Raises TypeError on bad if statements

This relates to issue #6.

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

hy/compiler.py

@@ -152,8 +152,16 @@ class HyASTCompiler(object):
     @builds("if")
     def compile_if_expression(self, expr):
         expr.pop(0)
-        test = self.compile(expr.pop(0))
-        body = self._code_branch(self.compile(expr.pop(0)))
+        try:
+            test = expr.pop(0)
+        except IndexError:
+            raise TypeError("if expects at least 2 arguments, got 0")
+        test = self.compile(test)
+        try:
+            body = expr.pop(0)
+        except IndexError:
+            raise TypeError("if expects at least 2 arguments, got 1")
+        body = self._code_branch(self.compile(body))
         orel = []
         if len(expr) > 0:
             orel = self._code_branch(self.compile(expr.pop(0)))

tests/compilers/test_ast.py

@@ -43,6 +43,29 @@ def test_ast_bad_type():
         pass
 
 
+def test_ast_bad_if_0_arg():
+    "Make sure AST can't compile invalid if"
+    try:
+        hy_compile(tokenize("(if)"))
+        assert False
+    except TypeError:
+        pass
+
+
+def test_ast_bad_if_1_arg():
+    "Make sure AST can't compile invalid if"
+    try:
+        hy_compile(tokenize("(if foobar)"))
+        assert False
+    except TypeError:
+        pass
+
+
+def test_ast_valid_if():
+    "Make sure AST can't compile invalid if"
+    hy_compile(tokenize("(if foo bar)"))
+
+
 def test_ast_expression_basics():
     """ Ensure basic AST expression conversion works. """
     code = hy_compile(tokenize("(foo bar)")).body[0]