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# Developing on Windows #

On Windows, Stack comes with an installation of [MSYS2](https://www.msys2.org/).
An environment of MSYS2 (by default, `CLANG64` on 64-bit Windows) will be used
by Stack to provide a Unix-like shell and environment for Stack. This may be
necessary for installing some Haskell packages, such as those which use
`configure` scripts, or if your project needs some additional tools during the
build phase.

No matter which terminal software you choose (Windows Terminal, Console Windows
Host, Command Prompt, PowerShell, Git bash or any other) you can use this
environment too by executing all programs through
`stack exec -- <program_name>`.

Executables and libraries can be installed with the MSYS2 package manager
`pacman`. All tools can be found in the [index](https://packages.msys2.org) to
MSYS2 packages. A [guide](https://www.msys2.org/docs/package-management/) to
package management with `pacman` is also available. `pacman` &mdash; like all
other tools in the [Stack environment](stack_environment.md) &mdash; should be
started with `stack exec -- pacman`. Help about `pacman` commands (operations)
can be obtained by `stack exec -- pacman --help`. Help about a specific `pacman`
operation can be obtained by using `--help` (or `-h`) with an operation. For
example, help about the operation `--sync` (or `-S`) can be obtained with
`stack exec -- pacman --sync --help` or, equivalently,
`stack exec -- pacman -Sh`.

Command `stack path --bin-path` to see the PATH in the
[Stack environment](stack_environment.md). If the relevant MSYS2 environment is
`CLANG64`, on Windows, it includes the `\clang64\bin`, `\usr\local\bin` and
`\usr\bin` directories of the Stack-supplied MSYS2. (It includes the
corresponding directory if the relevant MSYS2 environment is other than
`CLANG64`.) If your executable depends on files (for example, dynamic-link
libraries) in those directories and you want to run it outside of the Stack
environment, you will need to ensure copies of those files are on the PATH.

Command `stack path --extra-include-dirs` and `stack path --extra-library-dirs`
to see the extra directories searched for C header files or system libraries
files in the Stack environment. If the relevant MSYS2 environment is `CLANG64`,
on Windows, it includes the `\clang64\include` (include) and the `\clang64\lib`
and `\clang64\bin` directories (library) of the Stack-supplied MSYS2. (It
includes the corresponding directories if the relevant MSYS2 environment is
other than `CLANG64`.)

For further information about configuring the relevant MSYS2 environment, see
Stack's [`msys-environment`](../configure/yaml/non-project.md#msys-environment)
non-project specific configuration option documentation.

## Updating the Stack-supplied MSYS2 ##

The Stack-supplied MSYS2 can itself be updated with the Stack-supplied `pacman`.
See the MSYS2 guide [Updating MSYS2](https://www.msys2.org/docs/updating/). If
the Stack-supplied `pacman` has a version that is 5.0.1.6403 or greater (see
`stack exec -- pacman --version`) then the command to update is simply:

    stack exec -- pacman -Suy

This command may need to be run more than once, until everything is reported by
`pacman` as 'up to date' and 'nothing to do'.

## Setup.hs ##

`Setup.hs` is automatically run inside the Stack environment. So when you need
to launch another tool you do not need to prefix the command with
`stack exec --` within the custom `Setup.hs` file.

## Pacman packages to install for common Haskell packages ##

The following lists MSYS2 packages known to allow the installation of some
common Haskell packages on Windows. Feel free to submit additional entries via a
pull request.

*   For [text-icu](https://hackage.haskell.org/package/text-icu) install
    `mingw-w64-clang-x86_64-icu`.

*   For [zlib >= 0.7](https://hackage.haskell.org/package/zlib) the default
    Cabal flag `pkg-config` is `true` and requires executable `pkg-config` on
    the PATH. MSYS2 [defaults](https://www.msys2.org/docs/pkgconfig/) to
    [`pkgconf`](https://packages.msys2.org/package/pkgconf?repo=msys&variant=x86_64)
    as its `pkg-config` implementation. Installation:

        stack exec -- pacman -S pkgconf

    Alternatively, build with `--flag zlib:-pkg-config`.
