compiler: Make maps and lists able to look up their params

As a neat syntactic sugar, it's very neat if maps and lists are able
to work as if they were functions, and look up their arguments.

This implements just that, by translating (map key) to (index map key)
internally, and (list idx) to (index list idx).

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

hy/compiler/ast27.py

@@ -163,6 +163,10 @@ class AST27Converter(object):
             "for": self._ast_for,
             "kwapply": self._ast_kwapply,
         }
+        self.special_types = {
+            HYMap: self._ast_fn_index,
+            HYList: self._ast_fn_index,
+        }
         self.in_fn = False
 
     def _ast_index(self, node):
@@ -176,6 +180,13 @@ class AST27Converter(object):
             ast.Index(value=self.render(tar), ctx=ast.Load()),
             ast.Load())
 
+    def _ast_fn_index(self, node):
+        i = node.get_invocation()
+        cmd = ["index"]
+        cmd.append(i['function'])
+        cmd.extend(i['args'])
+        return self.render_expression(HYExpression(cmd))
+
     def _ast_dot(self, node):
         inv = node.get_invocation()
         args = inv['args']
@@ -370,6 +381,9 @@ class AST27Converter(object):
 
         inv = node.get_invocation()
 
+        if type(inv['function']) in self.special_types:
+            return self.special_types[type(inv['function'])](node)
+        
         if inv['function'] in self.native_cases:
             return self.native_cases[inv['function']](node)
 

tests/lang/colls.hy

@@ -0,0 +1,5 @@
+(defn test-map-index []
+  ({"foo" "bar"} "foo"))
+
+(defn test-list-index []
+  (["first" "second"] 1))

tests/lang/test_types_as_fn.py

@@ -0,0 +1,7 @@
+import tests.lang.colls
+
+def test_map_index():
+    assert tests.lang.colls.test_map_index() == "bar"
+
+def test_list_index():
+    assert tests.lang.colls.test_list_index() == "second"