fp_library/classes/
ref_semimonad.rs

1//! Contexts supporting by-reference monadic sequencing via [`bind`](crate::functions::bind).
2//!
3//! Like [`Semimonad::bind`](crate::classes::Semimonad::bind), but the closure
4//! receives `&A` instead of `A`. This enables memoized types like
5//! [`Lazy`](crate::types::Lazy) to participate in monadic sequencing without
6//! giving up their cached value.
7//!
8//! ### Examples
9//!
10//! ```
11//! use fp_library::{
12//! 	brands::*,
13//! 	classes::*,
14//! 	types::*,
15//! };
16//!
17//! let lazy = RcLazy::pure(5);
18//! let result = LazyBrand::<RcLazyConfig>::ref_bind(&lazy, |x: &i32| {
19//! 	Lazy::<_, RcLazyConfig>::new({
20//! 		let v = *x;
21//! 		move || v * 2
22//! 	})
23//! });
24//! assert_eq!(*result.evaluate(), 10);
25//! ```
26
27#[fp_macros::document_module]
28mod inner {
29	use {
30		crate::kinds::*,
31		fp_macros::*,
32	};
33
34	/// A type class for contexts supporting by-reference monadic sequencing.
35	///
36	/// The closure receives `&A` and decides what to do with it, including
37	/// whether to clone. No `Clone` bound is imposed by the trait.
38	#[kind(type Of<'a, A: 'a>: 'a;)]
39	pub trait RefSemimonad {
40		/// Sequences a computation using a reference to the contained value.
41		#[document_signature]
42		///
43		#[document_type_parameters(
44			"The lifetime of the values.",
45			"The type of the value inside the context.",
46			"The type of the value in the resulting context."
47		)]
48		///
49		#[document_parameters(
50			"The context containing the value.",
51			"A function that receives a reference to the value and returns a new context."
52		)]
53		///
54		#[document_returns("A new context produced by the function.")]
55		#[document_examples]
56		///
57		/// ```
58		/// use fp_library::{
59		/// 	brands::*,
60		/// 	classes::*,
61		/// 	types::*,
62		/// };
63		///
64		/// let lazy = RcLazy::pure(5);
65		/// let result = LazyBrand::<RcLazyConfig>::ref_bind(&lazy, |x: &i32| {
66		/// 	Lazy::<_, RcLazyConfig>::new({
67		/// 		let v = *x;
68		/// 		move || v * 2
69		/// 	})
70		/// });
71		/// assert_eq!(*result.evaluate(), 10);
72		/// ```
73		fn ref_bind<'a, A: 'a, B: 'a>(
74			fa: &Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
75			f: impl Fn(&A) -> Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, B>) + 'a,
76		) -> Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, B>);
77	}
78
79	/// Like [`ref_bind`](crate::functions::bind), but with the arguments flipped.
80	/// Collapses two nested layers of a by-ref semimonad into one.
81	///
82	/// Equivalent to `ref_bind(mma, |ma| ma.clone())`.
83	#[document_signature]
84	#[document_type_parameters(
85		"The lifetime of the computation.",
86		"The brand of the semimonad.",
87		"The type of the value inside the nested semimonad."
88	)]
89	#[document_parameters("The doubly-wrapped semimonadic value.")]
90	#[document_returns("The singly-wrapped semimonadic value.")]
91	#[document_examples]
92	///
93	/// ```
94	/// use fp_library::{
95	/// 	brands::*,
96	/// 	functions::{
97	/// 		explicit::join,
98	/// 		*,
99	/// 	},
100	/// 	types::*,
101	/// };
102	///
103	/// let inner = RcLazy::pure(5);
104	/// let outer = RcLazy::new({
105	/// 	let inner = inner.clone();
106	/// 	move || inner.clone()
107	/// });
108	/// let result = join::<LazyBrand<RcLazyConfig>, _, _>(&outer);
109	/// assert_eq!(*result.evaluate(), 5);
110	/// ```
111	pub fn ref_join<'a, Brand: RefSemimonad, A: 'a>(
112		mma: &Apply!(<Brand as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, Apply!(<Brand as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>)>)
113	) -> Apply!(<Brand as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>)
114	where
115		Apply!(<Brand as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>): Clone, {
116		Brand::ref_bind(mma, |ma| ma.clone())
117	}
118}
119
120pub use inner::*;
fp_library/classes/ref_semimonad.rs

fp_library/classes/
ref_semimonad.rs
