Building GHC: The package database

zw3rk
3 min readOct 30, 2017

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Over the next few posts, I will explore how the Glasgow Haskell Compilers build system works. I will try not to go into too much detail how make based build system, and focus on the high level process and how to arrive at the final compiler.

As we will have to deal with package databases, let us take a look at packages and package databases today.

Packages

While we usually build packages for use with GHC via cabal, the packages GHC knows about are those registered in the known (to GHC) package database(s). The global package database usually resides next to the ghc binary and is called package.conf.d.

To GHC, a package consists of a package specification and a collection of interface files .hi together with one or more archives .a of objects, .so / .dylib / .dll shared libraries and one merged .o object file for use with GHCi in a folder named $name-$version. (The $name-$version is also known as the installed package id or unit-id). The package specification will contain the package name and version, exposed modules, libraries, dependencies, and other package metadata. The details can be found in GHC User’s Guide.

Thus, if you don’t have or don’t want to use cabal, you could build your own package from scratch. Just collect the interface files .hi and the libary in a folder and write the package specification.

Package Database

The package database, is the package.conf.d folder as mentioned above. It contains all the package specification as $name-$version.conf. These contain the information about the library, search paths, where to find the archives and (shared) objects and more.

In addition to the .conf files, it also contains a cache file. If you simply drop your package specification into the package.conf.d folder, the package database won’t see it. For this we need the ghc-pkg tool to rebuild the cache.

The ghc-pkg Tool

To make dealing with package.conf.d easier, GHC comes with the ghc-pkg tool, which provides a rich set of commands to interact with the package database. For example, to update the cache after dropping a specification into the database by hand, one would run

$ ghc-pkg recache

to rebuild the package.cache file.

If you want to restrict the packages available to your GHC, you could for example create your own empty database, and copy over packages from a different database using:

$ ghc-pkg --expand-pkgroot describe base | \
ghc-pkg --global-package-db package.conf.d register -

This will read base from the global package db, dump the content of it’s specification to stdout and register that package in the database located at package.conf.d by reading the specification from stdin (via -).

The --expand-pkgroot is necessary because this will only copy the specification, it will not copy the contents (interface files, libraries, …). If the package used package database relative paths, we want to expand them, prior to registering the package in a different package database.

You will need to use some --force if you intent to register a package without it’s dependencies in a new package database. Because GHC can also deal with a stack of package databases, and you can control this stack either via GHC_PACKAGE_PATH or the -[no-]global-package-db, -[no-]user-package-db and package-db flags, your packages in one database can have dependencies on packages in another database.

Summary

GHC knows about packages through package database(s). Packages do not necessarily need to be created with cabal, we can create our own package from scatch if we really wanted to. (Though using cabal or the Cabal library makes dealing with it much easier).

ghc-pkg is the command line interface tool to interact with package database.

Next time we will look at a few more of the tools used in building GHC, to have the necessary foundation to build GHC.

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