diff -r a591de179931 -r a7a0115061ec lib/scripts/ghc
--- a/lib/scripts/ghc	Fri Mar 22 10:36:50 2019 +0100
+++ b/lib/scripts/ghc	Fri Mar 22 11:23:17 2019 +0100
@@ -4,7 +4,8 @@
 #
 # Invoke ghc via "stack".
 
-if [ -e "$ISABELLE_STACK_ROOT/config.yaml" ]; then
+if [ -f "$ISABELLE_STACK_ROOT/ISABELLE_GHC_EXE-$ISABELLE_PLATFORM_FAMILY" ]
+then
   isabelle_stack ghc -- "$@"
 else
   echo "Cannot execute ghc: missing Isabelle GHC setup" >&2