penum is a procedural macro that is used to make an enum conform to a
given pattern that can include generics with trait bounds, which then
allows for static dispatching. It's a tool for asserting how enums
should look and behave through simple expressive rust grammar.
Patterns — can be though of a toy shape sorter, where the enum
variants are shape pieces that are trying to fit the given pattern
we've expressed. There are 3 shapes to choose from, tuples (),
structs {} and units.
Trait bounds — are used in combination with generic parameters
to assert what the matched variants field types should implement, and
can be expressed like this where T: Trait<Type>. The generic
parameters actually needs to be introduced inside a pattern fragment.
Static dispatch — lets us express how an enum should behave in
respect to its variants. The symbol that is used to express this is
^ and should be put infront of the trait you wish to be dispatched,
e.g. (T) where T: ^AsRef<str>. This is currently limited to rust std
and core library traits, but there's plans to extend support for
custom trait definitions soon.
Impls — can be seen as a shorthand for a concrete type that
implements this trait, and are primarily used as a substitute for
regular generic trait bound expressions. They look something like
this, (impl Copy, impl Copy) | {name: impl Clone}
Placeholders — are single unbounded wildcards, or if you are
familiar with rust, it's the underscore _ identifier and usually
means that something is ignored, which means that they will satisfy
any type (_, _) | {num: _}.
Variadic — are similar to placeholders, but instead of only being
able to substitute one type, variadics can be substituted by 0 or more
types. Like placeholders, they are a way to express that we don't care
about the rest of the parameters in a pattern. The look something like
this(T, U, ..) | {num: T, ..}
Allowing developers to assert how an enum should look and behave.
This crate is available on crates.io
and can be used by adding the following to your project's Cargo.toml:
toml
[dependencies]
penum = "0.1.15"
Or run this command in your cargo project:
sh
$ cargo add penum
A Penum expression can look like this:
```text
^^^ ^^^^^^^^
| |
| Predicate bound.
|
Pattern fragment.
``` note that there can be multiple patterns fragments and predicate bounds.
Here's how it would look like if we wanted to dispatch a trait. ```text
|
Dispatch symbol
```
Here he have an enum with two unary tuple variants where the parameter
type Struct implements the trait Trait. The goal is to be able to
call the trait method through Foo. This can be accomplished
automatically marking the trait with a dispatch symbol ^.
```rust
enum Foo { V1(Struct), V2(Struct), } ```
rust
impl Trait for Foo {
fn method(&self, text: &str) {
match self {
V1(val) => val.method(text),
V2(val) => val.method(text),
}
}
}
rust
struct Struct;
trait Trait {
fn method(&self, text: &str);
}
impl Trait for Struct {}
Used penum to force every variant to be a tuple with one field that must
implement Copy.
```rust
enum Guard {
Bar(String),
^^^^^^
// ERROR: String doesn't implement Copy
Bor(&str),
^^^^
// ERROR: `&str` doesn't implement `Copy`
Bur(Vec<i32>),
^^^^^^^^
// ERROR: `Vec<i32>` doesn't implement `Copy`
Bir(i32, i32),
^^^^^^^^^^
// ERROR: `(i32, i32)` doesn't match pattern `(T)`
Byr(),
^^^^^
// ERROR: `Byr()` doesn't match pattern `(T)`
Bxr { name: usize },
^^^^^^^^^^^^^^^
// ERROR: `{ nname: usize }` doesn't match pattern `(T)`
Brr,
^^^
// ERROR: `Brr` doesn't match pattern `(T)`
Beer(i32) // Works!
} ```
```rust
enum Guard {
Bar(String),
^^^^^^
// ERROR: String doesn't implement Copy
Bor(Vec<&str>),
^^^^^^^^^
// ERROR: `Vec<&str>` doesn't implement `Copy`
Bur(Vec<i32>),
^^^^^^^^
// ERROR: `Vec<i32>` doesn't implement `Copy`
Bir(i32, i32),
^^^^^^^^^^
// ERROR: `(i32, i32)` doesn't match pattern `(T)`
Byr(),
^^^^^
// ERROR: `Byr()` doesn't match pattern `(T)`
Bxr { name: usize },
^^^^^^^^^^^^^^^
// ERROR: `{ nname: usize }` doesn't match pattern `(T)`
Brr,
^^^
// ERROR: `Brr` doesn't match pattern `(T)`
Beer(i32) // Works!
} ```
Static dispatch - auto implement core/std/custom traits (read
more).| Traits | Supported |
| ---------- | ------------- |
|Any| supported |
|Borrow| supported |
|BorrowMut| supported |
|Eq| supported |
|AsMut| supported |
|AsRef| supported |
|From| supported |
|Into| supported |
|TryFrom| supported |
|TryInto| supported |
|Default| supported |
|Binary| supported |
|Debug| supported |
|Display| supported |
|LowerExp| supported |
|LowerHex| supported |
|Octal| supported |
|Pointer| supported |
|UpperExp| supported |
|UpperHex| supported |
|Future| supported |
|IntoFuture| supported |
|FromIterator| supported |
|FusedIterator| supported |
|IntoIterator| supported |
|Product| supported |
|Sum| supported |
|Copy| supported |
|Sized| supported |
|ToSocketAddrs| supported |
|Add| supported |
|AddAssign| supported |
|BitAnd| supported |
|BitAndAssign| supported |
|BitOr| supported |
|BitOrAssign| supported |
|BitXor| supported |
|BitXorAssign| supported |
|Deref| supported |
|DerefMut| supported |
|Div| supported |
|DivAssign| supported |
|Drop| supported |
|Fn| supported |
|FnMut| supported |
|FnOnce| supported |
|Index| supported |
|IndexMut| supported |
|Mul| supported |
|MulAssign| supported |
|MultiMethod| supported |
|Neg| supported |
|Not| supported |
|Rem| supported |
|RemAssign| supported |
|Shl| supported |
|ShlAssign| supported |
|Shr| supported |
|ShrAssign| supported |
|Sub| supported |
|SubAssign| supported |
|Termination| supported |
|SliceIndex| supported |
|FromStr| supported |
|ToString| supported |