The purpose of the cabal-helper library is to give Haskell development tools
access to the same environment which build tools such as
cabal and stack
provide to the compiler.
In the Haskell ecosystem the most widely used build system is
the Cabal library, not to be
confused with the cabal build tool which is usually refered to by it's
package-name: cabal-install to disambiguate.
All contemporary meta build tools such as cabal and stack as well as some
custom Haskell build systems use the Cabal library as their
foundation. For example the Glasgow Haskell Compiler's bespoke GNU Make based
build system also utilises the Cabal library at its core.
We capitalize on this fact by using build information Cabal writes to disk as
the common denominator between all Haskell build tools. This allows us to
easily support a variety of build tools without incuring significant
additional complexity.
Essentially all Haskell packages implement
"The Haskell Cabal" (pdf)
packaging specification. The Cabal library and cabal-install build tool
are named after this specification. Yes we really love confusing naming in
Haskell land.
The specification revolves around this Setup.hs script file you might have
seen before. Basically the idea is a Haskell source package consists of, at the
very least, a Setup.hs file, which is a Haskell program that provides a well
defined command-line interface for configuring, building and installing
it. Developers can use build tools, such as cabal, which interface with
Setup.hs and provide functionality on top it.
Even though packages are in principle free to implement the Setup.hs interface
however they like in practice everybody just imports the Cabal library in
Setup.hs which provides a default implementation of this interface. The
Cabal library then in turn uses the <pkg-name>.cabal file you've likely seen
before to determine precisely what to do.
Since use of Cabal the library is pretty much a given we're just going to be
talking about this case from here on out.
So the first step of building a package in "The Haskell Cabal" is to call:
$ ./Setup.hs configure
The Cabal library will then go off and probe a bunch of stuff about the system
it's running on, such as the list of available Haskell packages, compiler type
and version, paths to build tools among other things. Using this information
Cabal then writes the concrete configuration of the package into a file called
setup-config. Subsequent steps (./Setup.hs build...) will then read this
file to avoid probing the system again.
It is this file that cabal-helper is primarily concerned with reading and
presenting in a usable format.
If you have any problems, suggestions, comments swing by #ghc-mod (web client) on Freenode. If you're reporting a bug please also create an issue here so we have a way to contact you if you don't have time to stay.
Do hang around for a while if no one answers and repeat your question if you still haven't gotten any answer after a day or so. You're most likely to get an answer during the day in GMT+1.