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

#[allow(dead_code)]
use std::rc::Rc;

pub trait Functor {
    type Elm;
    type M<B>;

    fn fmap<B, F>(self, f: F) -> Self::M<B>
    where
        F: Fn(&Self::Elm) -> B;
}

macro_rules! functor_numeric_impl {
    ($($t:ty)*) => ($(
        impl Functor for $t {
          type Elm = $t;
          type M<U> = U;

          fn fmap<B, F>(self, f: F) -> Self::M<B>
          where
            F: Fn(&Self::Elm) -> B,
          {
            f(&self)
          }
        }
    )*)
}

functor_numeric_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }

impl<A> Functor for Rc<A> {
    type Elm = A;
    type M<U> = Rc<U>;

    fn fmap<B, F>(self, f: F) -> Self::M<B>
    where
        F: FnOnce(&Self::Elm) -> B,
    {
        let v = f(&self);
        Rc::new(v)
    }
}

impl<A> Functor for Box<A> {
    type Elm = A;
    type M<U> = Box<U>;

    fn fmap<B, F>(self, f: F) -> Self::M<B>
    where
        F: FnOnce(&Self::Elm) -> B,
    {
        let v = f(&self);
        Box::new(v)
    }
}

// ---

impl<A> Functor for Option<A> {
    type Elm = A;
    type M<B> = Option<B>;

    fn fmap<B, F>(self, f: F) -> Self::M<B>
    where
        F: FnOnce(&Self::Elm) -> B,
    {
        match self {
            Some(ref v) => Some(f(v)),
            None => None,
        }
    }
}

impl<A, E> Functor for Result<A, E> {
    type Elm = A;
    type M<B> = Result<B, E>;

    fn fmap<B, F>(self, f: F) -> Self::M<B>
    where
        F: FnOnce(&Self::Elm) -> B,
    {
        match self {
            Ok(v) => Ok(f(&v)),
            Err(e) => Err(e),
        }
    }
}

impl<A> Functor for Vec<A> {
    type Elm = A;
    type M<B> = Vec<B>;

    fn fmap<B, F>(self, f: F) -> Self::M<B>
    where
        F: Fn(&Self::Elm) -> B,
    {
        self.iter().map(f).collect::<Vec<B>>()
    }
}

#[cfg(test)]
mod laws {
    mod option {
        use crate::Functor;
        use std::convert::identity;

        #[quickcheck]
        fn covariant_identity(n: Option<i32>) -> bool {
            n.fmap(|x| identity(*x)) == n
        }

        #[quickcheck]
        fn covariant_composition(n: Option<i32>) -> bool {
            let f1: fn(&i32) -> i32 = |x| *x * 2;
            let f2: fn(&i32) -> i32 = |x| *x + 4;
            n.fmap(f1).fmap(f2) == n.fmap(|x| f2(&f1(x)))
        }
    }

    mod result {
        use crate::Functor;
        use std::convert::identity;

        #[quickcheck]
        fn covariant_identity(n: Result<i32, String>) -> bool {
            let expected = n.clone();
            n.fmap(|x| identity(*x)) == expected
        }

        #[quickcheck]
        fn covariant_composition(n: Result<i32, String>) -> bool {
            let expected = n.clone();
            let f1: fn(&i32) -> i32 = |x| *x * 2;
            let f2: fn(&i32) -> i32 = |x| *x + 4;
            n.fmap(f1).fmap(f2) == expected.fmap(|x| f2(&f1(x)))
        }
    }

    mod vec {
        use crate::Functor;
        use std::convert::identity;

        #[quickcheck]
        fn covariant_identity(n: Vec<i32>) -> bool {
            let expected = n.clone();
            n.fmap(|x| identity(*x)) == expected
        }

        #[quickcheck]
        fn covariant_composition(n: Vec<i32>) -> bool {
            let expected = n.clone();
            let f1: fn(&i32) -> i32 = |x| *x * 2;
            let f2: fn(&i32) -> i32 = |x| *x + 4;
            n.fmap(f1).fmap(f2) == expected.fmap(|x| f2(&f1(x)))
        }
    }
}