GHC Commentary: Libraries

All GHC build trees contain a set of libraries, called the Boot Packages. These are the libraries that GHC's source code imports. Obviously you need the boot packages to build GHC at all. The boot packages are those packages in the file packages that have a - in the "tag" column.

The repository structure of a GHC source tree is described in Repositories.

You can see exactly which versions of what packages GHC depends on by looking in compiler/ The versions of the boot packages (including the base library) associated with each GHC release are tabulated in GHC Boot Library Version History.

Building packages that GHC doesn't depend on

You can make the build system build extra packages, on which GHC doesn't strictly depend, by adding them to the $(TOP)/packages file, with an extra tag. Then set BUILD_EXTRA_PKGS=YES in your mk/ file.

It should be exceptional, but you can make the build system provide per-package compiler flags, by adding some definitions in $(TOP)/, just below the comment

# Per-package compiler flags
# If you want to add per-package compiler flags, this 
# is the place to do it.  Do it like this for package <pkg>
#   libraries/<pkg>_dist-boot_HC_OPTS += -Wwarn
#   libraries/<pkg>_dist-install_HC_OPTS += -Wwarn

Classifying boot packages

A boot package is, by definition, a package that can be built by GHC's build system.

Boot packages can be classified in four different ways:

  • Required vs optional
  • Wired-in vs independent
  • Zero-boot vs not zero-boot
  • Installed vs not installed

These distinctions are described in the following sub-sections.

Required or optional

Most boot packages required to build ghc-stage2, or one of the supporting utilities such as ghc-pkg, hsc2hs, etc.

However a few are optional, and are built only

  • To ensure that they do indeed build cleanly; they are stress tests of GHC. E.g. dph
  • Because they are used in regression tests

Coupling to GHC

An important classification of the boot packages is as follows:

  • Wired in packages are totally specific to GHC. See the list in compiler/main/Packages.lhs function findWiredInPackages, and c.f. Commentary/Compiler/Packages. At the moment these are:
    • ghc-prim
    • integer-gmp, integer-simple
    • base
    • template-haskell
    • dph
  • Independent packages are loosely coupled to GHC, and often maintained by others. Most boot packages are independent; e.g. containers, binary, haskeline and so on.

Independent libraries may have a master repository somewhere separate from the GHC repositories. Whenever we release GHC, we ensure that the installed boot libraries (i.e. that come with GHC) that are also independent are precisely sync'd with a particular released version of that library.

Zero-boot packages

Since GHC's source code imports the boot packages, even the bootstrap compiler must have the boot packages available. (Or, more precisely, all the types and values that are imported must be available from some package in the bootstrap compiler; the exact set of packages does not need to be identical.)

For the most part we simply assume that the bootstrap compiler already has the boot packages installed. The Zero-boot Packages are a set of packages for which this assumption does not hold. Two reasons dominate:

  • For certain fast-moving boot packages (notably Cabal), we don't want to rely on the user having installed a bang-up-to-date version of the package.
  • The only packages that we can "assume that the bootstrap compiler already has" are those packages that come with GHC itself; i.e. the installed boot packages. So non-installed boot packages are also zero-boot packages. Example: bin-package-db or hoopl.

So we begin the entire build process by installing the zero-boot packages in the bootstrap compiler. (This installation is purely local to the build tree.) This is done in by setting PACKAGES_STAGE0 to the list of zero-boot packages; indeed this is the only way in which zero-boot packages are identified in the build system.

As time goes on, a Zero-boot package may become an ordinary boot package, because the bootstrap compiler is expected to have (a sufficiently up to date) version of the package already. Remember that we support bootstrapping with two previous versions of GHC.

To find out which packages are currently zero-boot packages, do the following in a GHC build:

$ make show VALUE=BOOT_PKGS

Some Zero-boot packages are maintained by other people. In order to avoid GHC being exposed to day-by-day changes in these packages, we maintain a "lagging" Git repository for each that we occasionally sync with the master repository. We never push patches to lagging repository; rather we push to the master (in discussion with the package maintainer), and pull the patches into the lagging repo. The current Zero-boot packages of this kind are:

  • Cabal: we frequently update Cabal and GHC in sync
  • binary (renamed to ghc-binary in the 6.12 branch): required by bin-package-db.

