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
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
//! Types that can be mapped over by receiving or returning references to their contents.
//!
//! ### Examples
//!
//! ```
//! use fp_library::{
//! brands::*,
//! functions::*,
//! types::*,
//! };
//!
//! let memo = Lazy::<_, RcLazyConfig>::new(|| 10);
//! let mapped = ref_map::<LazyBrand<RcLazyConfig>, _, _>(|x: &i32| *x * 2, memo);
//! assert_eq!(*mapped.evaluate(), 20);
//! ```
#[fp_macros::document_module]
mod inner {
use {
crate::kinds::*,
fp_macros::*,
};
/// A type class for types that can be mapped over, returning references.
///
/// This is a variant of `Functor` for types where `map` receives/returns references.
/// This is required for types like `Lazy` where `get()` returns `&A`, not `A`.
///
/// `RefFunctor` is intentionally independent from
/// [`SendRefFunctor`](crate::classes::SendRefFunctor). Although one might
/// expect `SendRefFunctor` to be a subtrait of `RefFunctor`, this is not the case because
/// `ArcLazy::new` requires `Send` on the closure, which a generic `RefFunctor` cannot
/// guarantee. As a result, `ArcLazy` implements only `SendRefFunctor`, not `RefFunctor`,
/// and `RcLazy` implements only `RefFunctor`, not `SendRefFunctor`.
///
/// ### Laws
///
/// `RefFunctor` instances must satisfy the following laws:
///
/// **Identity:** `ref_map(|x| x.clone(), fa)` is equivalent to `fa`, given `A: Clone`.
/// The `Clone` requirement arises because the mapping function receives `&A` but must
/// produce a value of type `A` to satisfy the identity law.
///
/// **Composition:** `ref_map(|x| g(&f(x)), fa)` is equivalent to
/// `ref_map(g, ref_map(f, fa))`.
#[document_examples]
///
/// RefFunctor laws for [`Lazy`](crate::types::Lazy):
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::*,
/// types::*,
/// };
///
/// // Identity: ref_map(|x| x.clone(), fa) evaluates to the same value as fa.
/// let fa = RcLazy::pure(5);
/// let mapped = ref_map::<LazyBrand<RcLazyConfig>, _, _>(|x: &i32| *x, fa.clone());
/// assert_eq!(*mapped.evaluate(), *fa.evaluate());
///
/// // Composition: ref_map(|x| g(&f(x)), fa) = ref_map(g, ref_map(f, fa))
/// let f = |x: &i32| *x * 2;
/// let g = |x: &i32| x + 1;
/// let fa = RcLazy::pure(5);
/// let composed = ref_map::<LazyBrand<RcLazyConfig>, _, _>(|x: &i32| g(&f(x)), fa.clone());
/// let sequential = ref_map::<LazyBrand<RcLazyConfig>, _, _>(
/// g,
/// ref_map::<LazyBrand<RcLazyConfig>, _, _>(f, fa),
/// );
/// assert_eq!(*composed.evaluate(), *sequential.evaluate());
/// ```
///
/// # Cache chain behavior
///
/// Chaining `ref_map` calls on memoized types like [`Lazy`](crate::types::Lazy) creates
/// a linked list of `Rc`/`Arc`-referenced cells. Each mapped value retains a reference to
/// its predecessor, so the entire chain of predecessor cells stays alive as long as any
/// downstream mapped value is reachable. Be aware that long chains can accumulate memory
/// that is only freed when the final value in the chain is dropped.
///
/// # Why `FnOnce`?
///
/// The `func` parameter uses `FnOnce` rather than `Fn` because memoized types like
/// `Lazy` create a new `Lazy` value capturing the closure. Since the resulting `Lazy`
/// will evaluate the closure at most once, `FnOnce` is sufficient and avoids imposing
/// unnecessary `Clone` or multi-call constraints on the caller.
#[kind(type Of<'a, A: 'a>: 'a;)]
pub trait RefFunctor {
/// Maps a function over the values in the functor context, where the function takes a reference.
#[document_signature]
///
#[document_type_parameters(
"The lifetime of the values.",
"The type of the value(s) inside the functor.",
"The type of the result(s) of applying the function."
)]
///
#[document_parameters(
"The function to apply to the value(s) inside the functor.",
"The functor instance containing the value(s)."
)]
///
#[document_returns(
"A new functor instance containing the result(s) of applying the function."
)]
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// classes::*,
/// types::*,
/// };
///
/// let memo = Lazy::<_, RcLazyConfig>::new(|| 10);
/// let mapped = LazyBrand::<RcLazyConfig>::ref_map(|x: &i32| *x * 2, memo);
/// assert_eq!(*mapped.evaluate(), 20);
/// ```
fn ref_map<'a, A: 'a, B: 'a>(
func: impl FnOnce(&A) -> B + 'a,
fa: Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
) -> Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, B>);
}
/// Maps a function over the values in the functor context, where the function takes a reference.
///
/// Free function version that dispatches to [the type class' associated function][`RefFunctor::ref_map`].
#[document_signature]
///
#[document_type_parameters(
"The lifetime of the values.",
"The brand of the functor.",
"The type of the value(s) inside the functor.",
"The type of the result(s) of applying the function."
)]
///
#[document_parameters(
"The function to apply to the value(s) inside the functor.",
"The functor instance containing the value(s)."
)]
///
#[document_returns("A new functor instance containing the result(s) of applying the function.")]
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::*,
/// types::*,
/// };
///
/// let memo = Lazy::<_, RcLazyConfig>::new(|| 10);
/// let mapped = ref_map::<LazyBrand<RcLazyConfig>, _, _>(|x: &i32| *x * 2, memo);
/// assert_eq!(*mapped.evaluate(), 20);
/// ```
pub fn ref_map<'a, Brand: RefFunctor, A: 'a, B: 'a>(
func: impl FnOnce(&A) -> B + 'a,
fa: Apply!(<Brand as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
) -> Apply!(<Brand as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, B>) {
Brand::ref_map(func, fa)
}
}
pub use inner::*;
#[cfg(test)]
mod tests {
use {
crate::{
brands::*,
functions::*,
types::*,
},
quickcheck_macros::quickcheck,
};
/// RefFunctor identity law: ref_map(Clone::clone, lazy) evaluates to the same value as lazy.
#[quickcheck]
fn prop_ref_functor_identity(x: i32) -> bool {
let lazy = RcLazy::pure(x);
let mapped = ref_map::<LazyBrand<RcLazyConfig>, _, _>(|v: &i32| *v, lazy.clone());
*mapped.evaluate() == *lazy.evaluate()
}
/// RefFunctor composition law: ref_map(|x| g(&f(x)), lazy) == ref_map(g, ref_map(f, lazy)).
#[quickcheck]
fn prop_ref_functor_composition(x: i32) -> bool {
let f = |v: &i32| v.wrapping_mul(2);
let g = |v: &i32| v.wrapping_add(1);
let lazy1 = RcLazy::pure(x);
let lazy2 = RcLazy::pure(x);
let composed = ref_map::<LazyBrand<RcLazyConfig>, _, _>(|v: &i32| g(&f(v)), lazy1);
let sequential = ref_map::<LazyBrand<RcLazyConfig>, _, _>(
g,
ref_map::<LazyBrand<RcLazyConfig>, _, _>(f, lazy2),
);
*composed.evaluate() == *sequential.evaluate()
}
}