: Record syntax for pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: mpickering Type: feature request | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: 5144 | Blocking: Related Tickets: | Differential Revisions: Phab:D1152 -------------------------------------+------------------------------------- Comment (by simonpj): I'm sorry but I'm honestly not sure what the specification of the new feature is. The [wiki:PatternSynonyms#Design design subsection] is helpful, but it uses only one example, and I can't tell what is supposed to happen in general. Let's try this. In general, a pattern synonym should behave exactly like its expansion. So given {{{ pattern P x = (x, True) }}} these two functions should behave the same: {{{ f1 (P False) = "yes" f2 (P True) = "no" f2 (False,True) = "yes" -- Replace (P pat) by its expansion f2 (True, True) = "no" }}} (Actually they are not quite the same; see [wiki:PatternSynonyms#DynamicSemantics dynamic semantics subsection]. But close enough for now.) But for your proposal, given {{{ pattern Foo{bar, baz} = (bar, baz) }}} how, exactly should pattern matches on `Foo` expand? I can't tell. Another way to get at this question would be to look at [https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-580003.1... Section 3.17 of the langauge definition]. What changes, precisely, are required to accommodate pattern synonyms? (It's a deficiency in the current user manual that it does not say.) To give the idea, here is my first stab at extending 3.17.2. Add an extra item to the list, saying * To match a pattern `P p1 .. pn` against a value, where `P` is a pattern synonym defined by `pattern P x1 .. xn = prhs`, * first match the pattern `prhs` against the value, thereby binding `x1`..`xn`. * and then match `p1` agianst `x1`, `p2` against `x2`, and so on in sequence. A similar modification to 3.17.3 would be needed. Now, how would you change those words if your proposal was adopted? Thanks. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:11> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#8582: Record syntax for pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: mpickering Type: feature request | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: 5144 | Blocking: Related Tickets: | Differential Revisions: Phab:D1152 -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:12 mpickering]:

{{{#!hs

pattern MkP :: Int -> Int -> Q pattern MkP{x, y} = MkQ x1 y1

data Q = MkQ { x1 :: Int, y1 :: Int } }}}

I imagine you meant {{{#!hs pattern MkP{x, y} = MkQ x y }}} Otherwise, I agree with what you've said above. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:13> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#8582: Record syntax for pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: mpickering Type: feature request | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: 5144 | Blocking: Related Tickets: | Differential Rev(s): Phab:D1152 -------------------------------------+------------------------------------- Comment (by mpickering): Simon, I didn't want to write an explicit specification for this patch because it would amount to copying the specification for records. Which bit did you find confusing in the example I gave? In retrospect, the phrase "relevant data constructor" is confusing. So to explain, by "relevant" I mean an isomorphic (normally defined) data constructor. Does the example not make this any clearer? The problem with the implementation is with the record updates. Pattern synonym builders have required constraints which normal data constructors don't have. When the record update is desugared, it is necessary to provide the dictionaries for these constraints to the builder. My question was, how was I meant to do this. I ended up adding a field to the RecordUpd constructor which carried around the HsWrapper which then applied the dictionaries. More details are to be found in the last comment on the phab ticket. I don't have much time anymore to work on this ticket but I would be very disappointed it if did make it into GHC 8.0 as I started working on it several months ago. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:15> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#8582: Record syntax for pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: mpickering Type: feature request | Status: closed Priority: high | Milestone: 8.0.1 Component: Compiler | Version: Resolution: fixed | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: 5144 | Blocking: Related Tickets: | Differential Rev(s): Phab:D1152 Wiki Page: | -------------------------------------+------------------------------------- Comment (by thomie): commit 2a74a64e8329ab9e0c74bec47198cb492d25affb: {{{ Author: Matthew Pickering <matthewtpickering@gmail.com> Date: Mon Oct 19 21:17:29 2015 +0100 Record pattern synonyms This patch implements an extension to pattern synonyms which allows user to specify pattern synonyms using record syntax. Doing so generates appropriate selectors and update functions. === Interaction with Duplicate Record Fields === The implementation given here isn't quite as general as it could be with respect to the recently-introduced `DuplicateRecordFields` extension. Consider the following module: {-# LANGUAGE DuplicateRecordFields #-} {-# LANGUAGE PatternSynonyms #-} module Main where pattern S{a, b} = (a, b) pattern T{a} = Just a main = do print S{ a = "fst", b = "snd" } print T{ a = "a" } In principle, this ought to work, because there is no ambiguity. But at the moment it leads to a "multiple declarations of a" error. The problem is that pattern synonym record selectors don't do the same name mangling as normal datatypes when DuplicateRecordFields is enabled. They could, but this would require some work to track the field label and selector name separately. In particular, we currently represent datatype selectors in the third component of AvailTC, but pattern synonym selectors are just represented as Avails (because they don't have a corresponding type constructor). Moreover, the GlobalRdrElt for a selector currently requires it to have a parent tycon. (example due to Adam Gundry) === Updating Explicitly Bidirectional Pattern Synonyms === Consider the following ``` pattern Silly{a} <- [a] where Silly a = [a, a] f1 = a [5] -- 5 f2 = [5] {a = 6} -- currently [6,6] ``` === Fixing Polymorphic Updates === They were fixed by adding these two lines in `dsExpr`. This might break record updates but will be easy to fix. ``` + ; let req_wrap = mkWpTyApps (mkTyVarTys univ_tvs) - , pat_wrap = idHsWrapper } +, pat_wrap = req_wrap } ``` === Mixed selectors error === Note [Mixed Record Field Updates] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider the following pattern synonym. data MyRec = MyRec { foo :: Int, qux :: String } pattern HisRec{f1, f2} = MyRec{foo = f1, qux=f2} This allows updates such as the following updater :: MyRec -> MyRec updater a = a {f1 = 1 } It would also make sense to allow the following update (which we reject). updater a = a {f1 = 1, qux = "two" } ==? MyRec 1 "two" This leads to confusing behaviour when the selectors in fact refer the same field. updater a = a {f1 = 1, foo = 2} ==? ??? For this reason, we reject a mixture of pattern synonym and normal record selectors in the same update block. Although of course we still allow the following. updater a = (a {f1 = 1}) {foo = 2} > updater (MyRec 0 "str") MyRec 2 "str" }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:17> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler