Documenting record pattern synonyms
Simon Peyton Jones
simonpj at microsoft.com
Fri Oct 30 11:15:53 UTC 2015
Excellent
Can I suggest that (as we discussed in person) you look at 3.17.2 of the language specification
https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-580003.17
and add new numbered items for pattern synonyms? And ditto 3.17.3.
The language there is chosen to be consistent, and at least somewhat precise and unambiguous. If you simply put your stuff in that language and form, as a delta to the existing semantics, that'll go a long way.
You can put that in the user manual! By way of an example https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/interactive-evaluation.html#extended-default-rules the "Type defaulting in GHCi" section explains how the rules in the report are changed.
Thanks
Simon
| -----Original Message-----
| From: Matthew Pickering [mailto:matthewtpickering at gmail.com]
| Sent: 29 October 2015 18:57
| To: Simon Peyton Jones
| Cc: ghc-devs at haskell.org
| Subject: Re: Documenting record pattern synonyms
|
| The documentation is in D1325 - https://phabricator.haskell.org/D1325
|
| I added a specification to the wiki page -
| https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms/RecordPatternSynonyms
|
|
| On Thu, Oct 29, 2015 at 12:03 PM, Simon Peyton Jones <simonpj at microsoft.com>
| wrote:
| > Matthew:
| >
| > Could you please write the user-manual sections that you have promised?
| That is, the *specification* this feature.
| >
| > Currently, so far as I can see, there is literally NO user manual
| documentation. No changes to docs/users_guide whatsoever.
| >
| > We have discussed this on several occasions, and you agreed to do it. I
| sat with you in person and showed you what I have in mind. There is a
| sketch on the wiki page.
| >
| > I did not intend this patch to land until the user documentation was done.
| Now I feel as if I've lost my leverage :-). But it really really really
| really needs doing.
| >
| > Please. I'm begging you. Compared to what you have already done, it's
| easy.
| >
| > Simon
| >
| > | -----Original Message-----
| > | From: ghc-commits [mailto:ghc-commits-bounces at haskell.org] On
| > | Behalf Of git at git.haskell.org
| > | Sent: 29 October 2015 11:25
| > | To: ghc-commits at haskell.org
| > | Subject: [commit: ghc] master: Record pattern synonyms (2a74a64)
| > |
| > | Repository : ssh://git@git.haskell.org/ghc
| > |
| > | On branch : master
| > | Link :
| > |
| https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fghc.haskell.
| > |
| > | org%2ftrac%2fghc%2fchangeset%2f2a74a64e8329ab9e0c74bec47198cb492d25a
| > | ffb%2fgh
| > |
| > | c&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c0deecf24e85641ff
| > | 3abb08d2
| > | e05387f2%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=nEAgwVmvFQNgN%
| > | 2bANGD4H
| > | z1flN%2fucKSRFoZMdhRqQKSM%3d
| > |
| > | >---------------------------------------------------------------
| > |
| > | commit 2a74a64e8329ab9e0c74bec47198cb492d25affb
| > | Author: Matthew Pickering <matthewtpickering at 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"
| > |
| > |
| > | >---------------------------------------------------------------
| > |
| > | 2a74a64e8329ab9e0c74bec47198cb492d25affb
| > | compiler/basicTypes/ConLike.hs | 86 +++++++-
