# 5. How to use Cabal in Windows

This document describes how to use Cabal in a Windows system. See the Further reading section for some other references that might provide some more explanations. For a TL;DR, jump to the Complete configuration.

## 5.1. Install the Haskell environment

Haskell development on Windows makes use of the MSYS2 tools.

The recommended way of setting up a Haskell environment in Windows is by using GHCup. Follow the steps outlined in its webpage to install at least GHC and Cabal. GHCup will install its own MSYS2 system in your computer unless told not to do so: refer to its documentation for more information.

Note

Stack is another tool you can use to set up a Haskell environment on Windows. Stack can be installed on its own or via GHCup. See Stack’s webpage and/or GHCup’s section on Stack integration, in particular the Windows related subsection.

## 5.2. MSYS2 environments and packages

A particular environment has to be chosen when using MSYS2. By default GHCup will use `MINGW64`. You can learn more about the different environments in the MSYS2 documentation.

GHCs before 9.4.1 are shipped with a minimal set of packages based on the `MINGW64` environment, and GHC 9.4.1 and newer are shipped with a minimal set of packages based on the `CLANG64` environment. It is in general advisable to work inside the same environment as your GHC uses, but (with some exceptions) it shouldn’t matter whether environments are mixed. Stay warned that it can sometimes lead to undecipherable errors.

We will refer to the chosen environment as `<environment>` through this documentation.

Third-party libraries and tools can be installed using the `pacman` package manager on the MSYS2 installation (see). If MSYS2 was installed via GHCup, check GHCup’s documentation on how to call `pacman`. Note that installing a package `mingw-w64-<environment>-x86_64-<pkg>` will install it in the `<msys-dir>\<environment>` tree of directories, and might not be visible if working on a different environment than `<environment>`. In general, it is advisable to install only packages for the environment that was chosen above.

Apart from these environments, there is the `msys` environment which is based on Cygwin. Some tools only exist for this environment. Tools from this environment are callable when working in any other environment. It is in general not possible to link to libraries installed in the `msys` environment.

## 5.3. Ensure that Cabal can call the tools it needs

Cabal sometimes needs to call tools that do not come with Windows (such as `make` or even `git`). The MSYS2 project makes many of them available on Windows. The directories where those are located need to be made visible in the `PATH` when executing `cabal`. For that, Cabal provides the `extra-prog-path` configuration option. Your global configuration should include this option:

    extra-prog-path: <msys-dir>\<environment>\bin
                     <msys-dir>\usr\bin

Where `<msys-dir>` points to the location of your MSYS2 installation. If MSYS2 was installed via GHCup, refer to GHCup’s documentation on the default location of this directory. If MSYS2 was installed system-wide this is usually `C:\msys64`.

Note

Unless told otherwise, the GHCup bootstrap script already adds these directories to extra-prog-path by default.

## 5.4. Ensure that Cabal can use system libraries

When installing a third party package its libraries and header files will (usually) be placed in `<msys-dir>\<environment>\{lib,include}` respectively. These directories need to be specified in the `extra-lib-dirs` and `extra-include-dirs` respectively. Your global configuration should include these options:

    extra-include-dirs: <msys-dir>\<environment>\include
    extra-lib-dirs: <msys-dir>\<environment>\lib

Note

Unless told otherwise, the GHCup bootstrap script already adds these directories to extra-include-dirs and extra-lib-dirs by default.

Warning

GHCs older than 9.4.1 will crash if a recent `mingw-w64-<environment>-x86_64-crt-git` is installed for whichever `<environment>` and these directories are set globally .

Effectively this means that if you have installed `mingw-w64-<environment>-x86_64-crt-git` (which you probably have if you are using `clang` in the `CLANG64` environment or `gcc` in the `UCRT64` or `MINGW64` environments outside of Haskell, as this package is part of the `mingw-w64-<environment>-x86_64-toolchain` meta-packages) and are using a GHC older than 9.4.1, you cannot simply depend on system libraries by adding these paths to the global config, and instead you will have to go through some other method to depend on those libraries like `pkgconfig-depends`.

## 5.5. Ensure that Cabal can call Haskell tools

Haskell tools are located in two places:

- `<ghcup-dir>\bin` for standard Haskell tools such as GHC, Cabal, Haddock, `hsc2hs`…

- The `installdir` that Cabal is configured with for user-installed Haskell tools.

For Cabal to be able to invoke these tools, those directories need to be made visible in the `PATH`. Your global configuration should include these options:

    installdir: <installdir>
    extra-prog-path: ...
                     <ghcup-dir>\bin
                     <installdir>

Note

Unless told otherwise, the GHCup bootstrap script already adds these directories to extra-prog-path by default.

## 5.6. Complete configuration

The complete global configuration should finally look like this:

    installdir: <installdir>
    extra-include-dirs: <msys-dir>\<environment>\include
    extra-lib-dirs: <msys-dir>\<environment>\lib
    extra-prog-path: <ghcup-dir>\bin
                     <installdir>
                     <msys-dir>\<environment>\bin
                     <msys-dir>\usr\bin

Note

Unless told otherwise, the GHCup bootstrap script already sets this configuration file to the right values by default.

## 5.7. Further reading

- MSYS2 homepage: https://www.msys2.org

- MinGW-W64 homepage: https://www.mingw-w64.org/

- Setting up Windows to build GHC: https://gitlab.haskell.org/ghc/ghc/-/wikis/building/preparation/windows

- Some definitions and useful tools: https://gitlab.haskell.org/ghc/ghc/-/wikis/surviving-windows

### 5.7.1. Outdated links

These links are outdated but still useful to understand the overall picture:

- GHC’s wiki about the Windows platform (outdated, GHC now uses MSYS2): https://gitlab.haskell.org/ghc/ghc/-/wikis/building/platforms/windows

- The Windows toolchain (outdated, GHC now uses the `CLANG64` environment): https://gitlab.haskell.org/ghc/ghc/-/wikis/working-conventions/windows-toolchain

- Haskell Wiki on Windows (outdated, it talks about MSYS and old tools such as the Haskell platform): https://wiki.haskell.org/Windows