[Haskell-cafe] 1-to-many preprocessor in stack/Cabal builds

Andrey Sverdlichenko blaze at ruddy.ru
Mon Mar 7 06:53:56 UTC 2016


I'm writing a package which uses custom preprocessor for creating some of
the source files. It takes one IDL file as input and writes several .hs
modules, depending on IDL contents. I need to compile this modules with a
few normal ones and link them as a library, preferably with generated
modules exposed. It doesn't look like a complex task and never was in with
make/etc, but I run into some problems now.

Obviously, cabal supports 1-to-1 preprocessors to create one Haskell module
from one source file (alex, happy, etc), but I can't find anything for
1-to-many preprocessors. So, I decided to use hookedPrograms and hooks and
do it myself. The call to preprocessor in the postConf hook worked just
fine and created source files and module list.

Cabal pre-* hooks seems to be right place to read this module list and use
it, but are presenting two problems: can only modify other-modules, not
exposed-modules, which is tolerable, and there is no access to build
configuration. The latter is fatal, I need to know build dir to read module
list. So I have to change action hooks.
As the additional annoyance, there is absolutely no way to modify
configuration once and for all, I have to override every single hook with
the same "read module list and change config" wrapper.

So, I have buildHook, copyHook and regHook replaced with wrappers which
change PackageDescription parameter and call original function. Library is
built and installed, but somewhere between build and copy actions there is
still a message "Warning: modules not listed in foo.cabal for library
component (add to other-modules)". I can't identify where it comes from.

I'm using stack 1.0.4 to build this library, but all the problems seems to
be cabal-related. Here is minimal example of what I ended up with:

Now I wonder if I missed an easier way to handle this code generation. If
not, at least how to get rid of warning about missing modules between build
and copy actions?
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