| > | compiler/basicTypes/ConLike.hs-boot | 18 ++
| > | compiler/basicTypes/Id.hs | 31 ++-
| > | compiler/basicTypes/IdInfo.hs | 21 +-
| > | compiler/basicTypes/PatSyn.hs | 51 +++--
| > | compiler/deSugar/Coverage.hs | 11 +-
| > | compiler/deSugar/DsExpr.hs | 79 ++++---
| > | compiler/deSugar/DsMeta.hs | 2 +-
| > | compiler/hsSyn/Convert.hs | 10 +-
| > | compiler/hsSyn/HsBinds.hs | 67 +++++-
| > | compiler/hsSyn/HsExpr.hs | 35 +++-
| > | compiler/hsSyn/HsUtils.hs | 21 +-
| > | compiler/hsSyn/PlaceHolder.hs | 6 +-
| > | compiler/iface/BuildTyCl.hs | 7 +-
| > | compiler/iface/IfaceSyn.hs | 17 +-
| > | compiler/iface/MkIface.hs | 11 +-
| > | compiler/iface/TcIface.hs | 12 +-
| > | compiler/main/HscTypes.hs | 15 +-
| > | compiler/parser/Parser.y | 28 ++-
| > | compiler/parser/RdrHsSyn.hs | 2 +-
| > | compiler/prelude/TysWiredIn.hs | 2 +-
| > | compiler/rename/RnBinds.hs | 18 +-
| > | compiler/rename/RnExpr.hs | 8 +-
| > | compiler/rename/RnNames.hs | 30 +--
| > | compiler/rename/RnSource.hs | 52 ++++-
| > | compiler/typecheck/TcBinds.hs | 6 +-
| > | compiler/typecheck/TcExpr.hs | 231
| +++++++++++++++---
| > | ---
| > | compiler/typecheck/TcHsSyn.hs | 10 +-
| > | compiler/typecheck/TcPat.hs | 6 +-
| > | compiler/typecheck/TcPatSyn.hs | 124 ++++++++---
| > | compiler/typecheck/TcPatSyn.hs-boot | 8 +-
| > | compiler/typecheck/TcRnDriver.hs | 3 +-
| > | compiler/typecheck/TcTyClsDecls.hs | 39 ++--
| > | compiler/types/TyCon.hs | 7 +-
| > | compiler/types/TypeRep.hs | 2 +-
| > | compiler/types/TypeRep.hs-boot | 1 +
| > | testsuite/tests/patsyn/should_compile/all.T | 7 +-
| > | .../tests/patsyn/should_compile/records-compile.hs | 11 +
| > | .../tests/patsyn/should_compile/records-poly.hs | 16 ++
| > | .../patsyn/should_compile/records-prov-req.hs | 26 +++
| > | .../patsyn/should_compile/records-req-only.hs | 16 ++
| > | .../tests/patsyn/should_compile/records-req.hs | 14 ++
| > | testsuite/tests/patsyn/should_fail/all.T | 7 +
| > | .../should_fail/mixed-pat-syn-record-sels.hs | 9 +
| > | .../should_fail/mixed-pat-syn-record-sels.stderr | 5 +
| > | .../tests/patsyn/should_fail/records-check-sels.hs | 10 +
| > | .../patsyn/should_fail/records-check-sels.stderr | 3 +
| > | .../tests/patsyn/should_fail/records-exquant.hs | 10 +
| > | .../patsyn/should_fail/records-exquant.stderr | 11 +
| > | .../patsyn/should_fail/records-mixing-fields.hs | 12 ++
| > | .../should_fail/records-mixing-fields.stderr | 17 ++
| > | .../patsyn/should_fail/records-no-uni-update.hs | 7 +
| > | .../should_fail/records-no-uni-update.stderr | 5 +
| > | .../patsyn/should_fail/records-no-uni-update2.hs | 11 +
| > | .../should_fail/records-no-uni-update2.stderr | 5 +
| > | .../patsyn/should_fail/records-poly-update.hs | 13 ++
| > | .../patsyn/should_fail/records-poly-update.stderr | 5 +
| > | testsuite/tests/patsyn/should_run/all.T | 1 +
| > | testsuite/tests/patsyn/should_run/records-run.hs | 14 ++
| > | .../tests/patsyn/should_run/records-run.stdout | 5 +
| > | 60 files changed, 1055 insertions(+), 262 deletions(-)
| > |
| > | Diff suppressed because of size. To see it, use:
| > |
| > | git diff-tree --root --patch-with-stat --no-color
| > | --find-copies-harder - -ignore-space-at-eol --cc
| > | 2a74a64e8329ab9e0c74bec47198cb492d25affb
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