| What I think you are proposing is to capture commas in a different way
| altogether before this stage, and I do not understand how or where
No. I'm proposing this. Insetad of
data HsRecFields id
= HsRecFields { rec_flds :: [HsRecField id arg],
rec_dotdot :: Maybe Int
have this:
data HsRecFields id
= HsRecFields { rec_flds :: HsCommaList (HsRecField id arg),
rec_dotdot :: Maybe Int
data HsCommaList a
= Empty
| ExtraComma (HsCommaList a)
| Cons a (HsCommaList a)
So that HsCommaList a is very like [a] but can express precisely where the
extra commas appear.
Now adapt the parser to produce a (HsCommaList (HsRecField RdrName (LHsExpr
RdrName))), rather than a list of those things.
The renamer then complains if it finds any ExtraComma constructors, unless the
-XExtraCommas is specified.
Does that help?
Simon
| -----Original Message-----
| From: Alexander Berntsen [mailto:alexan...@plaimi.net]
| Sent: 23 September 2014 10:40
| To: Simon Peyton Jones
| Subject: Re: ExtraCommas
|
| -----BEGIN PGP SIGNED MESSAGE-----
| Hash: SHA256
|
| On 23/09/14 11:28, Simon Peyton Jones wrote:
| > My advice was only: parse commas *always* as if they were part of
| the
| > language. (So the parser has no conditinoals.) Reject them later (in
| > the renamer) if the language extension is not enabled.
| I don't understand this. From my limited understanding commas are
| lexed, and then I can, in the parser, pattern match on things. Here's
| a very specific example, record field update/construction:
|
| fbinds :: { ([HsRecField RdrName (LHsExpr RdrName)], Bool) }
| : fbinds1 { $1 }
| | {- empty -} { ([], False) }
|
| fbinds1 :: { ([HsRecField RdrName (LHsExpr RdrName)], Bool) }
| : fbind ',' fbinds1 { case $3 of (flds, dd) ->
| ($1 : flds, dd) }
| | fbind { ([$1], False) }
| | '..' { ([], True) }
|
| What's happening here is that fbinds uses a helper fbinds1, to capture
| both fbinds1 & emptys. So to add support for leading/trailing commas,
| what I did is simply pattern match on them:
|
| fbinds :: { ([HsRecField RdrName (LHsExpr RdrName)], Bool) }
| : ',' fbinds1 { $2 }
| | fbinds1 { $1 }
|
| fbinds1 :: { ([HsRecField RdrName (LHsExpr RdrName)], Bool) }
| : fbind ',' fbinds1 { case $3 of
| (flds, dd) -> ($1 : flds, dd)
| }
| | fbind { ([$1], False) }
| | '..' { ([], True) }
| | {- empty -} { ([], False) }
|
| You can observe that I am now using fbinds to capture ',' fbinds1, and
| have moved the empty match to fbinds1 itself.
|
| This is the general pattern I have used for all of my patches so far.
|
|
| What I think you are proposing is to capture commas in a different way
| altogether before this stage, and I do not understand how or where
| this is meant to happen. And it also sounds like your suggestion will
| mean I need to handle the case where extra commas are *not* supposed
| to be handwaved, like tuples. So please elaborate, if you can.
|
| > Happy to talk. I'm simonpj0 on skype.
| Do you happen to use any non-proprietary tool or at least protocol?
| - --
| Alexander
| alexan...@plaimi.net
| https://secure.plaimi.net/~alexander
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