package ppx_here

title: ppx_here parent: ../README.md

A ppx rewriter that defines an extension node whose value is its source position.

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

Usage

This is normally used so exceptions can contain better positions. An example is Core_kernel.Std.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 filename given to the compiler (or overriden by line directive) is a relative path, then ppx_here rewrites it into an absolute path, unless a flag -dirname root is given, in which case relative filenames are made relative to that root filename instead. root can be a relative path.

The goal of this behavior is to

• avoid ambiguity: there are many files called server.ml, common.ml or config.ml in a tree

• when -dirname is passed, avoid being overly specific by giving a path that only exists on your machine, by allowing the build system to specify where the source file is, relative to the root of the project