feature: variable substitution

[zazedd]

Related to #12

This PR adds variable substitution to the %pcre extension.

Details

Allows defining variables with let%pcre, which could then be used inside a few patterns for substitution:

• (?&var) - Named substitution (doesn't capture)
• (?&name:var) - Named substitution with rename

Only 1 AST pass is needed.

If a user tries to define let%pcre var = (?N<var>), the ppx is smart enough to warn the user about having unbounded recursion.

Note

This updates the opam package to use ppxlib <= "0.35.0" only, as ppxlib.0.36.0 has breaking changes in the Parsetree AST.

[Drup]

If I understand correctly, this is similar to the call feature available for tyre ? IIRC, I tried to choose a fairly standard syntax, could you use the same one ?

[zazedd]

I tried to choose a fairly standard syntax, could you use the same one

Sure thing, I'll change the syntax!

[paurkedal]

Nice addition, one I recall this was on my wish-list if I had spent more time on it.

If a user tries to define let%pcre var = (?N<var>), the ppx is smart enough to warn the user about having unbounded recursion.

Shouldn't that, without rec qualification, shadow rather than recurse.

[zazedd]

Shouldn't that, without rec qualification, shadow rather than recurse.

You're right, I fixed the symptom and not the cause 😅.

---

The latest commit is an intermediate to a final version, there are still some improvements to be made.

I went back and looked at how ppx_tyre does it. The updated solution here uses Call and the syntax from ppx_tyre.

Important change: the AST is now translated into base Re calls, instead of into a string and then compiled using Re.Perl. This should make the change mentioned in #15 easier.

[paurkedal]

Important change: the AST is now translated into base Re calls, instead of into a string and then compiled using Re.Perl. This should make the #15 (comment) in #15 easier.

Nice! We should maybe merge this first to avoid conflicts. There seems to be a bug, though, involving group indexing:

$ OCAMLRUNPARAM=b dune runtest -p ppx_regexp
File "tests/dune", line 13, characters 9-24:
13 |   (names test_ppx_regexp test_ppx_regexp_unused)
              ^^^^^^^^^^^^^^^
(cd _build/default/tests && ./test_ppx_regexp.exe)
Fatal error: exception Not_found
Raised at Re__Group.offset in file "lib/group.ml", line 12, characters 12-27
Called from Re__Group.get in file "lib/group.ml", line 17, characters 15-25
Called from Test_ppx_regexp.test4 in file "tests/test_ppx_regexp.ml", lines 64-67, characters 12-20
Called from Test_ppx_regexp in file "tests/test_ppx_regexp.ml", line 94, characters 10-29

[zazedd]

Much thanks for catching this! Planning to update the branch this week and fix this issue.

[paurkedal]

In addition to the two corrections, can you add a test case to cover the three cases of new syntax?