Allow fn to have no body

Signed-off-by: Julien Danjou <julien@danjou.info>

hy/compiler.py

@@ -1027,7 +1027,7 @@ class HyASTCompiler(object):
             col_offset=expr.start_column)
 
     @builds("fn")
-    @checkargs(min=2)
+    @checkargs(min=1)
     def compile_fn_expression(self, expression):
         expression.pop(0)  # fn
 

tests/compilers/test_ast.py

@@ -300,6 +300,8 @@ def test_ast_anon_fns_basics():
     """ Ensure anon fns work. """
     code = hy_compile(tokenize("(fn (x) (* x x))")).body[0]
     assert type(code) == ast.FunctionDef
+    code = hy_compile(tokenize("(fn (x))")).body[0]
+    cant_compile("(fn)")
 
 
 def test_ast_non_decoratable():

tests/native_tests/language.hy

@@ -474,7 +474,9 @@
 (defn test-fn-return []
   "NATIVE: test function return"
   (setv fn-test ((fn [] (fn [] (+ 1 1)))))
-  (assert (= (fn-test) 2)))
+  (assert (= (fn-test) 2))
+  (setv fn-test (fn []))
+  (assert (= (fn-test) None)))
 
 
 (defn test-let []