lang: core: Simplify list and map lookup default functions

lang/core/lookup_default.go

@@ -108,16 +108,30 @@ func (obj *LookupDefaultFunc) Build(typ *types.Type) (*types.Type, error) {
 		return nil, fmt.Errorf("first arg must have a type")
 	}
 
+	name := ""
 	if tListOrMap.Kind == types.KindList {
-		obj.fn = &ListLookupDefaultFunc{} // set it
-		return obj.fn.Build(typ)
+		name = ListLookupDefaultFuncName
 	}
 	if tListOrMap.Kind == types.KindMap {
-		obj.fn = &MapLookupDefaultFunc{} // set it
-		return obj.fn.Build(typ)
+		name = MapLookupDefaultFuncName
+	}
+	if name == "" {
+		return nil, fmt.Errorf("we must lookup from either a list or a map")
 	}
 
-	return nil, fmt.Errorf("we must lookup from either a list or a map")
+	f, err := funcs.Lookup(name)
+	if err != nil {
+		// programming error
+		return nil, err
+	}
+	bf, ok := f.(interfaces.BuildableFunc)
+	if !ok {
+		// programming error
+		return nil, fmt.Errorf("not a BuildableFunc")
+	}
+	obj.fn = bf
+
+	return obj.fn.Build(typ)
 }
 
 // Validate tells us if the input struct takes a valid form.