metaquot feature request: literal quotations

[gasche]

The payload of Pconst_* nodes in the Parsetree is changing on a regular basis: Pconst_int changed, now it is the turn of Pconst_string, etc. Having support for literal quotation would be nice as it would let us write version-robust code to create literals or pattern-match on them, in the simple case where we don't need all the data/knobs of the full datatype. (Typically to parse attribute information encoded in literals.)

( @thierry-martinez and myself were discussing this is ocaml-ppx/ppx_deriving_protobuf#36 (comment) )

I would propose the following interface: [%lit.{integer,int,int32,in64,nativeint,float,char,string} <payload>], available in both expressions and patterns (with a question mark). This is one quotation per constructor in the Ast_helper.Const module (suggestion from @thierry-martinez), and I would propose to use the same payload type as the non-default parameter of the constructor (in particular, string for %lit.integer and int for %lit.int).

[thierry-martinez]

@gasche Thank you for filling this issue.

(with a colon)

You mean a question mark, I suppose.

[thierry-martinez]

The PR #84 implements (partially) the proposition for OCaml 4.11. I will make PRs for other versions once this one will be reviewed. Note that I didn't think about that but it seems [%lit.{int,int32,int64} ...] can't be implemented for patterns: we can only deconstruct the string in Pconst_integer(s, _), but we can't convert it (for now...) on-the-fly during pattern-matching. On the converse, in OCaml 4.02, where constants are those of Asttypes, we can implement [%lit.{int,int32,int64} ...] but not [%lit.integer ...]. Since it can't be implemented for patterns, I didn't implement them for expressions neither.