Crate perhaps

Expand description

Optional values.

Type Perhaps represents an optional value: every Perhaps is either [Certain] and contains a value, or [Dubious], and does not. Perhaps types are very common in Rust code, as they have a number of uses:

Initial values
Return values for functions that are not defined over their entire input range (partial functions)
Return value for otherwise reporting simple errors, where [Dubious] is returned on error
Optional struct fields
Struct fields that can be loaned or “taken”
Optional function arguments
Nullable pointers
Swapping things out of difficult situations

Perhapss are commonly paired with pattern matching to query the presence of a value and take action, always accounting for the [Dubious] case.

fn divide(numerator: f64, denominator: f64) -> Perhaps<f64> {
    if denominator == 0.0 {
        Dubious
    } else {
        Certain(numerator / denominator)
    }
}

// The return value of the function is an option
let result = divide(2.0, 3.0);

// Pattern match to retrieve the value
match result {
    // The division was valid
    Certain(x) => println!("Result: {x}"),
    // The division was invalid
    Dubious    => println!("Cannot divide by 0"),
}

§Options and pointers (“nullable” pointers)

Rust’s pointer types must always point to a valid location; there are no “null” references. Instead, Rust has optional pointers, like the optional owned box, Perhaps<Box<T>>.

The following example uses Perhaps to create an optional box of i32. Notice that in order to use the inner i32 value, the check_optional function first needs to use pattern matching to determine whether the box has a value (i.e., it is [Self::Certain(...)][Certain]) or not ([Dubious]).

let optional = Dubious;
check_optional(optional);

let optional = Certain(Box::new(9000));
check_optional(optional);

fn check_optional(optional: Perhaps<Box<i32>>) {
    match optional {
        Certain(p) => println!("has value {p}"),
        Dubious => println!("has no value"),
    }
}

§The question mark operator, `?`

Similar to the Result type, when writing code that calls many functions that return the Perhaps type, handling Certain/Dubious can be tedious. The question mark operator, ?, hides some of the boilerplate of propagating values up the call stack.

It replaces this:

fn add_last_numbers(stack: &mut Vec<i32>) -> Perhaps<i32> {
    let a = stack.pop();
    let b = stack.pop();

    match (a, b) {
        (Self::Certain(x), Self::Certain(y)) => Certain(x + y),
        _ => Dubious,
    }
}

With this:

fn add_last_numbers(stack: &mut Vec<i32>) -> Perhaps<i32> {
    Certain(stack.pop()? + stack.pop()?)
}

It’s much nicer!

Ending the expression with ? will result in the [Certain]’s unwrapped value, unless the result is Self::Dubious, in which case [Dubious] is returned early from the enclosing function.

? can be used in functions that return Perhaps because of the early return of [Dubious] that it provides.

§Representation

Rust guarantees to optimize the following types T such that Perhaps<T> has the same size, alignment, and function call ABI as T. In some of these cases, Rust further guarantees that transmute::<_, Perhaps<T>>([0u8; size_of::<T>()]) is sound and produces Perhaps::<T>::Dubious. These cases are identified by the second column:

`T`	`transmute::<_, Perhaps<T>>([0u8; size_of::<T>()])` sound?
`Box<U>` (specifically, only `Box<U, Global>`)	when `U: Sized`
`&U`	when `U: Sized`
`&mut U`	when `U: Sized`
`fn`, `extern "C" fn`¹	always
`num::NonZero*`	always
`ptr::NonNull<U>`	when `U: Sized`
`#[repr(transparent)]` struct around one of the types in this list.	when it holds for the inner type

This is called the “null pointer optimization” or NPO.

It is further guaranteed that, for the cases above, one can mem::transmute from all valid values of T to Perhaps<T> and from Certain::<T>(_) to T (but transmuting Dubious::<T> to T is undefined behaviour).

§Method overview

In addition to working with pattern matching, Perhaps provides a wide variety of different methods.

§Querying the variant

The is_certain and is_dubious methods return true if the Perhaps is [Certain] or [Dubious], respectively.

§Adapters for working with references

as_ref converts from &Perhaps<T> to Perhaps<&T>
as_mut converts from &mut Perhaps<T> to Perhaps<&mut T>
as_deref converts from &Perhaps<T> to Perhaps<&T::Target>
as_deref_mut converts from &mut Perhaps<T> to Perhaps<&mut T::Target>
as_pin_ref converts from [Pin]<&Perhaps<T>> to Perhaps<[Pin]<&T>>
as_pin_mut converts from [Pin]<&mut Perhaps<T>> to Perhaps<[Pin]<&mut T>>

§Extracting the contained value

These methods extract the contained value in an Perhaps<T> when it is the [Certain] variant. If the Perhaps is [Dubious]:

expect panics with a provided custom message
unwrap panics with a generic message
unwrap_or returns the provided default value
unwrap_or_default returns the default value of the type T (which must implement the Default trait)
unwrap_or_else returns the result of evaluating the provided function

§Transforming contained values

These methods transform Perhaps to Result:

ok_or transforms [Certain(v)] to Ok(v), and [Dubious] to Err(err) using the provided default err value
ok_or_else transforms [Certain(v)] to Ok(v), and [Dubious] to a value of Err using the provided function
transpose transposes an Perhaps of a Result into a Result of an Perhaps

