内容简介:One surprising feature of type inference in languages like Rust is defining functions with generic return types. The idea is that by specifying at some later point in the code which type you want your function to return, the compiler can go back and fill i
One surprising feature of type inference in languages like Rust is defining functions with generic return types. The idea is that by specifying at some later point in the code which type you want your function to return, the compiler can go back and fill in the blanks.
For example, let’s have a look at this function:
fn new<T: Default>() -> T { T::default() }
You pick the output
It has no value parameters, but one type parameter, T
.
That T
is its return type and also used in the function body.
You can call it like so:
let x: u32 = new();
Or, being explicit about the type parameter, like this:
let x = new::<i32>();
This is quite neat!
More generic: collect
A promising way to be more generic in Rust
is to use more traits!
Have a look at how the
Iterator::collect
method is defined:
fn collect<B: FromIterator<Self::Item>>(self) -> B // ...
You can read this type signature as
Consume self and return something
of a type that implements can be made From [an] Iterator
for the type of items we are iterating over.
Like above,
we call this by specifying what kind of output type want.
[Looking][ FromIterator
implementors] at some of the types FromIterator
is implemented for
is pretty revealing of the use cases.
You can get:
-
a
Vec
by collecting any items, -
a
BTreeMap
orHashMap
by collecting tuples, -
but also
PathBuf
by collectingPath
s, -
and
String
for strings and string slices.
Note: All these types are what you might call “container” types.
One more for the road
More generic? More traits.
There is one more gem hidden in FromIterator
:
impl<A, E, V> FromIterator<Result<A, E>> for Result<V, E> where V: FromIterator<A>, // ...
This means:
You can construct a
Result
containing
any type of container of items A
by collecting items that are Result
s of type A
.
(The first Err
will make the outer Result
be an Err
.)
Here’s an example, see the docs
for another one:
let input: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2)]; let output: Result<Vec<i32>, ()> = input.into_iter().collect();
Note: If you like type theory:
What we’re building is a Result<<T<A>, E>>
by collecting Result<A, E>
s and specifying T
.
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