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
//! A newtype wrapper whose [`Semigroup`](crate::classes::Semigroup) instance always keeps the first
//! (leftmost) value.
//!
//! ### Examples
//!
//! ```
//! use fp_library::{
//! functions::*,
//! types::First,
//! };
//!
//! assert_eq!(append(First(1), First(2)), First(1));
//! ```
#[fp_macros::document_module]
mod inner {
use {
crate::classes::*,
fp_macros::*,
};
/// A newtype wrapper whose [`Semigroup`] instance always keeps the first value.
///
/// `append(First(a), First(b))` returns `First(a)`, discarding `b`.
///
/// There is no [`Monoid`] instance because there is no identity element.
#[document_examples]
///
/// ```
/// use fp_library::{
/// functions::*,
/// types::First,
/// };
///
/// assert_eq!(append(First("hello"), First("world")), First("hello"));
/// ```
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct First<A>(
/// The wrapped value.
pub A,
);
#[document_type_parameters("The wrapped type.")]
impl<A> Semigroup for First<A> {
/// Returns the first (leftmost) value, discarding the second.
#[document_signature]
///
#[document_parameters("The first value (kept).", "The second value (discarded).")]
///
#[document_returns("The first value.")]
#[document_examples]
///
/// ```
/// use fp_library::{
/// functions::*,
/// types::First,
/// };
///
/// assert_eq!(append(First(1), First(2)), First(1));
/// ```
fn append(
a: Self,
_b: Self,
) -> Self {
a
}
}
}
pub use inner::*;
#[cfg(test)]
mod tests {
use {
super::*,
crate::functions::*,
quickcheck_macros::quickcheck,
};
#[quickcheck]
fn semigroup_associativity(
a: i32,
b: i32,
c: i32,
) -> bool {
let x = First(a);
let y = First(b);
let z = First(c);
append(x, append(y, z)) == append(append(x, y), z)
}
}