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from abc import abstractmethod
from typing import Callable, TypeVar, Generic
S = TypeVar("S")
R = TypeVar("R")
E = TypeVar("E")
class Either(Generic[S, E]):
@abstractmethod
def bind(self,
func): # type: (Callable[[S], Either[R, E]]) -> Either[R, E]
raise NotImplementedError
@abstractmethod
def __eq__(self, other): # type: (object) -> bool
raise NotImplementedError
def is_error(self): # type: () -> bool
return not self.is_valid()
def is_valid(self): # type: () -> bool
return {Left: False, Right: True}[type(self)]
def error(self): # type: () -> E
if isinstance(self, Left):
return self._error
else:
raise ValueError("Not an error: %s" % self)
def value(self):
if isinstance(self, Right):
return self._value
else:
raise ValueError("Not a value: %s" % self)
class Right(Either[S, E]):
def __init__(self, value): # type: (S) -> None
self._value = value
def bind(self,
func): # type: (Callable[[S], Either[R, E]]) -> Either[R, E]
return func(self._value)
def __eq__(self, other): # type: (object) -> bool
return isinstance(other, Right) and self._value == other._value
def __str__(self): # type: () -> str
return "Right %s" % self._value
class Left(Either[S, E]):
def __init__(self, error): # type: (E) -> None
self._error = error
def bind(self,
func): # type: (Callable[[S], Either[R, E]]) -> Either[R, E]
return Left(self._error)
def __eq__(self, other): # type: (object) -> bool
return isinstance(other, Left) and self._error == other._error
def __str__(self): # type: () -> str
return "Left: %s" % self._error
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