HyperThunk

Sometimes you want your build to do some additional work after compiling. For example, in my Erlang implementation of hamcrest, I dynamically generate the main header file after compiling to ensure that all the functions exported from the hamcrest_matchers module are imported when the header gets included and to ensure that wrapper macros for eunit are also generated for each of them.

Typically you’d just have a makefile deal with this, maybe like so:

compile:
./rebar compile; escript header_gen


But that kind of breaks other people’s builds if they want to use rebar themselves. The problem is that rebar doesn’t yet provide a mechanism for overriding the build/compile command for dependencies (at all, let alone individually) and thus if your build requires a makefile to be run then your consumers build has to incur the same kind of complexity. Fortunately there is a work-around. Rebar provides a compile_post_script hook which you can specify in your rebar.config file. You specify a shell command and rebar will run this hook immediately after compilation. Unfortunately it also runs it when the compile command is applied to all your dependencies (again this is something which isn’t controllable yet) which typically will break because your shell command is not relevant to these apps. I spent a little time trying to do something clever with erl --eval '...' but in the end that didn’t play nicely at all. I then tried to encode the whole thing in a compound shell command like if [ -f hrlgen ]; then escript hrl; fi but then discovered that rebar_utils:sh/1 (which runs the shell command) wraps the supplied command text in an exec system call (which obviously isn’t going to work for a complex if..then expression). In the end, it was a comedic reptile that came to the rescue:

{compile_post_script,
"env ERL_LIBS=deps python -c \"import os; os.path.isfile('hrlgen') and os.system('escript hrlgen') or None\""}.