Add commonSubtype()

1 files changed, 14 insertions, 0 deletions

diff --git a/src/main/java/org/shedlang/compiler/types/Types.java b/src/main/java/org/shedlang/compiler/types/Types.java
index 9ab5c634..ab48f77e 100644
--- a/src/main/java/org/shedlang/compiler/types/Types.java
+++ b/src/main/java/org/shedlang/compiler/types/Types.java
@@ -89,4 +89,18 @@ public class Types {
             return TOP;
         }
     }
+
+    public static Type commonSubtype(List<Type> supertypes) {
+        var uniqueSubtypes = supertypes.stream()
+            .filter(subtype -> subtype != Types.TOP)
+            .collect(Collectors.toSet());
+
+        if (uniqueSubtypes.isEmpty()) {
+            return TOP;
+        } else if (uniqueSubtypes.size() == 1) {
+            return uniqueSubtypes.iterator().next();
+        } else {
+            return BOTTOM;
+        }
+    }
 }