Have a variable to point to installed library paths

[samoht]

Related to #1185 but a bit simpler.

In a few places I would like to get the place of where the current library (or one of the already present libraries) is installed. To do this I have come up with this very inelegant solution, which seems to work fine:

(rule
  (target install-dir)
  (action (with-stdout-to install-dir
    (bash "echo $(dirname %{lib:<lib>:META})"))))

And now other rules could just (:include install-dir) when they want to refer to the path where <lib> is installed.

Is there any way to make it simpler? E.g. by expanding %{lib:<lib>} to the parent directory of %{lib:<lib>:META} or %{lib:<lib>:dune-package} ?

[ghost]

There is no way at the moment. We considered it in the past, however it is a bit dodgy as it provides an easy way for users to mess up their build. What is your use case?

[samoht]

I want my project to install the C flags that users should use when compiling C objects using that library.

For instance, I want <my-lib> to install a file named cflags that can be included easily:

(-isystem %{lib:<my-lib>} -L%{lib:<my-lib>})

And users will just have to write:

(foreign_library ... (flags (:include %{lib:<my-lib>:cflags}))

Note that %{lib:<my-lib>} coud either be part of cflags and be expended when it is used (but currently dune complains with Error: %{lib:..} isn't allowed in this position) or be expanded directly when installed the file (I'm doing that at the moment and it seems to work). Note also that (:include %{lib:<my-lib>:cflags}) doesn't work as you need to make a copy first.

[ghost]

I see. So you basically want to provide a set of flags as a library? I'm wondering if we shouldn't provide this high-level feature directly. After all, the compiler itself provide a way to distribute link flags with a library.

And also, I'm generally not excited by giving too many low-level features as it makes it very easy to mess things up :)

[samoht]

Installing C flags for a library would be very useful. In my use-case I need to use these to compile a C project, so storing them in the cma/cmxa is not very practical, but having an easier way to handle these would be very useful. Not that you usually 2 sets of flags, one for compile-time, one for link-time.

[ghost]

Alright, let's look into this then. @voodoos I'm assigning you this issue since this seems useful for Tarides