There are two cases when you may need haskell-packages: if you are writing a Haskell compiler (or any tool that processes Haskell source code, such as a static analyzer), or if you want to integrate with an existing Haskell compiler that uses haskell-packages.
Writing a compiler
If you are writing a compiler, you typically want to integrate it with Cabal, to be able to build ordinary Haskell packages.
If you go the hard way, this involves:
Parsing command line parameters. Sounds easy — just take a list of files to compile. In reality you also need to handle package ids and package dbs, CPP options (
-DFOO=1), language extension flags (
To integrate with Cabal, you also need to tell it the list of installed packages, supported languages and extensions etc.
Actual integration with Cabal means understanding how Cabal works and hard-coding support for your compiler. And then getting it accepted and waiting for the next Cabal release.
You may pretend that you are GHC or other compiler that is already supported by Cabal. It might work, but often it won't, for various reasons. Also, GHC's command line protocol is quite complex.
Package management. You need to implement a package db mechanism, which would let you to keep track of installed packages. Then you'd have to implement a
ghc-pkg-like tool to manage those databases, for both Cabal and your users.
Or, you can simply use this library!
It already has command line options parsing, Cabal support, and package
management. All you need to do is to provide the function to do actual
compilation and tell a couple of other things about your compiler. See
Distribution.HaskellSuite.Compiler module for details.
Using other compilers
Some compilers produce artifacts that you may want to use. A good example is the
hs-gen-iface compiler from haskell-names which generates an interface for each
module — the set of all named entities (functions, types, classes) that are
exported by that module. This information can be then used either by other
compilers, or by tools like IDEs.
Assuming the compiler uses haskell-packages and exports its database type, it's
very easy to use its artifacts. See
package resolution and
Distribution.HaskellSuite.Modules for module
It doesn't matter what Cabal version you use together with haskell-packages, but if you want to invoke a haskell-packages compiler from Cabal (see Usage), you need to build our fork of Cabal. Eventually it will be merged into Cabal.
To compile a Haskell package using a haskell-packages tool:
cabal install --haskell-suite -w $TOOL
$TOOL may be either a full path to the compiler, or just an executable
name if it's in