1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161

/*
 * mini-haskell: light-weight Haskell for fun
 * Copyright (C) 2021  Xie Ruifeng
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

//! Control flow utilities.

/// The uninhabited type `Void`.
#[derive(Copy, Clone, Debug)]
pub enum Void {}

impl Void {
    /// Consumes a [`Void`] value, serves as an unreachable.
    pub fn absurd(self) -> ! { match self {} }
}

/// Result type for parsing.
#[derive(Debug, Copy, Clone, Eq, PartialEq)]
pub enum Result3<T, E, M> {
    /// succeed with a result
    Success(T),
    /// fail with an error, no recovery
    FailFast(E),
    /// fail without error, allowing future recovery
    RetryLater(M),
}

/// Named after `Maybe`, `Just`, and `Nothing` from Haskell.
/// Use this for success/failure semantics, since [`Either`] is used to model the control flow.
pub trait Maybe {
    /// The success type in a `Just`.
    type Just;
    /// Construct a success.
    fn just(x: Self::Just) -> Self;
    /// Check whether it is a success.
    fn is_just(&self) -> bool;
    /// Check whether it is a failure.
    fn is_nothing(&self) -> bool { !self.is_just() }
    /// Convert into an [`Option`].
    fn into_optional(self) -> Option<Self::Just>;
}

impl<T> Maybe for Option<T> {
    type Just = T;
    fn just(x: T) -> Self { Some(x) }
    fn is_just(&self) -> bool { self.is_some() }
    fn into_optional(self) -> Option<Self::Just> { self }
}

impl<T, E> Maybe for Result<T, E> {
    type Just = T;
    fn just(x: T) -> Self { Ok(x) }
    fn is_just(&self) -> bool { self.is_ok() }
    fn into_optional(self) -> Option<T> { self.ok() }
}

impl<T, E, M> Maybe for Result3<T, E, M> {
    type Just = T;
    fn just(x: T) -> Self { Self::Success(x) }
    fn is_just(&self) -> bool { matches!(self, Self::Success(_)) }
    fn into_optional(self) -> Option<Self::Just> {
        match self {
            Self::Success(x) => Some(x),
            _ => None,
        }
    }
}

/// The [`std::ops::Try`] trait is not yet stable. We roll up our own for now.
/// It is named after `Either`, `Left`, and `Right` from Haskell.
pub trait Either {
    /// The type to propagate in a `Left`.
    type Left;
    /// The type to continue with in a `Right`.
    type Right;
    /// Construct a `Left`:
    /// - [`None`] for [`Option`]
    /// - [`Err`] for [`Result`]
    fn left(x: Self::Left) -> Self;
    /// Construct a `Right`:
    /// - [`Some`] for [`Option`]
    /// - [`Ok`] for [`Result`]
    fn right(x: Self::Right) -> Self;
    /// Consumes the value and makes a [`Result`].
    ///
    /// Note that by design (to follow the convention in Haskell), the
    /// [`Err`] is for `Left` and the [`Ok`] is for `Right`.
    fn into_result(self) -> Result<Self::Right, Self::Left>;
}

macro_rules! unwrap {
    ($e: expr) => {
        match $crate::utils::Either::into_result($e) {
            Ok(x) => x,
            Err(e) => return $crate::utils::Either::left(e),
        }
    }
}

impl<T> Either for Option<T> {
    type Left = Option<Void>;
    type Right = T;

    fn left(_: Option<Void>) -> Self { None }
    fn right(x: T) -> Self { Some(x) }
    fn into_result(self) -> Result<T, Option<Void>> {
        match self {
            Some(x) => Ok(x),
            None => Err(None),
        }
    }
}

impl<T, E> Either for Result<T, E> {
    type Left = E;
    type Right = T;

    fn left(x: E) -> Self { Err(x) }
    fn right(x: T) -> Self { Ok(x) }
    fn into_result(self) -> Result<T, E> { self }
}

impl<T, E> From<T> for Result3<T, E, Void> {
    fn from(x: T) -> Self { Self::Success(x) }
}

impl<T, E, M> Either for Result3<T, E, M> {
    type Left = M;
    type Right = Result3<T, E, Void>;

    fn left(m: M) -> Self { Self::RetryLater(m) }

    fn right(x: Result3<T, E, Void>) -> Self {
        match x {
            Result3::Success(x) => Self::Success(x),
            Result3::FailFast(e) => Self::FailFast(e),
            Result3::RetryLater(m) => m.absurd(),
        }
    }

    fn into_result(self) -> std::result::Result<Result3<T, E, Void>, M> {
        match self {
            Self::Success(x) => Ok(Result3::Success(x)),
            Self::FailFast(e) => Ok(Result3::FailFast(e)),
            Self::RetryLater(m) => Err(m),
        }
    }
}