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Stack and Visual Studio Code

Visual Studio Code (VS Code) is a popular source code editor, and 'Haskell' 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, 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 build tool), 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 build 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 YAML configuration: setup-info). 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 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 documentation.

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 documentation and the system-ghc documentation.

For this workaround to work, each time that a resolver 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 resolver: lts-20.19 (GHC 9.2.7), the command ghcup install ghc 9.2.7 must have been used to install GHC 9.2.7. That may be a minor inconvenience for some people, as one the primary benefits of Stack over other Haskell build tools 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 for the VS Code extension:

"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 -UseBasicParsing))) -ArgumentList $true,$false


HLS may need a 'cradle' - an hie.yaml file - in the project's root directory in order to work well.

The gen-hie tool can help generate such a cradle.


It has been suggested that a project must have been successfully built before the VS code extension (and HLS) is first activated on the project, for HLS to work reliably.