The stack setup command

stack setup [GHC_VERSION] [--[no-]reinstall] [--ghc-bindist URL]
            [--ghcjs-boot-options GHCJS_BOOT] [--[no-]ghcjs-boot-clean]

stack setup attempts to install a version of GHC - by default, the version required by the project and only if it is not already available to Stack. For example:

stack setup
stack will use a sandboxed GHC it installed
For more information on paths, see 'stack path' and 'stack exec env'
To use this GHC and packages outside of a project, consider using:
stack ghc, stack ghci, stack runghc, or stack exec

Alternatively, the version of GHC to be installed can be specified as an argument. For example stack setup 9.0.2.

Set the --reinstall flag (disabled by default) to attempt to install the version of GHC regardless of whether it is already available to Stack.

The --ghc-bindist <url> option can be used to specify the URL of the GHC to be downloaded and installed. This option requires the use of the --ghc-variant option specifying a custom GHC variant.

If Stack is configured not to install GHC (install-ghc: false or passing the --no-install-ghc flag) then stack setup will warn that the flag and the command are inconsistent and take no action.