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#
# Copyright Applied Language Solutions 2012
#
# This file is part of Pypeline.
#
# Pypeline is free software: you can redistribute it and/or modify
# it under the terms of the GNU Lesser General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# Pypeline is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public License
# along with Pypeline. If not, see <http://www.gnu.org/licenses/>.
#
import types
from pypeline.core.arrows.arrow import Arrow
#
# Kleisli arrows of a monad
#
# See:
# Publications:
# http://www.cse.chalmers.se/~rjmh/Papers/arrows.pdf
#
class KleisliArrow(Arrow):
#
# patcher :: Monad m => a -> m a
# function :: Monad m => a -> m b
#
def __init__(self, patcher, f):
if not isinstance(patcher, types.FunctionType) and not isinstance(patcher, types.MethodType):
raise ValueError("Patcher must be a function")
if f and (not isinstance(f, types.FunctionType) and
not isinstance(f, types.MethodType)):
raise ValueError("Function must be a function")
self._patcher = patcher
self._func = f
# arr f = K(\b ->return(f b))
def arr(self, f):
ka = KleisliArrow(self._patcher, None)
ka._func = lambda b: ka._patcher(f(b))
return ka
# K f >>> K g = K(\b -> f b >>= g)
def __rshift__(self, other):
if not isinstance(other, KleisliArrow):
raise ValueError("Must be an KleisliArrow")
return KleisliArrow(other._patcher, lambda b: self._func(b) >= other._func)
# first (K f) = K(\(b, d) -> f b >>= \c -> return (c, d))
def first(self):
ka = KleisliArrow(self._patcher, None)
ka._func = lambda t: self._func(t[0]) >= (lambda c: self._patcher((c, t[1])))
return ka
# second (K f) = K(\(d, b) -> f b >>= \c -> return (d, c))
def second(self):
ka = KleisliArrow(self._patcher, None)
ka._func = lambda t: self._func(t[1]) >= (lambda c: self._patcher((t[0], c)))
return ka
@staticmethod
def runKleisli(k, a):
if not isinstance(k, KleisliArrow):
raise ValueError("Arrow must be a Kleisli arrow")
return k._func(a)
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