Module B0_srcs

Select source files.

FIXME. This will need a few more design rounds. Here are a few things:

• Review w.r.t. b0.
• We likely want combinators represenging sels and producing t and ways union ts (for `Fut users).
• The `Fut case should return a t.
• Support for watermaking should likely occur here.

This module provides a type to select source files for build units in b0 files. To support generated source files, selections can depend on the build.

In a nutshell the declaration:

let srcs =
  Fpath.[ `Dir (v "src-exe"); `Dir_rec (v "src"); `X (v "src/not.ml");
          `X (v "src/not")]

instructs to:

• Select all the files in directory src-exe.
• Select all the files in the file hierarchy rooted at directory src.
• Remove from the files found in directories the files whose paths segments are prefixed by src/not.ml and src/not.

Relative file paths are expressed relative to the build unit's scope directory.

The prefix relation for exclusions respects path segments boundaries. In the example any file whose path matches src/not.ml, src/not.ml/*, src/not or src/not/* is excluded from the selection. But for example src/not.c is not.

The relative order of directory selections and exclusions doesn't matter, the semantics is to select all the files via `Dir and `Dir_rec and then apply the exclusion `X on the resulting set. Exclusions affect only directory selections, not file `File and future `Future selections.

When a directory is selected via `Dir or `Dir_rec, all its files are, modulo exclusions. It is expected that build units themselves filter the final result by file extension or additional mechanisms. Consult the documentation of build units for more information.