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 normally 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. Haskell developers can use build tools, such as
cabal, which interface with Setup.hs and provide functionality on top
it.
Note that even though originally packages were meant to be free to
implement the Setup.hs interface however they like, this hasn't been
supported by build tools for a long time. In practice use of the Cabal
library in Setup.hs is mandatory.
Now, the first step of building a package in "The Haskell Cabal" is to call
Setup.hs's configure command:
$ runhaskell Setup.hs configure
When invoking Setup.hs the default behaviour of the Cabal library is to
first read the <pkg-name>.cabal configuration file to determine how the
project is structured and what dependencies it has. In the case of
configure it will then probe the system it's running on about:
- the list of available Haskell packages,
- system package dependencies (using pkg-config) and
- Haskell compiler type, version and supported language extensions (among other things).
Finally Haskell package dependency resolution is also run.
Cabal then writes all the gathered information on the concrete
configuration of the package into a file called setup-config. Subsequent
steps, such asrunhaskell Setup.hs build, will then read this state file
instead of <package-name>.cabal to avoid having to probe the system or
run dependency resolution again.
It is this file that cabal-helper is primarily concerned with reading and
presenting in a usable manner. Reading this file essentially means that all
the complicated work has already been done for us and we can get straight
to running the compiler.
So far so good. That's pretty much the end of the story for the traditional
cabal build commands but what about cabal new-build and Stack I hear
you asking?
Well, essentially both new-build and Stack simply build on top of the
traditional Setup.hs interface. So the setup-config file is still there
in all it's glory, we just have to deal with more than one of it since both
build-tools support multiple packages and Setup.hs only knows how to deal
with a single package at a time.
To support this cabal-helper has grown a representation of what a project is in it's API starting with the 1.0 series. We currently support both new-build and Stack. The API is designed to allow extending support to custom build systems such as GHC's but we have not done this yet.
In the API docs you will find frequent mentions of "the helper executable" so I'll explain what that is here because it is quite fundamental to how things work in the codebase.
The fundamental problem cabal-helper solves is the fact that in order to
access the data Cabal stores in the setup-config file we have to link
against lib:Cabal. However the binary format of this file is unstable and
there is no backwards compatibility mechanism in the library. So to read a
setup-config file produced by a certain version of Cabal we have to link
against exactly that version.
Not only that but usally the cabal commandline tool controls which Cabal
library version is used, so we really just have to deal with whatever we
get.
To solve this problem the cabal-helper library builds a small executable at
runtime who's only purpose is to link against lib:Cabal, read the
contents of setup-config and present the data there in a Cabal version
independent format for consumption by the cabal-helper library.
Recently some work was merged into cabal to have Setup.hs to do this
natively (haskell/cabal#5954), we're planning to
use this eventually to replace our "helper".
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