[wg-camlp4] Improvements to the OCaml Parsetree

[Alain Frisch]

Dear all,

Another useful topic for discussion related to -ppx are improvements to 
the OCaml Parsetree itself.  Of course, since the Parsetree is "going 
public" with -ppx, it make sense to improve it before too much code 
depend on it.

Let me list of few points I have in mind:

  - Get rid of some tuples / n-ary constructors in favor of proper 
record types (self-documented, more future proof).  Similarly, prefer 
explicit constructors to "bool" flags (although if we use more records, 
this is less of a problem).

  - Get rid of weird encodings in favor of more explicit representation. 
For instance, Pexp_when: guard expressions should be explicitly put next 
to patterns (or maybe allowed within the pattern algebra with some 
type-error if they are not at the toplevel).  Also, I'm not sure about 
the special "Pexp_assertfalse" node.  Do you have another examples?

  - Be more liberal about where Ptyp_poly must/might appear (the 
type-checker currently expect some dummy Ptyp_poly wrapper in some 
places, and the parser know that).

  - Avoid doing too much desugaring in the parser.  For instance, we 
should probably distinguish between "let open M in e" and "M.(e)" in the 
Parsetree.  Where to stop exactly, I'm not sure.  For instance, I don't 
have a clear opinion on whether record field punning should be handled 
purely in the parser or reflected in the Parsetree.  What do people here 
think?

  - This needs to be discussed, but what about remove those Ptyp_, 
Pexp_, ...., prefixes everywhere?  And similarly for record fields 
(ptype_, ...). Now that we have type-based disambiguation of 
constructors/labels, we don't really need those prefixes and they 
increase the verbosity of AST rewriters.  Some experimentation is 
needed, though.


Don't hesitate to comment on those points and add more!


Alain