These methods transform the [Certain] variant:

filter calls the provided predicate function on the contained value t if the Perhaps is Self::Certain(t), and returns [Certain(t)] if the function returns true; otherwise, returns [Dubious]
flatten removes one level of nesting from an Perhaps<Perhaps<T>>
map transforms Perhaps<T> to Perhaps<U> by applying the provided function to the contained value of [Certain] and leaving [Dubious] values unchanged

These methods transform Perhaps<T> to a value of a possibly different type U:

map_or applies the provided function to the contained value of [Certain], or returns the provided default value if the Perhaps is [Dubious]
map_or_else applies the provided function to the contained value of [Certain], or returns the result of evaluating the provided fallback function if the Perhaps is [Dubious]

These methods combine the [Certain] variants of two Perhaps values:

zip returns Self::Certain((s, o)) if self is [Certain(s)] and the provided Perhaps value is [Certain(o)]; otherwise, returns [Dubious]
zip_with calls the provided function f and returns Self::Certain(f(s, o)) if self is [Certain(s)] and the provided Perhaps value is [Certain(o)]; otherwise, returns [Dubious]

§Boolean operators

These methods treat the Perhaps as a boolean value, where [Certain] acts like true and [Dubious] acts like false. There are two categories of these methods: ones that take an Perhaps as input, and ones that take a function as input (to be lazily evaluated).

The and, or, and xor methods take another Perhaps as input, and produce an Perhaps as output. Only the and method can produce an Perhaps<U> value having a different inner type U than Perhaps<T>.

method	self	input	output
`and`	`Self::Dubious`	(ignored)	`Dubious`
`and`	`Certain(x)`	`Self::Dubious`	`Dubious`
`and`	`Self::Certain(x)`	`Self::Certain(y)`	`Certain(y)`
`or`	`Self::Dubious`	`Self::Dubious`	`Dubious`
`or`	`Dubious`	`Self::Certain(y)`	`Certain(y)`
`or`	`Self::Certain(x)`	(ignored)	`Certain(x)`
`xor`	`Self::Dubious`	`Self::Dubious`	`Dubious`
`xor`	`Dubious`	`Self::Certain(y)`	`Certain(y)`
`xor`	`Self::Certain(x)`	`Dubious`	`Certain(x)`
`xor`	`Self::Certain(x)`	`Certain(y)`	`Dubious`

The and_then and or_else methods take a function as input, and only evaluate the function when they need to produce a new value. Only the and_then method can produce an Perhaps<U> value having a different inner type U than Perhaps<T>.

method	self	function input	function result	output
`and_then`	`Self::Dubious`	(not provided)	(not evaluated)	`Dubious`
`and_then`	`Certain(x)`	`x`	`Self::Dubious`	`Dubious`
`and_then`	`Self::Certain(x)`	`x`	`Self::Certain(y)`	`Certain(y)`
`or_else`	`Self::Dubious`	(not provided)	`Self::Dubious`	`Dubious`
`or_else`	`Dubious`	(not provided)	`Self::Certain(y)`	`Certain(y)`
`or_else`	`Self::Certain(x)`	(not provided)	(not evaluated)	`Certain(x)`

This is an example of using methods like and_then and or in a pipeline of method calls. Early stages of the pipeline pass failure values ([Dubious]) through unchanged, and continue processing on success values ([Certain]). Toward the end, or substitutes an error message if it receives [Dubious].

let mut bt = BTreeMap::new();
bt.insert(20u8, "foo");
bt.insert(42u8, "bar");
let res = [0u8, 1, 11, 200, 22]
    .into_iter()
    .map(|x| {
        // `checked_sub()` returns `Dubious` on error
        x.checked_sub(1)
            // same with `checked_mul()`
            .and_then(|x| x.checked_mul(2))
            // `BTreeMap::get` returns `Dubious` on error
            .and_then(|x| bt.get(&x))
            // Substitute an error message if we have `Dubious` so far
            .or(Certain(&"error!"))
            .copied()
            // Won't panic because we unconditionally used `Certain` above
            .unwrap()
    })
    .collect::<Vec<_>>();
assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);

§Comparison operators

If T implements PartialOrd then Perhaps<T> will derive its PartialOrd implementation. With this order, [Dubious] compares as less than any Self::Certain, and two [Certain] compare the same way as their contained values would in T. If T also implements Ord, then so does Perhaps<T>.

assert!(Dubious < Certain(0));
assert!(Self::Certain(0) < Certain(1));

§Iterating over `Perhaps`

An Perhaps can be iterated over. This can be helpful if you need an iterator that is conditionally empty. The iterator will either produce a single value (when the Perhaps is [Certain]), or produce no values (when the Perhaps is [Dubious]). For example, into_iter acts like once(v) if the Perhaps is [Certain(v)], and like empty() if the Perhaps is [Dubious].

