Substitute Install Paths?

[japendergrass]

Is there a mechanism to substitute install paths (e.g., $prefix) into a generated file during build?

I expected to be able to accomplish this using a rule with like

(rule 
     ((targets (project.conf))
       (deps    (project.conf.in))
       (action (run sed -e /\${prefix}/%{prefix}/g %{deps} > %{target})))

But %{prefix} (or bindir, libdir, etc) is not listed as a substituted variable in the documentation.

[ghost]

Not currently, dune doesn't know the value of these parameters. Related ticket: #1185

[ghost]

And this one as well: #680

[rgrinberg]

Well $ dune install works without rebuilding the artifacts. To substitute installation directories, we'd need to configure these variables and then run the build. This would be a fairly significant change to dune, do you have some motivation for why you'd like this variables to be substitutable?

[kit-ty-kate]

I stubbled upon the same issue about a lack of %{prefix} (or similar) variables in mirage/mirage-solo5#61 which currently uses opam config var lib when compiling the package to know where a required directory (which is not a library) is.

[ghost]

@kit-ty-kate looking at the mirage-solo PR makes me wonder: shouldn't it be opam's job to extend the PKG_CONFIG_PATH variable?

Regarding the original question, during the build we don't know the installation prefix, so we'd need to introduce a configure step to make that possible, which as @rgrinberg pointed out is a significant change. Perhaps we should add that one day though.

On a related note, @bobot is working on a general solution to the problem of data needed at runtime: #3104. That seems worse looking at.

[ejgallego]

@bobot can we close this?

[rgrinberg]

I think we can. @bobot can re-open if he disagrees.