Other Zero-boot packages are maintained by us. There is just one Git repo for each, the master. When we make a GHC release, we simultaneously tag and release each of these packages. They are:

  • hpc
  • extensible-exceptions: this is a shim that provides an API to older versions of GHC that is compatible with what the current base package now exports. So, unusually, extensible-exceptions is a zero-boot package, but not a boot package.
  • bin-package-db: a GHC-specific package that provides binary serialisation of the package database, use by ghc-pkg and GHC itself.


When we build a distribution of GHC, it includes at least some libraries, otherwise it would be utterly useless. Since GHC is part of the Haskell Platform, any library that is installed with GHC is necessarily part of the Haskell Platform, so we have to be a bit careful what we include.

Alas, since the ghc package (implementing the GHC API) is certainly an installed package, all the packages on which it depends must also be installed, and hence willy-nilly become part of the Haskell Platform. In practice that means that almost all the Boot Packages are installed. In some cases that is unfortunate. For example, we currently have a special version of the binary library, which we don't really expect Haskell users to use; in this case, we call it ghc-binary, and informally discourage its use.

Currently the Boot Packages that are not installed are haskeline, mtl, and terminfo; these are needed to build the GHC front-end, but not to build the ghc package.

QUESTION: where in the build system is the list of installed packages defined?

Boot packages dependencies

  • At the root of the hierarchy we have ghc-prim. As the name implies, this package contains the most primitive types and functions. It only contains a handful of modules, including GHC.Prim (which contains Int#, +#, etc) and GHC.Bool, containing the Bool datatype. See "WARNING: pattern matching" below.
  • Above ghc-prim are the packages
    • integer-gmp
    • integer-simple

The two have the same interface, and only one of the two is used. (When we want to be vague about which one, we call it integer-impl.) They provide a definition of the Integer type (on top of the C gmp library, or in plain Haskell, respectively). Which functionality is provided in ghc-prim is mostly driven by what functionality the integer-impl packages need. By default integer-gmp is used; to use integer-simple define INTEGER_LIBRARY=integer-simple in mk/

See "WARNING: pattern matching" below.

  • Next is the base package. This contains a large number of modules, many of which are in one big cyclic import knot, mostly due to the Exception type.
  • On top of base are a number of other, more specialised packages, whose purpose is generally clear from their name. If not, you can get more detail from the descriptions in their Cabal files. The up-to-date list of packages can be found in the file packages.

The haskell98, old-time, old-locale and random packages are mostly only needed for Haskell 98 support, although dph currently uses random too.

WARNING: Pattern matching in ghc-prim, integer-simple, and integer-gmp

Note that ghc-prim and integer-impl are below the dependency chain from Exception (in base), which means they must not raise generate code to raise an exception (it's not enough that this code will never run). One particularly subtle case of GHC exception-raising code is in the case of (complete!) pattern matches. Consider the unboxed form of Integers, which has the constructor S# or J#.

f (S# _) (S# _) = ...
f x (S# _) = ...
f (S# _) y = ...
f (J# _ _) (J# _ _) = ...

GHC will incorrectly generate core that pattern matches against the second argument twice, the second match being a partial one with (dead) exception raising code. When compiled with optimizations, the dead code is eliminated. However, this breaks with -O0, thus:

Loading package integer-simple ... linking ... 
ghc: /usr/local/ghc/ 
unknown symbol `base_ControlziExceptionziBase_patError_info'

The fix is to explicitly spell out the constructor in the second and third line, so that GHC does not generate calls to patError:

f (S# _) (S# _) = ...
f (J# _ _) (S# _) = ...
f (S# _) (J# _ _) = ...
f (J# _ _) (J# _ _) = ...

GHC-internal libraries (ghc-boot-*)

GHC ships with two few libraries which exist to share code between components of the compiler: ghc-boot and ghc-boot-th.

Previously there was one ghc-boot library to allow us to share types and functions between the ghc library, the ghci library, and the template-haskell library. This situation was suboptimal (see #12052) since it is important that template-haskell has a minimal set of dependencies (as it is depended upon by a large set of user code) yet ghc-boot has dependencies on binary and bytestring.

To reduce the transitive dependency set of template-haskell it was decided that ghc-boot would be split into two separate libraries. Those definitions required by template-haskell live in ghc-boot-th and everything else lives in ghc-boot. Modules provided by ghc-boot-th are re-exported by `ghc-boot.


The list of repository locations has moved to Repositories.

Last modified 16 months ago Last modified on May 20, 2016 5:32:39 PM

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