diff -r 3ed262d72468 -r b979e3044d8e lib/scripts/ghc
--- a/lib/scripts/ghc	Wed Mar 20 21:15:20 2019 +0100
+++ b/lib/scripts/ghc	Wed Mar 20 21:54:08 2019 +0100
@@ -4,7 +4,7 @@
 #
 # Invoke ghc via "stack".
 
-if [ -d "$ISABELLE_STACK_ROOT" ]; then
+if [ -e "$ISABELLE_STACK_ROOT/config.yaml" ]; then
   isabelle_stack ghc -- "$@"
 else
   echo "Cannot execute ghc: missing Isabelle GHC setup" >&2