Integrate stack better

hooks/stack/ghc-install.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+set -eu
+
+case $HOOK_GHC_TYPE in
+	bindist)
+		ghcup --offline whereis ghc $HOOK_GHC_VERSION || {
+			ghcup --cache install ghc $HOOK_GHC_VERSION && ghcup whereis ghc $HOOK_GHC_VERSION
+		}
+		;;
+	git)
+		>&2 echo "Hook doesn't support installing from source."
+		>&2 echo "Consider enabling stack GHC installs for this project, via:"
+		>&2 echo "    stack config set install-ghc true"
+		exit 1
+		;;
+	*)
+		>&2 echo "Unsupported GHC installation type: ${HOOK_GHC_TYPE}."
+		>&2 echo "Consider enabling stack GHC installs for this project, via:"
+		>&2 echo "    stack config set install-ghc true"
+		exit 2
+		;;
+esac