ExtraCommas

[Alan & Kim Zimmerman]

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 <simonpj at microsoft.com>
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 at 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 at plaimi.net
> |  https://secure.plaimi.net/~alexander
> |  -----BEGIN PGP SIGNATURE-----
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