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
162
163
164
165
166

//! Kleene logic within Rust's type system
//!
//! Values are `True`, `False` and `Unknown`. Operations are `Not`, `BitAnd`
//! and `BitOr` from `std::ops`. There is also the `Ternary` enum which 
//! represents the values at runtime and the `ToTernary` trait that adds the
//! `to_ternary()` methods to our value types.
//!
//! Examples:
//!
//! `Same` and `Not`
//!
//! ```
//!# use ternary::{True, False, Unknown, Same};
//!# use std::ops::Not;
//! type NotTrue = <<True as Not>::Output as Same<False>>::Output;
//! type NotFalse = <<False as Not>::Output as Same<True>>::Output;
//! type NotUnknown = <<Unknown as Not>::Output as Same<Unknown>>::Output;
//! ```
//! 
//! Transforming Values to Runtime, `BitAnd` and `BitOr`
//!
//! ```
//!# use ternary::{True, False, Unknown, Same, Ternary, ToTernary};
//!# use std::ops::{BitAnd, BitOr};
//! assert_eq!(Ternary::T, <True as BitOr<<Unknown as BitAnd<False>>::Output>>::Output::to_ternary());
//! ```
use std::ops::{Not, BitAnd, BitOr};

/// Our True type value
pub enum True {}

/// Our False type value
pub enum False {}

/// Our Unknown type value
pub enum Unknown {}

/// runtime representation
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
pub enum Ternary { 
    /// A True-ish value
    T,
    /// An Unknown value
    U,
    /// A False-ish value
    F,
}

/// conversion to runtime enum
///
/// examples:
/// ```
///# use ternary::{True, False, Unknown, Ternary, ToTernary};
/// assert_eq!(True::to_ternary(), Ternary::T);
/// assert_eq!(False::to_ternary(), Ternary::F);
/// assert_eq!(Unknown::to_ternary(), Ternary::U);
/// ```
pub trait ToTernary {
    fn to_ternary() -> Ternary;
}

impl ToTernary for True {
    #[inline] fn to_ternary() -> Ternary { Ternary::T }
}

impl ToTernary for False {
    #[inline] fn to_ternary() -> Ternary { Ternary::F }
}

impl ToTernary for Unknown {
    #[inline] fn to_ternary() -> Ternary { Ternary::U }
}

/// Not

/// !True == False
impl Not for True {
    type Output = False;
    fn not(self) -> Self::Output { unreachable!() }
}

///!False == True
impl Not for False {
    type Output = True;
    fn not(self) -> Self::Output { unreachable!() }
}

///!Unknown == Unknown
impl Not for Unknown {
    type Output = Unknown;
    fn not(self) -> Self::Output { unreachable!() }
}

/// BitAnd

/// True & X == X
impl<X: ToTernary> BitAnd<X> for True {
    type Output = X;
    fn bitand(self, _: X) -> Self::Output { unreachable!() }
}

/// False & X == False
impl<X: ToTernary> BitAnd<X> for False {
    type Output = False;
    fn bitand(self, _: X) -> Self::Output { unreachable!() }
}

/// Unknown & True == Unknown
impl BitAnd<True> for Unknown {
    type Output = Unknown;
    fn bitand(self, _: True) -> Self::Output { unreachable!() }
}

/// Unknown & Unknown == Unknown
impl BitAnd<Unknown> for Unknown {
    type Output = Unknown;
    fn bitand(self, _: Unknown) -> Self::Output { unreachable!() }
}

/// Unknown & False == False
impl BitAnd<False> for Unknown {
    type Output = False;
    fn bitand(self, _: False) -> Self::Output { unreachable!() }
}


/// BitOr

/// True | X == True
impl<X: ToTernary> BitOr<X> for True {
    type Output = True;
    fn bitor(self, _: X) -> Self::Output { unreachable!() }
}

/// False | X == X
impl<X: ToTernary> BitOr<X> for False {
    type Output = X;
    fn bitor(self, _: X) -> Self::Output { unreachable!() }
}

/// Unknown | True == True
impl BitOr<True> for Unknown {
    type Output = True;
    fn bitor(self, _: True) -> Self::Output { unreachable!() }
}

/// Unknown | Unknown == Unknown
impl BitOr<Unknown> for Unknown {
    type Output = Unknown;
    fn bitor(self, _: Unknown) -> Self::Output { unreachable!() }
}

/// Unknown | False == Unknown
impl BitOr<False> for Unknown {
    type Output = Unknown;
    fn bitor(self, _: False) -> Self::Output { unreachable!() }
}

/// shamelessly copied from typenum
pub trait Same<Rhs = Self> {
    type Output;
}

impl<T> Same<T> for T {
    type Output = T;
}