data_defs.py

# h/t Dave Long

from collections import namedtuple

data = lambda s: {ws[0]: namedtuple(ws[0], ws[1:])
                  for term in s.split('|') for ws in [term.split()]}
Data = lambda s: globals().update(data(s))

Data("Nil | Cons head tail")

Nil .length = lambda xs: 0
Cons.length = lambda xs: 1 + xs.tail.length()

## Cons(42, Cons(137, Nil())).length()
#. 2