On Unix-like operating systems and Windows, Stack's installation procedure can be fully customised by placing a sh shell script (a 'hook') in the Stack root directory at hooks/ghc-install.sh. On Unix-like operating systems, the script file must be made executable. The script is run by the sh application (which is provided by MSYS2 on Windows).
The script must return an exit code of 0 and the standard output must be the absolute path to the GHC binary that was installed. Otherwise Stack will ignore the script and possibly fall back to its own installation procedure.
When system-ghc: true, the script is not run. That is because the two mechanisms reflect distinct concepts, namely:
system-ghc: true causes Stack to search the PATH for a version of GHC; and
hooks/ghc-install.sh causes Stack to execute a script that is intended to send to standard output a path to a version of GHC. The path in question may or may not be in the PATH. The script may also do other things, including installation.
When install-ghc: false, the script is still run. That allows you to ensure that only your script will install GHC and Stack will not default to its own installation logic, even when the script fails.
The following environment variables are always available to the script:
HOOK_GHC_TYPE = "bindist" | "git" | "ghcjs"For "bindist", additional variables are:
HOOK_GHC_VERSION = <ver>For "git", additional variables are:
HOOK_GHC_COMMIT = <commit>HOOK_GHC_FLAVOR = <flavor>For "ghcjs", additional variables are:
HOOK_GHC_VERSION = <ver>HOOK_GHCJS_VERSION = <ver>An example script is:
#!/bin/sh
set -eu
case $HOOK_GHC_TYPE in
bindist)
# install GHC here, not printing to stdout, e.g.:
# command install $HOOK_GHC_VERSION >/dev/null
;;
git)
>&2 echo "Hook does not support installing from source"
exit 1
;;
*)
>&2 echo "Unsupported GHC installation type: $HOOK_GHC_TYPE"
exit 2
;;
esac
echo "location/to/ghc/executable"
If the following script is installed by GHCup, GHCup makes use of it, so that if Stack needs a version of GHC, GHCup takes over obtaining and installing that version:
#!/bin/sh
set -eu
case $HOOK_GHC_TYPE in
bindist)
ghcdir=$(ghcup whereis --directory ghc "$HOOK_GHC_VERSION" || ghcup run --ghc "$HOOK_GHC_VERSION" --install) || exit 3
printf "%s/ghc" "${ghcdir}"
;;
git)
# TODO: should be somewhat possible
>&2 echo "Hook does not support installing from source"
exit 1
;;
*)
>&2 echo "Unsupported GHC installation type: $HOOK_GHC_TYPE"
exit 2
;;
esac
--file-watch post-processing¶
On Unix-like operating systems and Windows, Stack's build --file-watch post-processing can be fully customised by specifying an executable or a sh shell script (a 'hook') using the file-watch-hook non-project specific configuration option. On Unix-like operating systems, the script file must be made executable. A script is run by the sh application (which is provided by MSYS2 on Windows).
The following environment variables are always available to the executable or script:
HOOK_FW_RESULT (Equal to "" if the build did not fail. Equal to the result of displayException e, if exception e thown during the build.)An example script is:
#!/bin/sh
set -eu
if [ -z "$HOOK_FW_RESULT" ]; then
echo "Success! Waiting for next file change."
else
echo "Build failed with exception:"
echo $HOOK_FW_RESULT
fi
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