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Insomniac Typed Programming in Python

From: andrew cooke <andrew@...>

Date: Tue, 12 Apr 2011 02:51:49 -0300

Just got the following code to run (it will be in the next version of pytyp).
Note how there is dispatch by type defined in the TypedProperty subclass
(instances of which occur in both Tree and Node) and how functors are
naturally supported.

  def tree_functor(leaf_type):

      tree_type = Delayed()

      class TreeProperty(TypedProperty):

	  def __init__(self, value):
	      super(TreeProperty, self).__init__(value, tree_type)

	  def size(value, spec):
	      return spec.on(value,
			     none=lambda _: 0,
			     leaf=lambda l: 1,
			     node=lambda n: len(n))

	  def set_add(value, spec, leaf:leaf_type):
	      return spec.on(value,
			     none=lambda _: leaf,
			     leaf=lambda l: Node(l).add(leaf),
			     node=lambda n: n.add(leaf))

      class Node(Typed):

	  value = TypedProperty(leaf_type)
	  left = TreeProperty(None)
	  right = TreeProperty(None)

	  def __init__(self, value:leaf_type):
	      super(Node, self).__init__()
	      self.value = value

	  def add(self, value:leaf_type):
	      if value < self.value:
	      return self

	  def __len__(self):
	      return 1 + self.p.left.size() + self.p.right.size()

      class Tree(Typed):

	  root = TreeProperty(None)

	  def add(self, value:leaf_type):

	  def __len__(self):
	      return self.p.root.size()

      tree_type += Alt(none=None, leaf=leaf_type, node=Node)

      return Tree

  # create a functor that supports trees of integers
  Tree = tree_functor(int)
  t1 = Tree()
      assert False, 'Expected error'
  except TypeError:
  for n in [8,3,6,5,9,2]:
  assert len(t1) == 7, len(t1)

A Little More Detail

From: andrew cooke <andrew@...>

Date: Tue, 12 Apr 2011 03:18:55 -0300

To expand on the above, the Tree class contains a tree_type value called
root.  From the definition of tree_type we can see that root is either None
(empty tree), leaf_type (a single value), or Node.

Similarly, Node contains both a value (leaf_type) and two sub-nodes of

Because the add and size methods are defined on the typed parameter they can
be used (to extend the tree and count the number of nodes) from within both

And although the logic for those methods is quite complex (because of the
different types involved), the dispatch by type gives a clear defintion.

To explain displatch by types, here is the size function:

        def size(value, spec):
            return spec.on(value,
                           none=lambda _: 0,
                           leaf=lambda l: 1,
                           node=lambda n: len(n))

and the tree_type:

        Alt(none=None, leaf=leaf_type, node=Node)

The arguments in the "on" call match the names for the alternate types that
may be present.  When size() is invoked, the value is matched against the spec
and the correct expression evaluated.  So, for example, if value is None, the
result is 0.

The only other important detail is how TypedProperties are instantiated within
a class.  They are defined at the class level, but installed into each
instance via the Typed suprrclass.  When installed they are visible in two
ways.  First, as a type-verified value.  Second, under the ".p" attribute,
with the methods defined in the TypedParameter class (value and spec
automatically substituted).

So code like

    class Tree(Typed):
        root = TreeProperty(None)

supports the creation of tree instances for which self.root returns a value of
type tree_type (and which is verified when assigned to).  Those instances also
have self.p.root which has size() and set_add() methods (the "set_" prefix
means that the result of the method is used to update the attribute).


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