Something else to throw into the mix, once the Located replacement by Genlocated is done for hsSyn I intend to capture the locations of all non-captured syntactic elements, such as commas. So this proposed mechanism would help, provided the location of each comma is also tracked. Alan On Fri, Sep 26, 2014 at 5:46 PM, Simon Peyton Jones wrote:

| 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:alexander@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 | alexander@plaimi.net | https://secure.plaimi.net/~alexander | -----BEGIN PGP SIGNATURE----- | Version: GnuPG v2 | Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ | | iF4EAREIAAYFAlQhP9sACgkQRtClrXBQc7U0+gD/TvT+9iVA6JqiopTfeYEc7NU2 | USD2bf+bKwhy9MNUjssA/3YUjA3PVYRpw1RFoRQF15zBFF6WDH5PQbS/8/5q7h+P | =saJc | -----END PGP SIGNATURE----- _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs