Capturing commas in Api Annotations (D297)

[Carter Schonwald]

One small question I have is this: why's it called a comma list?
On Oct 8, 2014 12:33 PM, "Alan & Kim Zimmerman" <alan.zimm at gmail.com> wrote:

> I am currently working annotations into the parser, provided them as a
> separate structure at the end of the parse, indexed to the original by
> SrcSpan and AST element type.
>
> The question I have is how to capture commas and semicolons in lists of
> items.
>
> There are at least three ways of doing this
>
> 1. Make sure each of the items is Located, and add the possible comma
> location to the annotation structure for it.
>
> This has the drawback that all instances of the AST item annotation have
> the possible comma location in them, and it does not cope with multiple
> separators where these are allowed.
>
>
> 2. Introduce a new hsSyn structure to explicitly capture comma-separated
> lists.
>
> This is the current approach I am taking, modelled on the OrdList
> implementation, but with an extra constructor to capture the separator
> location.
>
> Thus
>
> ```
> data HsCommaList a
>   = Empty
>   | Cons a (HsCommaList a)
>   | ExtraComma SrcSpan (HsCommaList a)
>        -- ^ We need a SrcSpan for the annotation
>   | Snoc (HsCommaList a) a
>   | Two (HsCommaList a) -- Invariant: non-empty
>         (HsCommaList a) -- Invariant: non-empty
> ```
>
>
> 3. Change the lists to be of type `[Either SrcSpan a]` to explicitly
> capture the comma locations in the list.
>
>
> 4. A fourth way is to add a list of SrcSpan to the annotation for the
> parent structure of the list, simply tracking the comma positions. This
> will make working with the annotations complicated though.
>
>
> I am currently proceeding with option 2, but would appreciate some comment
> on whether this is the best approach to take.
>
> Option 2 will allow the AST to capture the extra commas in record
> constructors, as suggested by SPJ in the debate on that feature.
>
>
> Regards
>   Alan
>
>
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