diff --git a/src/pebblisp.h b/src/pebblisp.h
index bb3c753..51e00c5 100644
--- a/src/pebblisp.h
+++ b/src/pebblisp.h
@@ -10,17 +10,21 @@
 
 #define array_length(_array) (sizeof(_array) / sizeof((_array)[0]))
 
-#define fn(_name, _docs, ...) static const char * const _name ## Doc = _docs; \
+#define UNPACK(...) __VA_ARGS__
+
+#define fn(_name, _docs, ...) \
+static const char * const _name ## Doc = _docs; \
 static const char * const _name ## Tests[] = {__VA_ARGS__}; \
 static_assert(array_length(_name ## Tests) % 2 == 0, "Array of test strings must have exactly one expected result for each test."); \
 Object _name(Object* params, int length, struct Environment* env)
 
-#define fnn(_name, _symbol, _docs, ...)\
-static const char * const _name ## Doc = _docs;\
-static const char * const _name ## Symbol = _symbol;\
-static const char * const _name ## Tests[] = {__VA_ARGS__}; \
-static_assert(array_length(_name ## Tests) % 2 == 0, "Array of test strings must have exactly one expected result for each test."); \
-Object _name(Object* params, int length, struct Environment* env)
+#define tfn(_name, _type, _docs, ...) \
+static const Type _name ## Type[] = UNPACK _type; \
+fn(_name, _docs, __VA_ARGS__)
+
+#define fnn(_name, _symbol, _docs, ...) \
+static const char * const _name ## Symbol = _symbol; \
+fn(_name, _docs, __VA_ARGS__)
 
 struct Slice {
     const char* text;
diff --git a/src/plfunc.h b/src/plfunc.h
index 5d98465..a9f6b4e 100644
--- a/src/plfunc.h
+++ b/src/plfunc.h
@@ -52,12 +52,12 @@ fn(prepend,
    "(pre (2 3) 1)", "( 1 2 3 )",
 );
 
-/// LIST => NUMBER
-fn(len,
-   "Returns the length of the given list, or a NOT_A_LIST error if the expression is not a list.",
-   "(len (2 3))", "2",
-   "(len ())", "0",
-   "(len \"string\")", "NOT_A_LIST",
+tfn(len,
+    ({ TYPE_LIST, TYPE_NUMBER }),
+    "Returns the length of the given list, or a NOT_A_LIST error if the expression is not a list.",
+    "(len (2 3))", "2",
+    "(len ())", "0",
+    "(len \"string\")", "NOT_A_LIST",
 );
 
 /// LIST, FUNCY, ANY => ANY