Additions to extra-source-files do not trigger rebuild

[andreasabel]

If you change a file listed in extra-source-files, cabal rebuilds the project, but not if you add a file there.

I discovered this when playing with the embedDir function of the file-embed package. It uses template haskell to quote into a Haskell source all files contained in a directory, so when you add a file to this directory, you want a rebuild.
I thought that listing all these files in extra-source-files would do the trick. When adding a new file to the directory, I would add it to extra-source-files, and cabal would be doing a rebuild.
But while cabal correctly triggers a rebuild if I modify any of the files in extra-source-files, it does not seem to care about additions to the list.

I first thought only Stack has this problem, but Cabal does so as well. For reference, here is the Stack companion issue:

• file-embed: Additions to extra-source-files don't trigger rebuild of module commercialhaskell/stack#6689

To reproduce, play with this little project:
embed-datadir-0.1.tar.gz

• do a cabal run
• change data/README.md and again do a cabal run and observe that changes did propagate
• add a file data/Bar.md, also to extra-source-files, and try a cabal run; observe that no rebuild takes place defying expectation

[mpilgrem]

I would characterise this experience slightly differently:

• Cabal (and Stack) do rebuild when a new file is added to extra-source-files; but
• in this use case (use of package file-embed) there is nothing in that to trigger the recompilation of a module that has already been built once (unlike when the content of an existing file changes, and file-embed's use of qAddDependentFile does its job, causing the relevant module to be recompiled).

For most users, having a rebuild also recompile everything (just in case) would likely be suboptimal.

[andreasabel]

Ok, I guess then I was on a wild goose chase.