|
| 1 | +"""Algebraic data types for OpenFisca. |
| 2 | +
|
| 3 | +An algebraic data type is a structured type that’s formed by composing other |
| 4 | +types. [...] Product types allow you to have more than one value in a single |
| 5 | +structure, at the same time. [...] Sum types are types where your value must |
| 6 | +be one of a fixed set of options. |
| 7 | +
|
| 8 | +.. _See: |
| 9 | + https://jrsinclair.com/articles/2019/algebraic-data-types-what-i-wish-someone-had-explained-about-functional-programming/ |
| 10 | +
|
| 11 | +""" |
| 12 | + |
| 13 | +from __future__ import annotations |
| 14 | + |
| 15 | +from collections.abc import Callable |
| 16 | +from typing import Generic, TypeVar, cast, final |
| 17 | +from typing_extensions import Never |
| 18 | + |
| 19 | +import dataclasses |
| 20 | + |
| 21 | +#: Type variable representing an error. |
| 22 | +E = TypeVar("E") |
| 23 | + |
| 24 | +#: Type variable representing a value. |
| 25 | +A = TypeVar("A") |
| 26 | + |
| 27 | + |
| 28 | +@dataclasses.dataclass(frozen=True) |
| 29 | +class Either(Generic[E, A]): |
| 30 | + """The Either monad. |
| 31 | +
|
| 32 | + The Either monad specifies the Either data type as well as several |
| 33 | + functions that operate on top of it. The Either data type represents the |
| 34 | + result of a computation that may fail. |
| 35 | +
|
| 36 | + """ |
| 37 | + |
| 38 | + #: The value or state passed on. |
| 39 | + _value: E | A |
| 40 | + |
| 41 | + @property |
| 42 | + @final |
| 43 | + def is_failure(self) -> bool: |
| 44 | + """bool: Whether this instance represents a failure.""" |
| 45 | + return isinstance(self, Failure) |
| 46 | + |
| 47 | + @property |
| 48 | + @final |
| 49 | + def is_success(self) -> bool: |
| 50 | + """bool: Whether this instance represents a success.""" |
| 51 | + return isinstance(self, Success) |
| 52 | + |
| 53 | + @final |
| 54 | + def unwrap(self) -> E | A: |
| 55 | + """Return the value of this instance. |
| 56 | +
|
| 57 | + Examples: |
| 58 | + >>> Either.fail("error").unwrap() |
| 59 | + 'error' |
| 60 | +
|
| 61 | + >>> Either.succeed(1).unwrap() |
| 62 | + 1 |
| 63 | +
|
| 64 | + Returns: |
| 65 | + E | A: The value of this instance. |
| 66 | +
|
| 67 | + """ |
| 68 | + |
| 69 | + return self._value |
| 70 | + |
| 71 | + @final |
| 72 | + def then( |
| 73 | + self, f: Callable[[A], Failure[E] | Success[A]] |
| 74 | + ) -> Failure[E] | Success[A]: |
| 75 | + """Apply a flatMap to input stream. |
| 76 | +
|
| 77 | + Examples: |
| 78 | + >>> Either.fail("error").then(lambda x: Either.succeed(x)).unwrap() |
| 79 | + 'error' |
| 80 | +
|
| 81 | + >>> Either.succeed(1).then(lambda x: Either.succeed(x + 1)).unwrap() |
| 82 | + 2 |
| 83 | +
|
| 84 | + Args: |
| 85 | + f: A function that takes a value and returns a new Either instance. |
| 86 | +
|
| 87 | + Returns: |
| 88 | + _Failure[E] | _Success[A]: The result of applying f. |
| 89 | +
|
| 90 | + """ |
| 91 | + |
| 92 | + if self.is_success: |
| 93 | + return f(cast(A, self.unwrap())) |
| 94 | + return Either.fail(cast(E, self.unwrap())) |
| 95 | + |
| 96 | + @staticmethod |
| 97 | + @final |
| 98 | + def fail(value: E) -> Failure[E]: |
| 99 | + """_Failure[E]: Create a failing result.""" |
| 100 | + return Failure(value) |
| 101 | + |
| 102 | + @staticmethod |
| 103 | + @final |
| 104 | + def succeed(value: A) -> Success[A]: |
| 105 | + """_Success[A]: Create a successful result.""" |
| 106 | + return Success(value) |
| 107 | + |
| 108 | + |
| 109 | +@final |
| 110 | +class Failure(Either[E, Never]): |
| 111 | + """A failing result in an Either ADT.""" |
| 112 | + |
| 113 | + |
| 114 | +@final |
| 115 | +class Success(Either[Never, A]): |
| 116 | + """A successful result in an Either ADT.""" |
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