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Expand Up @@ -10,112 +10,159 @@ is an extension for VS Code that is popular with Haskell coders.
The 'Haskell' extension can be used with Stack but there are some things to be
aware of, set out below.

## Haskell Language Server

The VS Code extension makes use of the Haskell Language Server (HLS). To work,
HLS has to be built with the same version of GHC that it will support. That is,
a version of HLS is required for each version of GHC in use. It is possible that
the most recent versions of GHC are not supported by HLS.

By default, the VS Code extension uses tools that are in the PATH. However, the
extension's settings (under 'Haskell: Manage HLS') allow a user to specify
that the extension should use a separate application,
[GHCup](https://www.haskell.org/ghcup/), to download and install the versions of
HLS that it needs. GHCup can download and install things other than HLS,
including GHC, MSYS2 (on Windows), Cabal (a tool for building Haskell code), and
Stack itself. GHCup can also update itself. On Windows, GHCup has the capability
of using the Stack-supplied MSYS2 rather than installing a duplicate copy. Cabal
(the tool), like Stack, depends on the Cabal (the library). Cabal (the tool),
unlike Stack, does not have the capability to automatically install necessary
versions of GHC, and (as well as supporting the extension) GHCup fills a
important gap for users of the Cabal tool.

If the VS Code extension is set not to use GHCup, its user needs to ensure that
each version of HLS that the extension needs is on the PATH.

For the most part, the versions of HLS provided by GHCup are built with the same
versions of GHC that Stack downloads from its default `setup-info` dictionary
(see the [`setup-info`](../configure/yaml/non-project.md#setup-info) non-project
specific configuration option documentation). Stack's default is to mirror the
'official' binary distributions published by GHC. However, in some cases, it is
possible that a GHCup-supplied and GHCup-selected HLS has been built with a
different binary distribution of GHC than the one which Stack has installed.

One example of that occurred with the release of GHC 9.0.2. For some Linux users
(Debian 9 and Fedora 27), the version of GHC 9.0.2 linked on GHC’s download
[web page](https://www.haskell.org/ghc/download_ghc_9_0_2.html) was broken. The
GHC developers made alternative ‘9.0.2a’ versions available. For a while, Stack
referred to the versions published by GHC on its download web page while the
GHCup-supplied versions of HLS were built using alternative versions. This
incompatibility led to problems. It was resolved by Stack's default also being
changed to refer to the '9.0.2a' versions. (Where Stack has already installed
GHC 9.0.2, it is necessary to delete GHC 9.0.2 from the `stack path --programs`
directory. This will cause Stack to reinstall the alternative version, when it
first needs GHC 9.0.2. Stack should distinguish what it builds with the
alternative from what it has built, and cached, with the original GHC 9.0.2.)

### GHCup and Stack >= 2.9.1

From Stack 2.9.1, GHCup can configure Stack so that if Stack needs a version of
GHC, GHCup takes over obtaining and installing that version. By default, the
script to install GHCup (which can be run more than once) configures Stack in
that way. For further information about how GHCup configures Stack, see the GHC
installation customisation
## GHCup

The separate [GHCup](https://www.haskell.org/ghcup/) project provides a tool
that can be used to install various tools useful for developing Haskell
projects. Those tools include:

* GHC,
* HLS (see further below),
* MSYS2 (on Windows; see the developing on Windows
[documentation](developing_on_windows.md)),
* Stack, and
* Cabal (the tool).

Stack itself can be used to install GHC and MSYS2. Stack can also be used to
upgrade, or downgrade, Stack.

GHCup can configure Stack so that if Stack needs a version of GHC, GHCup takes
over obtaining and installing that version. By default, the script to install
GHCup (which can be run more than once) configures Stack in that way. For
further information about how GHCup configures Stack, see the GHC installation
customisation
[documentation](../configure/customisation_scripts.md#ghc-installation-customisation).

### Workaround #1

If GHCup does not configure Stack in the way described above, one workaround is
to allow GHCup to install versions of GHC on the PATH and to cause Stack to use
those versions of GHC, by making use of Stack's `install-ghc` option (which
needs to be disabled) and Stack's `system-ghc` option (which needs to be
enabled). For further information about these options, see the
[`install-ghc`](../configure/yaml/non-project.md#install-ghc) documentation and
the [`system-ghc`](../configure/yaml/non-project.md#system-ghc) documentation.

For this workaround to work, each time that a snapshot is used that references a
different version of GHC, then GHCup must be used to install it (if GHCup has
not already installed that version). For example, to use `snapshot: lts-22.28`
(GHC 9.6.6), the command `ghcup install ghc 9.6.6` must have been used to
install GHC 9.6.6. That may be a minor inconvenience for some people, as one the
primary benefits of Stack over other tools for building Haskell code has been
that Stack automatically ensures that the necessary version of GHC is available.

### Workaround #2

If GHCup does not configure Stack, another partial workaround is to install
GHCup so that it is 'empty' except for the current version of HLS, allow the
VS Code extension to use GHCup to manage HLS requirements only, and to ignore
any messages (if any) from the extension on start-up that installation of GHC,
Cabal (the tool) and/or Stack are also necessary (they are not, if only Stack is
being used).

For this workaround to work, however, there can be no differences between the
version of GHC that the GHCup-supplied HLS was built with and the version that
Stack has installed. A slight inconvenience here is also the possibility of
false messages from the start-up that need to be ignored. In principle, those
messages can be disabled by
[setting the following](https://github.com/haskell/vscode-haskell#setting-a-specific-toolchain)
for the VS Code extension:

~~~yaml
"haskell.toolchain": {
"ghc": null,
"cabal": null,
"stack": null
}
~~~

To install a version of GHCup that is 'empty' is a little more complicated than
a default installation of GHCup.

