algebraic datatypes

[Marcin 'Qrczak' Kowalczyk]

Sat, 23 Jun 2001 10:32:46 GMT, Ronald Legere <legere at adelphia.net> pisze:

> Essentially,
> datatype Tree = Node Tree Tree | Leaf Int
> should translate into a class structure like this:

from __future__ import nested_scopes
import new, types

def data(con, con_args):
    if isinstance(con_args, types.StringType): con_args = (con_args,)
    def __init__(self, *args, **kwargs):
        i = 0
        for arg in con_args:
            if kwargs.has_key(arg):
                self.__dict__[arg] = kwargs[arg]
                del kwargs[arg]
            else:
                try:
                    self.__dict__[arg] = args[i]
                except IndexError:
                    raise 'Too few arguments of %s provided' % con
                i += 1
        if i < len(args):
            raise 'Too many arguments of %s provided' % con
        if kwargs:
            raise 'Unknown keyword arguments of %s' % con
    globals()[con] = new.classobj(con, (), {'__init__': __init__})
    # Perhaps it should be put in a different namespace?

# Example of usage:

data('Node', ('l', 'r'))
data('Leaf', ('anint'))

tree1 = Node(Leaf(1), Leaf(2))
tree2 = Node(l = Leaf(3), r = Leaf(4))

if isinstance(tree1, Node) and isinstance(tree1.r, Leaf):
    print tree1.r.anint

# Types of fields, as well as the type of the tree itself,
# are implicit in Python.

# Exceptions should be better; I'm not familiar with existing
# conventions.

-- 
 __("<  Marcin Kowalczyk * qrczak at knm.org.pl http://qrczak.ids.net.pl/
 \__/
  ^^                      SYGNATURA ZASTĘPCZA
QRCZAK