"""Monad combinators for a list/generator monad in Python"""
from itertools import chain
# Infix class by Ferdinand Jamitzky
class Infix:
def __init__(self, function):
self.function = function
def __ror__(self, other):
return Infix(lambda x, self=self, other=other: self.function(other, x))
def __or__(self, other):
return self.function(other)
def __call__(self, value1, value2):
return self.function(value1, value2)
# Monad bind, return, fail and zero
@Infix
def bind(g, f):
for v in g:
for v2 in f(v):
yield v2
@Infix
def mplus(g1, g2):
return chain(g1, g2)
mzero = (v for v in [])
fail = lambda _ : mzero
mreturn = lambda x : (v for v in [x])
# Ambiguous parser example from
# http://www.nomaware.com/monads/html/listmonad.html
class Parsed:
DIGIT="Digit"
HEX="Hex"
WORD="Word"
def __init__(self, tokenType, tokenValue):
self.tokenType = tokenType
self.tokenValue = tokenValue
def __repr__(self):
return "(%s, %s)" % (self.tokenType, str(self.tokenValue))
def parseHexDigit(parsed, char):
if parsed.tokenType==Parsed.HEX:
val = "0123456789ABCDEF".find(char)
if val>-1:
return mreturn(Parsed(Parsed.HEX,
(parsed.tokenValue * 16) + val))
else:
return mzero
else:
return mzero
def parseDigit(parsed, char):
if parsed.tokenType==Parsed.DIGIT:
if char.isdigit():
return mreturn(Parsed(Parsed.DIGIT,
(parsed.tokenValue * 10) + int(char)))
else:
return mzero
else:
return mzero
def parseWord(parsed, char):
if parsed.tokenType==Parsed.WORD:
if char.isalpha():
return mreturn(Parsed(Parsed.WORD,
parsed.tokenValue + char))
else:
return mzero
else:
return mzero
def parse(parsed, char):
return parseHexDigit(parsed, char) |mplus| parseDigit(parsed, char) |mplus| parseWord(parsed, char)
def foldM(p, v, i):
c = i[:1]
cs = i[1:]
if len(i)==1:
return p(v, c)
else:
return p(v, c) |bind| (lambda r : foldM(p, r, cs))
def parseArg(string):
hexParser = mreturn(Parsed(Parsed.HEX, 0))
digitParser = mreturn(Parsed(Parsed.DIGIT, 0))
wordParser = mreturn(Parsed(Parsed.WORD, ""))
parser = hexParser |mplus| digitParser |mplus| wordParser
return parser |bind| (lambda v : foldM(parse, v, string))
