Unable to find dependency from parent directory when building exe

[samharrison7]

Description

I am trying to build an executable from a project with the following structure:

src/
  a/
    a_mod.f90
  b_mod.f90
  main.f90

In the code, a_mod uses b_mod, and main uses a_mod. When I build just as a library, it works as expected. However, when I specify to build an executable from main.f90, the build fails with the error <ERROR>*cmd_build*:target error:Unable to find source for module dependency: "b_mod" used by "./src/a/a_mod.f90".

The issue seems to occur when a module depends on a module that is in a parent directory (i.e. a_mod depending on b_mod from its parent directory) and only when build an executable.

I have placed a minimum viable example of the project structure that causes this issue in this repo: https://github.com/samharrison7/fpm-test-exe-issues

Expected Behaviour

I expected fpm to build the executable from this project structure.

Version of fpm

0.6.0, alpha

Platform and Architecture

Ubuntu 20.04

Additional Information

No response

[zoziha]

Thanks for your feedback @samharrison7 , it seems that this problem does exist.

In fpm.f90, fpm first parses the main program source files, and labels these source files with FPM_SCOPE_APP, FPM_SCOPE_EXAMPLE, FPM_SCOPE_TEST, then parses the module source files, and labels FPM_SCOPE_LIB, FPM_SCOPE_DEP, and only FPM_SCOPE_LIB, FPM_SCOPE_DEP labelled source files will be queried if provide the module names (see find_module_dependency).

A feasible solution is to parse the dependency source files first, and then parse those main programs. (See #737)

[samharrison7]

Thanks for the quick response and fix @zoziha! I've just tested your patch on my code (including a much more complicated version of the minimum example I posted) and it compiles the exe without problems now.

[LKedward]

Hi @samharrison7 - this behaviour is currently part of the intended design of fpm. Modules that are not in the main src directory are not part of the library and have a use scope limited to the folder they are in and subfolders. Apologies that this is not documented anywhere for users. It is documented for developers here.

[samharrison7]

Ah I did wonder whether it was a design thing. That seems a logical restriction and is probably a good motivation for me to do some src directory restructuring of my projects!

[samharrison7]

Though I do wonder whether having a flag to override this restriction would be a useful addition for legacy projects where directory restructuring isn't possible/desirable?

[LKedward]

Though I do wonder whether having a flag to override this restriction would be a useful addition for legacy projects where directory restructuring isn't possible/desirable?

Yes, this is a good point, though it'll have to be an option in the manifest file instead of a flag.

[zoziha]

@LKedward ，if I'm not mistaken, #737 doesn't seem to change the original intention of the design you mentioned.

1. Library sources (FPM_SCOPE_LIB) may only use modules also with library scope. This includes library modules from dependencies.
2. Executable sources (FPM_SCOPE_APP,FPM_SCOPE_TEST) may use library modules (including dependencies) as well as any modules corresponding to source files in the same directory or a subdirectory of the executable source file.

src/
    a/
        a_mod.f90
    b_mod.f90
    main.f90
---
# fpm.toml
[[executable]]
name = "main"
source-dir = "src"
main = "main.f90"

The above demo can be compiled normally in the #737 patch because src defaults to the library file path and a_mod.f90 obeys the rule 1 and can access the parent folder (src), both a_mod.f90 and b_mod.f90 belong to the same FPM_SCOPE_LIB.

app/
    a/
        a_mod.f90
    b_mod.f90
    main.f90
---
# fpm.toml
[[executable]]
name = "main"
source-dir = "app"
main = "main.f90"

The above demo cannot be compiled normally in the #737 patch because the app defaults to the executable program file path. Both a_mod.f90 and b_mod.f90 belong to FPM_SCOPE_APP, and an error will appear:

<ERROR>*cmd_build*:target error:Unable to find source for module dependency: "b_mod" used by "./app/a/a_mod.f90"

The reason is that a_mod.f90 obeys the rule 2 and cannot access the parent folder.

(PS. These 2 rules are really worth documenting in the fpm-doc documentation after we discuss here.)

[LKedward]

Thanks for explaining @zoziha, that makes sense to me now.

(PS. These 2 rules are really worth documenting in the fpm-doc documentation after we discuss here.)

Agreed!