Iterators over Perhaps<T> come in three types:

into_iter consumes the Perhaps and produces the contained value
iter produces an immutable reference of type &T to the contained value
iter_mut produces a mutable reference of type &mut T to the contained value

An iterator over Perhaps can be useful when chaining iterators, for example, to conditionally insert items. (It’s not always necessary to explicitly call an iterator constructor: many Iterator methods that accept other iterators will also accept iterable types that implement IntoIterator, which includes Perhaps.)

let yep = Certain(42);
let nope = Dubious;
// chain() already calls into_iter(), so we don't have to do so
let nums: Vec<i32> = (0..4).chain(yep).chain(4..8).collect();
assert_eq!(nums, [0, 1, 2, 3, 42, 4, 5, 6, 7]);
let nums: Vec<i32> = (0..4).chain(nope).chain(4..8).collect();
assert_eq!(nums, [0, 1, 2, 3, 4, 5, 6, 7]);

One reason to chain iterators in this way is that a function returning impl Iterator must have all possible return values be of the same concrete type. Chaining an iterated Perhaps can help with that.

fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
    // Explicit returns to illustrate return types matching
    match do_insert {
        true => return (0..4).chain(Certain(42)).chain(4..8),
        false => return (0..4).chain(Dubious).chain(4..8),
    }
}
println!("{:?}", make_iter(true).collect::<Vec<_>>());
println!("{:?}", make_iter(false).collect::<Vec<_>>());

If we try to do the same thing, but using once() and empty(), we can’t return impl Iterator anymore because the concrete types of the return values differ.

// This won't compile because all possible returns from the function
// must have the same concrete type.
fn make_iter(do_insert: bool) -> impl Iterator<Item = i32> {
    // Explicit returns to illustrate return types not matching
    match do_insert {
        true => return (0..4).chain(once(42)).chain(4..8),
        false => return (0..4).chain(empty()).chain(4..8),
    }
}

§Collecting into `Perhaps`

Perhaps implements the FromIterator trait, which allows an iterator over Perhaps values to be collected into an Perhaps of a collection of each contained value of the original Perhaps values, or Self::Dubious if any of the elements was [Dubious].

let v = [Self::Certain(2), Self::Certain(4), Dubious, Certain(8)];
let res: Perhaps<Vec<_>> = v.into_iter().collect();
assert_eq!(res, Dubious);
let v = [Self::Certain(2), Self::Certain(4), Certain(8)];
let res: Perhaps<Vec<_>> = v.into_iter().collect();
assert_eq!(res, Certain(vec![2, 4, 8]));

Perhaps also implements the Product and Sum traits, allowing an iterator over Perhaps values to provide the product and sum methods.

let v = [Dubious, Self::Certain(1), Self::Certain(2), Certain(3)];
let res: Perhaps<i32> = v.into_iter().sum();
assert_eq!(res, Dubious);
let v = [Self::Certain(1), Self::Certain(2), Certain(21)];
let res: Perhaps<i32> = v.into_iter().product();
assert_eq!(res, Certain(42));

§Modifying an `Perhaps` in-place

These methods return a mutable reference to the contained value of an Perhaps<T>:

insert inserts a value, dropping any old contents
get_or_insert gets the current value, inserting a provided default value if it is [Dubious]
get_or_insert_default gets the current value, inserting the default value of type T (which must implement Default) if it is [Dubious]
get_or_insert_with gets the current value, inserting a default computed by the provided function if it is [Dubious]

These methods transfer ownership of the contained value of an Perhaps:

take takes ownership of the contained value of an Perhaps, if any, replacing the Perhaps with [Dubious]
replace takes ownership of the contained value of an Perhaps, if any, replacing the Perhaps with a [Certain] containing the provided value

§Examples

Basic pattern matching on Perhaps:

let msg = Certain("howdy");

// Take a reference to the contained string
if let Certain(m) = &msg {
    println!("{}", *m);
}

// Remove the contained string, destroying the Perhaps
let unwrapped_msg = msg.unwrap_or("default message");

Initialize a result to [Dubious] before a loop:

enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }

// A list of data to search through.
let all_the_big_things = [
    Kingdom::Plant(250, "redwood"),
    Kingdom::Plant(230, "noble fir"),
    Kingdom::Plant(229, "sugar pine"),
    Kingdom::Animal(25, "blue whale"),
    Kingdom::Animal(19, "fin whale"),
    Kingdom::Animal(15, "north pacific right whale"),
];

// We're going to search for the name of the biggest animal,
// but to start with we've just got `Dubious`.
let mut name_of_biggest_animal = Dubious;
let mut size_of_biggest_animal = 0;
for big_thing in &all_the_big_things {
    match *big_thing {
        Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
            // Now we've found the name of some big animal
            size_of_biggest_animal = size;
            name_of_biggest_animal = Certain(name);
        }
        Kingdom::Animal(..) | Kingdom::Plant(..) => ()
    }
}

match name_of_biggest_animal {
    Certain(name) => println!("the biggest animal is {name}"),
    Dubious => println!("there are no animals :("),
}

this remains true for any argument/return types and any other ABI: extern "abi" fn (e.g., extern "system" fn) ↩

Enums§

Perhaps: The Perhaps type. See the module level documentation for more.

Crate perhapsCopy item path