On Unix-like operating systems, the following environment variable must be set
before GHCup's installation `sh` script is run: `BOOTSTRAP_HASKELL_MINIMAL`.

On Windows, the second argument to the PowerShell script must be set to
`$false`, namely:

Set-ExecutionPolicy Bypass -Scope Process -Force;[System.Net.ServicePointManager]::SecurityProtocol = [System.Net.ServicePointManager]::SecurityProtocol -bor 3072;Invoke-Command -ScriptBlock ([ScriptBlock]::Create((Invoke-WebRequest https://www.haskell.org/ghcup/sh/bootstrap-haskell.ps1 -UseBasicParsing))) -ArgumentList $true,$false
On Windows, GHCup has the capability of using the Stack-supplied MSYS2 rather
than installing a duplicate copy.

## HLS

The VS Code extension makes use of
[HLS](https://github.com/haskell/haskell-language-server) (the Haskell Language
Server). To work, HLS has to be built with the same version of GHC that it will
support. That is, a version of HLS is required for each version of GHC in use.
It is possible that the most recent versions of GHC are not supported by HLS.

VS Code with the 'Haskell' extension can be configured in a number of ways:

=== "GHCup manages HLS"

The VS Code extension's settings (under 'Haskell: Manage HLS') allow a
user to specify that the extension should use GHCup, to download and install
the versions of HLS that it needs.

If GHCup manages versions of HLS, versions of GHC can be managed in a number
of ways:

=== "Stack manages GHC using GHCup"

As identified above, GHCup can configure Stack to use GHCup to manage
versions of GHC.

=== "Stack manages GHC directly"

It is possible to install GHCup so that it is 'empty' except for the
current version of HLS, allow the VS Code extension to use GHCup to
manage HLS requirements only, and to disable messages from the extension
on start-up that installation of GHC, Cabal (the tool)
and/or Stack are also necessary (they are not, if only Stack is being
used).

To install a version of GHCup that is 'empty' is a little more
complicated than a default installation of GHCup.

=== "Unix-like"

The following environment variable must be set before GHCup's
installation `sh` script is run: `BOOTSTRAP_HASKELL_MINIMAL`.

=== "Windows"

The second argument to the PowerShell script must be set to
`$false`, namely:

Set-ExecutionPolicy Bypass -Scope Process -Force;[System.Net.ServicePointManager]::SecurityProtocol = [System.Net.ServicePointManager]::SecurityProtocol -bor 3072;Invoke-Command -ScriptBlock ([ScriptBlock]::Create((Invoke-WebRequest https://www.haskell.org/ghcup/sh/bootstrap-haskell.ps1 -UseBasicParsing))) -ArgumentList $true,$false

There is the possibility of false messages from the extension on
start-up that need to be ignored. Those messages can be disabled by
[setting the following](https://github.com/haskell/vscode-haskell#setting-a-specific-toolchain)
for the VS Code extension:

~~~yaml
"haskell.toolchain": {
"ghc": null,
"cabal": null,
"stack": null
}
~~~

There can be no differences between the version of GHC that the
GHCup-supplied HLS was built with and the version that Stack has
installed.

For the most part, the versions of HLS provided by GHCup are built with
the same versions of GHC that Stack downloads from its default
`setup-info` dictionary (see the
[`setup-info`](../configure/yaml/non-project.md#setup-info) non-project
specific configuration option documentation). Stack's default is to
mirror the 'official' binary distributions published by GHC.

However, in some cases, it is possible that a GHCup-supplied and
GHCup-selected HLS has been built with a different binary distribution
of GHC than the one which Stack has installed.

??? question "When have the GHCup- and Stack-supplied GHCs differed?"

An example occurred with the release of GHC 9.0.2. For some Linux
users (Debian 9 and Fedora 27), the version of GHC 9.0.2 linked on
GHC’s download
[web page](https://www.haskell.org/ghc/download_ghc_9_0_2.html) was
broken. The GHC developers made alternative ‘9.0.2a’ versions
available. For a while, Stack referred to the versions published by
GHC on its download web page while the GHCup-supplied versions of
HLS were built using alternative versions. This incompatibility led
to problems.

It was resolved by Stack's default also being changed to refer to
the '9.0.2a' versions. Where Stack has already installed GHC 9.0.2,
it is necessary to delete GHC 9.0.2 from the `stack path --programs`
directory. This will cause Stack to reinstall the alternative
version, when it first needs GHC 9.0.2. Stack should distinguish
what it builds with the alternative from what it has built, and
cached, with the original GHC 9.0.2.

=== "Stack uses a GHCup-supplied GHC"

GHCup is used to manage versions of GHC and Stack is configured to use
the version of GHC on the PATH.

That is, GHCup is used to install a version of GHC on the PATH. Stack is
configured to make use of that version, by making use of Stack's
`install-ghc` option (which needs to be disabled) and Stack's
`system-ghc` option (which needs to be enabled).

For further information about these options, see the
[`install-ghc`](../configure/yaml/non-project.md#install-ghc)
documentation and the
[`system-ghc`](../configure/yaml/non-project.md#system-ghc)
documentation.

Each time that a snapshot is used that references a different version of
GHC, then GHCup must be used to install it (if GHCup has not already
installed that version). For example, to use `snapshot: lts-22.28`
(GHC 9.6.6), the command `ghcup install ghc 9.6.6` must have been used
to install GHC 9.6.6. That may be a minor inconvenience for some people,
as one the primary benefits of Stack over other tools for building
Haskell code has been that Stack automatically ensures that the
necessary version of GHC is available.

=== "User manages HLS"

By default, the VS Code extension uses tools that are in the PATH.

If the VS Code extension is set not to use GHCup, its user needs to ensure
that each version of HLS that the extension needs is on the PATH.

### Cradle

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