Libraries requiring linking

[ivan-pi]

I'm working an interface to the FLANN library for nearest neighbor searches: https://github.com/ivan-pi/fortran-flann/
I haven't put the manifest in yet, but was just wondering about the current status of build dependencies. Say my project must be linked to a system-installed FLANN library with the flag -lflann. If I put the code below in my manifest file,

[build]
link = ["flann"]

will the linkage dependency get propagated forward to any fpm project that uses my interface like shown below?

[dependencies]
fortran-flann = { git = "https://github.com/ivan-pi/fortran-flann" }

I've seen in Cargo that crates are able to communicate some meta-data for such cases, see here.

[awvwgk]

Yes, this is how it is supposed to work. The link dependencies are collected from the complete dependency tree for the current project.

Note that linking is only supported in Fortran fpm, but not in the Haskell version (https://github.com/fortran-lang/fpm/issues/288).

[ivan-pi]

Great!

If the user doesn't have the library installing, I'm assuming the build will break with a linker message like /usr/bin/ld: cannot find -lflann.

Have there been any plans to have a more "gentle" break down? What I mean is like in CMake and the find_package command. If the requirements to build a package are not satisfied, CMake will issue some kind of error. For newcomers who potentially only interact with fpm and not the compiler or linker, the message as given above might appear very cryptic.

[awvwgk]

Compiler errors are currently escalated to an error stop rather directly, so no gentle break down here yet.

Native library support is difficult, usually pkg-config files and CMake package files can be used to find projects in non-standard locations. Something we definitely need at some point. But beyond that, you are usually plain out of luck.

[ivan-pi]

I think supporting pkg-config should be done as a new fpm package (not necessarily within fortran-lang). At least this is way it is done in the rust community: https://crates.io/crates/pkg-config

I am not close enough to these tools to say if the same can be said for CMake, or does it require closer access to the fpm internals. From a quick glance at the rust cmake package, in their case it is only used to build external C/C++ dependencies. What I think we want fpm to do in #69 is totally different, in that we want to convert an fpm package into a functional CMake target.