to decrease build time, sources should be in separate src/ folder

[jwaldmann]

The derive.cabal spec contains executable and library. When cabal install, (most) library files are recompiled. This wastes time.

The executable should depend on the library, then recompilation is avoided.
This only works when the sources are in a separate directory.

(I don't know why, and I think this is seriously underdocumented, but I know it from experience.)

It is a trivial change to make, I can do it.

[ndmitchell]

Go for it. Although given your other large pending change, I suggest you either wait til after that, or do this change first then merge into that one.

[ndmitchell]

I saw your changes and liked them, so merged them all over - so this is now fixed. Thanks!