# 2. Introduction to GHC¶

This is a guide to using the Glasgow Haskell Compiler (GHC): an interactive and batch compilation system for the Haskell 2010 language.

GHC has two main components: an interactive Haskell interpreter (also known as GHCi), described in Using GHCi, and a batch compiler, described throughout Using GHC. In fact, GHC consists of a single program which is just run with different options to provide either the interactive or the batch system.

The batch compiler can be used alongside GHCi: compiled modules can be loaded into an interactive session and used in the same way as interpreted code, and in fact when using GHCi most of the library code will be pre-compiled. This means you get the best of both worlds: fast pre-compiled library code, and fast compile turnaround for the parts of your program being actively developed.

GHC supports numerous language extensions, including concurrency, a foreign function interface, exceptions, type system extensions such as multi-parameter type classes, local universal and existential quantification, functional dependencies, scoped type variables and explicit unboxed types. These are all described in GHC Language Features.

GHC has a comprehensive optimiser, so when you want to Really Go For It (and you’ve got time to spare) GHC can produce pretty fast code. Alternatively, the default option is to compile as fast as possible while not making too much effort to optimise the generated code (although GHC probably isn’t what you’d describe as a fast compiler :-).

GHC’s profiling system supports “cost centre stacks”: a way of seeing the profile of a Haskell program in a call-graph like structure. See Profiling for more details.

GHC comes with a number of libraries. These are described in separate documentation.

## 2.1. Obtaining GHC¶

Alternatively, if you want to build GHC yourself, head on over to the GHC Building Guide to find out how to get the sources, and build it on your system. Note that GHC itself is written in Haskell, so you will still need to install GHC in order to build it.

## 2.2. Meta-information: Web sites, mailing lists, etc.¶

On the World-Wide Web, there are several URLs of likely interest:

We run the following mailing lists about GHC. We encourage you to join, as you feel is appropriate.

glasgow-haskell-users

This list is for GHC users to chat among themselves. If you have a specific question about GHC, please check the FAQ first.

Subscribers can post to the list by sending their message to glasgow-haskell-users@haskell.org. Further information can be found on the Mailman page.

ghc-devs

The GHC developers hang out here. If you are working with the GHC API or have a question about GHC’s implementation, feel free to chime in.

Subscribers can post to the list by sending their message to ghc-devs@haskell.org. Further information can be found on the Mailman page.

There are several other Haskell and GHC-related mailing lists served by www.haskell.org. Go to http://www.haskell.org/mailman/listinfo/ for the full list.

## 2.3. Reporting bugs in GHC¶

Glasgow Haskell is a changing system so there are sure to be bugs in it. If you find one, please see this wiki page for information on how to report it.

## 2.4. GHC version numbering policy¶

As of GHC version 6.8, we have adopted the following policy for numbering GHC versions:

Stable branches are numbered x.y, where ⟨y⟩ is even. Releases on the stable branch x.y are numbered x.y.z, where ⟨z⟩ (>= 1) is the patchlevel number. Patchlevels are bug-fix releases only, and never change the programmer interface to any system-supplied code. However, if you install a new patchlevel over an old one you will need to recompile any code that was compiled against the old libraries.

The value of __GLASGOW_HASKELL__ (see Options affecting the C pre-processor) for a major release x.y.z is the integer ⟨xyy⟩ (if ⟨y⟩ is a single digit, then a leading zero is added, so for example in version 6.8.2 of GHC we would have __GLASGOW_HASKELL__==608).

We may make snapshot releases of the current stable branch available for download, and the latest sources are available from the git repositories.

Stable snapshot releases are named x.y.z.YYYYMMDD. where YYYYMMDD is the date of the sources from which the snapshot was built, and x.y.z+1 is the next release to be made on that branch. For example, 6.8.1.20040225 would be a snapshot of the 6.8 branch during the development of 6.8.2.

We may make snapshot releases of the HEAD available for download, and the latest sources are available from the git repositories.

Unstable snapshot releases are named x.y.YYYYMMDD. where YYYYMMDD is the date of the sources from which the snapshot was built. For example, 6.7.20040225 would be a snapshot of the HEAD before the creation of the 6.8 branch.

The value of __GLASGOW_HASKELL__ for a snapshot release is the integer ⟨xyy⟩. You should never write any conditional code which tests for this value, however: since interfaces change on a day-to-day basis, and we don’t have finer granularity in the values of __GLASGOW_HASKELL__, you should only conditionally compile using predicates which test whether __GLASGOW_HASKELL__ is equal to, later than, or earlier than a given major release.

The version number of your copy of GHC can be found by invoking ghc with the --version flag (see Verbosity options).

The compiler version can be tested within compiled code with the MIN_VERSION_GLASGOW_HASKELL CPP macro (defined only when -XCPP is used). See Standard CPP macros for details.