4 releases
0.2.1 | May 27, 2021 |
---|---|
0.2.0 | Apr 9, 2021 |
0.1.1 | Mar 30, 2021 |
0.1.0 | Mar 30, 2021 |
#1774 in Rust patterns
20KB
270 lines
Overview
rovv
is a crate to provide a "poor-man's" row-polymorphism for rust base on lens-rs
.
What is row poly?
In programming language type theory, row polymorphism is a kind of polymorphism that allows one to write programs that are polymorphic on record field types (also known as rows, hence row polymorphism).
-- wikipedia
Considering a function in PureScript:
\r -> r.x
you can pass a record into the function above
{ x: 1 }
or even
{ x: 1, y: 2, z: 3 }
That is what "row-poly" means: a record can be passed into the function above as long as it contains a field .x
.
The type of the function is:
{ x: a | l } -> a
-- The label `l` represents the rest fields of a record.
Now you can do the same(not exactly) in rust.
Usage
restrict the parameter r
contains a field .x
fn take_x<T>(r: row! { x: T, .. }) -> T {
r.view(optics!(x))
}
// &row! { x: T, .. } is ok
fn take_x_ref<T>(r: &row! { ref x: T, .. }) -> &T {
r.view_ref(optics!(x))
}
// &mut row! { x: T, .. } is ok
fn take_x_mut<T>(r: &mut row! { mut x: T, .. }) -> &mut T {
r.view_mut(optics!(x))
}
let foo = Foo { // have been derived Lens for Foo
x: String::from("this is Foo"),
y: 1234
}
let bar = Bar { // have been derived Lens for Bar
x: 0,
z: Some(1)
}
assert_eq!(&*take_x_ref(&foo), "this is Foo");
assert_eq!(take_x(bar), 0);
You can also describe a type may have a field:
fn or_take_y<T>(r: row! { y: T?, .. }) -> Option<T> {
r.preview(optics!(y))
}
assert_eq!(or_take_y(foo), Some(1234));
assert_eq!(or_take_y(bar), None);
Desugar
The function take_x
is equivalent to
fn take_x<T, R>(r: R) -> T
where
R: Lens<Optics![x], T>
{
r.view(optics!(x))
}
In fact the row! { .. }
will be desugared to the impl trait, the placeholder of a type satisfied the lens trait.
And the dyn_row! { .. }
will be desugared to the dyn trait, the dynamic version of row!
.
fn sum_field(n: i32) -> Box<dyn_row! { Ok: i32?, Err: String?, Some: ()? , ..}> {
match n%3 {
0 => Box::new(Result::<_, String>::Ok(0)),
1 => Box::new(Result::<i32, _>::Err(String::from("no!"))),
2 => Box::new(Some(())),
_ => Box::new(Option::<()>::None)
}
}
Limitations
- Cannot pass the genreric arguments explicitly into the function when
row!
is used in argument position now, because therow!
is the impl trait. dyn_row!
will lose some polymorphism e.g.dyn_row! { x: i32, .. }
does not satisfydyn_row! { ref x: i32, .. }
, because the trait object cannot convert to the others, thought Trait1: Trait2.- Cannot move out of a field from a
dyn_row!
.
Cargo.toml
Please add the following in your Cargo.toml
[dependencies]
lens-rs = "0.3"
rovv = "0.2"
[package.metadata.inwelling]
lens-rs_generator = true
rovv = true
Enjoy it!
Dependencies
~1.2–1.8MB
~43K SLoC