diff --git a/packages/codemirror-lang-kcl/src/kcl.grammar b/packages/codemirror-lang-kcl/src/kcl.grammar
index c8a6ad1aa..179d535be 100644
--- a/packages/codemirror-lang-kcl/src/kcl.grammar
+++ b/packages/codemirror-lang-kcl/src/kcl.grammar
@@ -17,7 +17,7 @@
 
 statement[@isGroup=Statement] {
   ImportStatement { kw<"import"> ImportItems ImportFrom String } |
-  FunctionDeclaration { kw<"export">? kw<"fn"> VariableDefinition Equals ParamList Arrow Body } |
+  FunctionDeclaration { kw<"export">? kw<"fn"> VariableDefinition Equals? ParamList Arrow? Body } |
   VariableDeclaration { kw<"export">? (kw<"var"> | kw<"let"> | kw<"const">)? VariableDefinition Equals expression } |
   ReturnStatement { kw<"return"> expression } |
   ExpressionStatement { expression }
diff --git a/packages/codemirror-lang-kcl/test/fn.txt b/packages/codemirror-lang-kcl/test/fn.txt
new file mode 100644
index 000000000..b77946cc4
--- /dev/null
+++ b/packages/codemirror-lang-kcl/test/fn.txt
@@ -0,0 +1,60 @@
+# full
+
+fn two = () => {
+  return 2
+}
+
+==>
+
+Program(FunctionDeclaration(fn,
+                            VariableDefinition,
+                            Equals,
+                            ParamList,
+                            Arrow,
+                            Body(ReturnStatement(return,
+                                                 Number))))
+
+# = is optional
+
+fn one () => {
+  return 1
+}
+
+==>
+
+Program(FunctionDeclaration(fn,
+                            VariableDefinition,
+                            ParamList,
+                            Arrow,
+                            Body(ReturnStatement(return,
+                                                 Number))))
+
+# => is optional
+
+fn one = () {
+  return 1
+}
+
+==>
+
+Program(FunctionDeclaration(fn,
+                            VariableDefinition,
+                            Equals,
+                            ParamList,
+                            Body(ReturnStatement(return,
+                                                 Number))))
+
+# terse
+
+fn two() {
+  return 2
+}
+
+==>
+
+Program(FunctionDeclaration(fn,
+                            VariableDefinition,
+                            ParamList,
+                            Body(ReturnStatement(return,
+                                                 Number))))
+