0003926: Unneeded limitation on the CamplP4 quotation system

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0003926

OCaml

Camlp4

public

2005-12-16 10:46

2009-03-31 13:07

Reporter

alexbaretta

Assigned To

ertai

Priority

normal

Severity

feature

Reproducibility

always

Status

closed

Resolution

fixed

Platform

OS Version

Product Version

3.09.0

Target Version

Fixed in Version

3.10.0

Summary

0003926: Unneeded limitation on the CamplP4 quotation system

Description

I use the CamlP4 quotation system to embed languages completely different from Ocaml in Ocaml code. Namely, I use the quotation system to compile SQL queries towards Ocaml *module* definitions. Thus, for example, the SQL code of a SELECT query called Select_foos is accessible from Ocaml namespace as

Select_foos.query_text ()

and the recordset can be obtained with

Select_foos.records ().

The Embedded SQL provides many other features which justify packaging the generated Ocaml code within a module definition. Sadly, camlp4 does not allow quotations to expand to module definitions. IIRC, only expressions and patterns are allowed as results of a quotation expander. This is not an insurmountable problem, since, instead of preprocessing my source files with camlp4, I can preprocess them with ocpp, which is not so picky about what quotations expand to. Yet, this, too, is not an adequate solution, as it does not produce the appropriate hash-directives for ocamlc to figure out where a preprocessed line of code actually appears in the original source file.

These seems to be a limitation of the pa_o original Ocaml syntax for Camlp4. Here are my reasons. The quotation result is parsed by the following code in pcaml.ml

value parse_quotation_result entry loc shift name str =
  let cs = Stream.of_string str in
  try Grammar.Entry.parse entry cs with
    ...

As I would expect, the mechanimism seems to be completely agnostic about the kind of subtree that is being parsed, as Grammar.Entry.parse knows the grammar being recognized only through the entry parameter. By perusing pa_o.ml, I have been able to verify that the QUOTATION symbol appears only in the productions pertaining to the expr and patt nonterminals.

In my opinion this is a spurious limitation, which could easily be overcome by allowing the the QUOTATION symbol to appear in productions of other nonterminals, such as module_expr and str_iterm, thus allowing respectively the following constructs:

module Foo_query = <:sql< SELECT * FROM foo >>

and

<:sql< QUERY Foo_query IS SELECT * FROM foo>>

I am willing to write the necessary glue code in pa_o.ml. Will the caml team accept the patch?

Alex

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Notes
(0003996) ertai (developer) 2007-03-29 16:42	This is will be available in OCaml 3.10.

Issue History
Date Modified	Username	Field	Change
2005-12-16 10:46	alexbaretta	New Issue
2006-01-05 14:03	doligez	Assigned To	=> mauny
2006-01-05 14:03	doligez	Severity	major => feature
2006-01-05 14:03	doligez	Status	new => acknowledged
2007-03-29 16:42	ertai	Note Added: 0003996
2007-03-29 16:43	ertai	Assigned To	mauny => ertai
2007-03-29 16:43	ertai	Status	acknowledged => resolved
2007-03-29 16:43	ertai	Resolution	open => fixed
2007-03-29 16:43	ertai	Fixed in Version	=> 3.10+dev
2009-03-31 13:07	xleroy	Status	resolved => closed
2009-03-31 13:07	xleroy	Fixed in Version	3.10+dev => 3.10.0