wordladder.py

import nltk
nltk.download('words')
from nltk.corpus import words
tetrawords = set(w for w in words.words() if len(w) == 4)

from constraint import *

wordladder = Problem()
for v in ['b','c','d','e','f','g','h'] : 
    wordladder.addVariable(v, Domain(tetrawords))
wordladder.addVariable('a', ["earn"])
wordladder.addVariable('i', ["free"])

# Constraint is: zip chars |> piecewise compare |> count matches |> matches == length - 1
singleLetterDifferenceFn = lambda init, xform : sum(map(lambda t : t[0] == t[1], zip(init,xform))) == len(init) - 1

for x in range(ord('a'), ord('i')) : 
    wordladder.addConstraint(singleLetterDifferenceFn, [chr(x),chr(x+1)])

for soln in wordladder.getSolutionIter() : 
    print(soln)