moved let to a plugin with extended capabilities

@@ -1040,43 +1040,6 @@ class Expression (ArcAnnotation) :
     def __hash__ (self) :
         return hash(self._str.strip())
 
-def let (**args) :
-    """A dirty trick to allow side effects in expressions
-
-    Assignement takes place when method `bind` of the expression is
-    called. Assigned variables are then stored in the `Substitution`
-    passed to `bind`. This is useful in some cases when you want to
-    repeat a computation in several output arcs: you do it once in the
-    guard and then use the bounded variable on the output arcs.
-
-    >>> n = PetriNet('n')
-    >>> n.globals['let'] = let
-    >>> t = Transition('t', Expression('x is not None and let(foo=42)'))
-    >>> n.add_transition(t)
-    >>> n.add_place(Place('p', [dot]))
-    >>> n.add_input('p', 't', Variable('x'))
-    >>> t.modes() == [Substitution(x=dot, foo=42)]
-    True
-
-    Note in the example above that `let` is not known by default, you
-    have to import it explicitly in the global environment of your
-    `PetriNet` (line 2). This is intended so it is clear that you
-    known what you do.
-
-    @param args: a list of `name=value` to assign
-    @return: `True` if assignment could be made, `False` otherwise
-    @rtype: `bool`
-    @warning: use with care, sides effects are nasty tricks, you may
-        get unexpected results while playing with `let`
-    """
-    try :
-        __binding__ = inspect.stack()[1][0].f_locals["__binding__"]
-        for name, value in args.items() :
-            __binding__[name] = value
-    except :
-        return False
-    return True
-
 class MultiArc (ArcAnnotation) :
     """A collection of other annotations, allowing to consume or produce
     several tokens at once.

+"""A dirty trick to allow side effects in expressions
+
+Assignement takes place when method `bind` of the expression is
+called. Assigned variables are then stored in the `Substitution`
+passed to `bind`. This is useful in some cases when you want to
+repeat a computation in several output arcs: you do it once in the
+guard and then use the bounded variable on the output arcs.
+
+>>> import snakes.plugins
+>>> snakes.plugins.load('let', 'snakes.nets', 'snk')
+<module ...>
+>>> from snk import *
+>>> n = PetriNet('n')
+>>> t = Transition('t', Expression('x is not None and let("egg, spam = iter(str(foo))", foo=42, __raise__=True)'))
+>>> n.add_transition(t)
+>>> n.add_place(Place('p', [dot]))
+>>> n.add_input('p', 't', Variable('x'))
+>>> t.modes() == [Substitution(x=dot, foo=42, egg='4', spam='2')]
+True
+
+@param args: a list of `name=value` to assign
+@return: `True` if assignment could be made, `False` otherwise
+@rtype: `bool`
+@warning: use with care, sides effects are nasty tricks, you may
+  get unexpected results while playing with `let`
+"""
+
+import inspect
+import snakes.plugins
+
+from snakes.lang.python.parser import ast, parse
+from snakes.lang import unparse
+
+class DropLet (ast.NodeTransformer) :
+    def __init__ (self, names) :
+        ast.NodeTransformer.__init__(self)
+        self.names = set(names)
+        self.calls = []
+    def visit_Call (self, node) :
+        func = node.func
+        if func.__class__.__name__ == "Name" and func.id in self.names :
+            self.calls.append((func.id,
+                               node.args and node.args[0].s or None,
+                               [(k.arg, unparse(k.value))
+                                for k in node.keywords]))
+            return ast.Name(id="True")
+        return node
+
+class DropTrue (ast.NodeTransformer) :
+    def visit_BoolOp (self, node) :
+        if node.op.__class__.__name__ == "And" :
+            values = [self.visit(v) for v in node.values
+                      if v.__class__.__name__ != "Name" or v.id != "True"]
+            if values :
+                node.values[:] = values
+                return node
+            else :
+                return ast.Name(id="True")
+        else :
+            return self.visit(node)
+
+def unlet (expr, *names) :
+    if not names :
+        names = ["let"]
+    drop = DropLet(names)
+    new = DropTrue().visit(drop.visit(parse(expr)))
+    return unparse(new), drop.calls
+
+class MakeLet (object) :
+    def __init__ (self, globals) :
+        self.globals = globals
+    def match (self, match, binding) :
+        env = binding.dict()
+        env.update(iter(self.globals))
+        exec("", env)
+        old = set(env)
+        exec(match, env)
+        for name in set(env) - old :
+            binding[name] = env[name]
+    def __call__ (self, __match__=None, __raise__=False, **args) :
+        try :
+            __binding__ = inspect.stack()[1][0].f_locals["__binding__"]
+            for name, value in args.items() :
+                __binding__[name] = value
+            if __match__ :
+                self.match(__match__, __binding__)
+        except :
+            if __raise__ :
+                raise
+            return False
+        return True
+
+@snakes.plugins.plugin("snakes.nets")
+def extend (module) :
+    class PetriNet (module.PetriNet) :
+        def __init__ (self, name, **args) :
+            module.PetriNet.__init__(self, name, **args)
+            self.globals["let"] = MakeLet(self.globals)
+    return PetriNet, unlet