Copyright Digital Asset 2018-2019.
The GHC API allows you to use the GHC compiler as a library, so you can parse, analyze and compile Haskell code. The GHC API comes preinstalled with GHC, and is tied to that GHC version - if you are using GHC 8.6.3, you get version 8.6.3 of the API, and can't change it. The
ghc-lib package solves that problem, letting you mix and match versions of the GHC compiler and GHC API. Why might you want that?
- Imagine you are writing a tool to work with several versions of the GHC compiler. The GHC API changes significantly between each version, so doing this would require writing a lot of C preprocessor code to support it. An alternative is to use one version of
ghc-libwhich works across multiple versions of GHC.
- Imagine you are modifying the GHC API or want features from GHC HEAD. With
ghc-libyou can depend on the revised GHC API, without upgrading the compiler used to build everything, speeding up iteration.
ghc-lib provides the full GHC API, it doesn't contain a runtime system, nor does it create a package database. That means you can't run code produced by
ghc-lib (no runtime), and compiling off-the-shelf code is very hard (no package database containing the
base library). What you can do:
- Parse Haskell code, making
ghc-liba potential replacement for
haskell-src-exts. See the demo
mini-hlintin this repo;
- Compile Haskell code as far as GHC's Core language, which includes renaming and type checking. See the demo
mini-compilein this repo, and the carefully tailored file it compiles.
There are some downsides to
- The lack of runtime means you can't run code, which includes running code at compile time, e.g.
ghc-libisn't tied to any specific GHC versions, it can only read package databases and
.hifiles for one particular version of GHC. That means your existing package database probably can't be consumed by
ghc-lib(unless you happen to perfectly match the GHC version, in which case you could just have used the GHC API), and it doesn't ship with a package database so you'd have to painfully build your own.
- Compilation times for
ghc-libare not small, taking approximately 5 minutes on our CI machines.
ghc-lib is available on Hackage, and can be used like any normal package, e.g.
cabal install ghc-lib. Since
ghc-lib conflicts perfectly with the GHC API and
template-haskell, the package is hidden by default : use the language extension
PackageImports to do
import "ghc-lib" GHC. There are two release streams within the
- Version 8.8.1 will be the version of
ghc-libproduced against the released GHC 8.8.1, once it comes out;
- Version 0.20190204 is the version of
ghc-libusing GHC HEAD on the date 2019-02-04.
ghc-lib Hackage package is licensed under the BSD-3-Clause license, just like GHC itself. This repo, including the examples and the script that generates
ghc-lib, are licensed under the BSD-3-Clause OR Apache-2.0 license.
ghc-lib by taking a checkout of GHC, and combining the
ghc package with the various dependencies it is tightly tied to (e.g.
template-haskell) in one new
ghc-lib.cabal. That new package depends on a few generated outputs (which we build using the GHC build system) and some Cmm files (which we hack around by editing the Haskell source, because Cabal support for Cmm files is lacking). The
ghc-lib-gen directory contains a script that puts all the pieces together. Because GHC itself is capable of being bootstrapped with older GHC versions (its Stage0 build), the generated
ghc-lib also compiles with multiple GHC versions.
ghc-lib you'll need clones of this repository and the GHC repository. In a bash shell, build with the following commands.
git clone email@example.com:digital-asset/ghc-lib.git cd ghc-lib git clone git://git.haskell.org/ghc.git --recursive cabal run ghc cd ghc cabal install
ghc-lib is known to work on all of MacOS, Linux and Windows. A distribution produced with
cabal sdist on Linux/MacOS builds on Windows, but a
cabal sdist produced on Windows does not build on MacOS/Linux.
ghc-lib (notes for maintainers)
First prepare with:
cd ghc-lib git clone git://git.haskell.org/ghc.git --recursive cabal run ghc cd ghc
ghc-lib.cabal to fix the version number (e.g. 0.20190204)