implement let

NEWS

@@ -49,6 +49,7 @@ Changes from 0.13.0
    [ Misc. Improvements ]
    * `read`, `read_str`, and `eval` are exposed and documented as top-level
      functions in the `hy` module
+   * Experimental `let` macro in `hy.contrib.walk`
 
 Changes from 0.12.1
 

docs/contrib/walk.rst

@@ -10,7 +10,7 @@ Functions
 .. _walk:
 
 walk
------
+----
 
 Usage: `(walk inner outer form)`
 
@@ -36,7 +36,7 @@ Example:
     97
 
 postwalk
----------
+--------
 
 .. _postwalk:
 
@@ -116,7 +116,7 @@ each sub-form, uses ``f`` 's return value in place of the original.
             HyInteger(7)])])])])
 
 prewalk
---------
+-------
 
 .. _prewalk:
 
@@ -194,3 +194,57 @@ each sub-form, uses ``f`` 's return value in place of the original.
           HyInteger(6),
           HyList([
             HyInteger(7)])])])])
+
+macroexpand-all
+---------------
+
+Usage: `(macroexpand-all form &optional module-name)`
+
+Recursively performs all possible macroexpansions in form, using the ``require`` context of ``module-name``.
+`macroexpand-all` assumes the calling module's context if unspecified.
+
+Macros
+======
+
+let
+---
+
+``let`` creates lexically-scoped names for local variables.
+A let-bound name ceases to refer to that local outside the ``let`` form.
+Arguments in nested functions and bindings in nested ``let`` forms can shadow these names.
+
+.. code-block:: hy
+
+    => (let [x 5]  ; creates a new local bound to name 'x
+    ...  (print x)
+    ...  (let [x 6]  ; new local and name binding that shadows 'x
+    ...    (print x))
+    ...  (print x))  ; 'x refers to the first local again
+    5
+    6
+    5
+
+Basic assignments (e.g. ``setv``, ``+=``) will update the local variable named by a let binding,
+when they assign to a let-bound name.
+
+But assignments via ``import`` are always hoisted to normal Python scope, and
+likewise, ``defclass`` will assign the class to the Python scope,
+even if it shares the name of a let binding.
+
+Use ``__import__`` and ``type`` (or whatever metaclass) instead,
+if you must avoid this hoisting.
+
+The ``let`` macro takes two parameters: a list defining *variables*
+and the *body* which gets executed. *variables* is a vector of
+variable and value pairs.
+
+``let`` executes the variable assignments one-by-one, in the order written.
+
+.. code-block:: hy
+
+    => (let [x 5
+    ...      y (+ x 1)]
+    ...  (print x y))
+    5 6
+
+It is an error to use a let-bound name in a ``global`` or ``nonlocal`` form.

hy/cmdline.py

@@ -125,7 +125,7 @@ def ast_callback(main_ast, expr_ast):
 
 
 @macro("koan")
-def koan_macro():
+def koan_macro(ETname):
     return HyExpression([HySymbol('print'),
                          HyString("""
   Ummon asked the head monk, "What sutra are you lecturing on?"
@@ -143,7 +143,7 @@ def koan_macro():
 
 
 @macro("ideas")
-def ideas_macro():
+def ideas_macro(ETname):
     return HyExpression([HySymbol('print'),
                          HyString(r"""
 

hy/compiler.py

@@ -831,6 +831,7 @@ def compile_raise_expression(self, expr):
     @builds("try")
     @checkargs(min=2)
     def compile_try_expression(self, expr):
+        expr = copy.deepcopy(expr)
         expr.pop(0)  # try
 
         # (try something…)
@@ -1125,6 +1126,7 @@ def compile_yield_expression(self, expr):
 
     @builds("import")
     def compile_import_expression(self, expr):
+        expr = copy.deepcopy(expr)
         def _compile_import(expr, module, names=None, importer=asty.Import):
             if not names:
                 names = [ast.alias(name=ast_str(module), asname=None)]
@@ -2005,14 +2007,15 @@ def rewire_init(expr):
         expressions.pop(0)  # class
 
         class_name = expressions.pop(0)
+        if not isinstance(class_name, HySymbol):
+            raise HyTypeError(class_name, "Class name must be a symbol.")
 
         bases_expr = []
         bases = Result()
         if expressions:
             base_list = expressions.pop(0)
             if not isinstance(base_list, HyList):
-                raise HyTypeError(expressions,
-                                  "Bases class must be a list")
+                raise HyTypeError(base_list, "Base classes must be a list.")
             bases_expr, bases, _ = self._compile_collect(base_list)
 
         body = Result()
@@ -2054,62 +2057,6 @@ def rewire_init(expr):
             bases=bases_expr,
             body=body.stmts)
 
-    def _compile_time_hack(self, expression):
-        """Compile-time hack: we want to get our new macro now
-        We must provide __name__ in the namespace to make the Python
-        compiler set the __module__ attribute of the macro function."""
-
-        hy.importer.hy_eval(copy.deepcopy(expression),
-                            compile_time_ns(self.module_name),
-                            self.module_name)
-
-        # We really want to have a `hy` import to get hy.macro in
-        ret = self.compile(expression)
-        ret.add_imports('hy', [None])
-        return ret
-
-    @builds("defmacro")
-    @checkargs(min=1)
-    def compile_macro(self, expression):
-        expression.pop(0)
-        name = expression.pop(0)
-        if not isinstance(name, HySymbol):
-            raise HyTypeError(name, ("received a `%s' instead of a symbol "
-                                     "for macro name" % type(name).__name__))
-        name = HyString(name).replace(name)
-        for kw in ("&kwonly", "&kwargs", "&key"):
-            if kw in expression[0]:
-                raise HyTypeError(name, "macros cannot use %s" % kw)
-        new_expression = HyExpression([
-            HyExpression([HySymbol("hy.macros.macro"), name]),
-            HyExpression([HySymbol("fn")] + expression),
-        ]).replace(expression)
-
-        ret = self._compile_time_hack(new_expression)
-
-        return ret
-
-    @builds("deftag")
-    @checkargs(min=2)
-    def compile_tag_macro(self, expression):
-        expression.pop(0)
-        name = expression.pop(0)
-        if name == ":" or name == "&":
-            raise NameError("%s can't be used as a tag macro name" % name)
-        if not isinstance(name, HySymbol) and not isinstance(name, HyString):
-            raise HyTypeError(name,
-                              ("received a `%s' instead of a symbol "
-                               "for tag macro name" % type(name).__name__))
-        name = HyString(name).replace(name)
-        new_expression = HyExpression([
-            HyExpression([HySymbol("hy.macros.tag"), name]),
-            HyExpression([HySymbol("fn")] + expression),
-        ]).replace(expression)
-
-        ret = self._compile_time_hack(new_expression)
-
-        return ret
-
     @builds("dispatch_tag_macro")
     @checkargs(exact=2)
     def compile_dispatch_tag_macro(self, expression):