Better error messages for fn/defn w/o arglists

When (fn) or (defn) does not get an arglist as first/second parameter,
emit a more descriptive error message, rather than an ugly traceback.

Fixes #716.

Reported-by: Joakim Tall
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

hy/compiler.py

@@ -1893,6 +1893,9 @@ class HyASTCompiler(object):
         called_as = expression.pop(0)
 
         arglist = expression.pop(0)
+        if not isinstance(arglist, HyList):
+            raise HyTypeError(expression,
+                              "First argument to (fn) must be a list")
         ret, args, defaults, stararg, kwargs = self._parse_lambda_list(arglist)
 
         if PY34:

hy/core/bootstrap.hy

@@ -44,6 +44,8 @@
   "define a function `name` with signature `lambda-list` and body `body`"
   (if (not (= (type name) HySymbol))
     (macro-error name "defn/defun takes a name as first argument"))
+  (if (not (isinstance lambda-list HyList))
+    (macro-error name "defn/defun takes a parameter list as second argument"))
   `(setv ~name (fn ~lambda-list ~@body)))
 
 

tests/native_tests/language.hy

@@ -52,6 +52,15 @@
   (try (eval '(defn lambda [] (print "hello")))
        (catch [e [TypeError]] (assert (in "Can't assign to a builtin" (str e))))))
 
+(defn test-fn-corner-cases []
+  "NATIVE: tests that fn/defn handles corner cases gracefully"
+  (try (eval '(fn "foo"))
+       (catch [e [Exception]] (assert (in "to (fn) must be a list"
+                                          (str e)))))
+  (try (eval '(defn foo "foo"))
+       (catch [e [Exception]]
+         (assert (in "takes a parameter list as second" (str e))))))
+
 (defn test-for-loop []
   "NATIVE: test for loops"
   (setv count 0)