ppx_here

A ppx rewriter that defines extension nodes whose values are source positions.

Syntax

ppx_here rewrites the extension [%here] in expressions, by replacing it by a value of type Source_code_position.t (i.e. Lexing.position) corresponding to the current position. It respects line number directives.

For instance:

let _ =
  print_endline [%here].Lexing.pos_fname

becomes:

let _ =
  print_endline
    {
      Lexing.pos_fname = ppx/ppx_here/test/test.ml";
      pos_lnum = 2;
      pos_cnum = 26;
      pos_bol = 8;
    }.Lexing.pos_fname

The extension let%with_pos emits a companion value with the source position of a let binding.

For instance:

let%with_pos foo = ...

defines the value foo__pos equal to the position of foo.

See test/test_ppx_here_expander.ml for more examples.

Usage

This is normally used so exceptions can contain better positions. An example is Core.Option.value_exn, which takes an optional position so that if you have a stack trace, you can get still the origin of the exception.

It can also be used in cases where stack traces are useless (for instance in monads with a complicated control flow).

Command line flag

If the flag -dirname <dir> is given, relative filenames are made relative to <dir>. <dir> can be a relative path.

<dir> can be chosen as the path from the root of the repository to the directory of the source, to make filenames unique within the repository (which avoids ambiguities as there can be many files called server.ml, common.ml or config.ml).