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0.1.0	Jan 12, 2020

#2446 in #macro

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spez

Macro to specialize on the type of an expression.

This crate implements auto(de)ref specialization: A trick to do specialization in non-generic contexts on stable Rust.

For the details of this technique, see:

What it can and cannot do

The auto(de)ref technique—and therefore this macro—is useless in generic functions, as Rust resolves the specialization based on the bounds defined on the generic context, not based on the actual type when instantiated. (See the example below for a demonstration of this.)

In non-generic contexts, it's also mostly useless, as you probably already know the exact type of all variables.

The only place where using this can make sense is in the implementation of macros that need to have different behaviour depending on the type of a value passed to it. For example, a macro that prints the Debug output of a value, but falls back to a default when it doesn't implement Debug. (See the example below for a demonstration of that.)

How to use it

The basic syntax of the macro is:

spez! {
    for <expression>;
    match <type> { <body> }
    [match <type> { <body> }]
    [...]
}

The examples below show more details.

Simple specialization

In the most simple case, you use this macro to match specific types:

let x = 0;
spez! {
    for x;
    match i32 {
        println!("x is a 32-bit integer!");
    }
    match &str {
        println!("x is a string slice!");
        assert!(false);
    }
}

Return types

Values can be returned from the matches, but have to be explicitly specified for each match. They do not have to be the same for every match.

let x = 0;
let result = spez! {
    for x;
    match i32 -> &'static str {
        "x is a 32-bit integer!"
    }
    match &str -> i32 {
        123
    }
};
assert_eq!(result, "x is a 32-bit integer!");

Generic matches

Generic matches are also possible. Generic variables can be defined on the match, and a where clause can be added after the type.

The matches are tried in order. The first matches get priority over later ones, even if later ones are perfect matches.

let x = 123i32;
let result = spez! {
    for x;
    match<T> T where i8: From<T> -> i32 {
        0
    }
    match<T: std::fmt::Debug> T -> i32 {
        1
    }
    match i32 -> i32 {
        2
    }
};
assert_eq!(result, 1);

Consuming the input

The input (after the for) is consumed and made available to the match bodies.

(If you don't want to consume the input, take a reference and also prepend a & to the types you're matching.)

let x = Box::new(123);
let result = spez! {
    for x;
    match<T: Deref<Target = i32>> T -> i32 {
        *x
    }
    match i32 -> i32 {
        x
    }
};
assert_eq!(result, 123);

Expressions as input

Not just variable names, but full expressions can be given as input. However, if you want to refer to them from the match bodies, you need to prepend name = to give the input a name.

let result = spez! {
    for 1 + 1;
    match i32 -> i32 { 0 }
    match i64 -> i32 { 1 }
};
assert_eq!(result, 0);

let result = spez! {
    for x = 1 + 1;
    match i32 -> i32 { x }
    match i64 -> i32 { 1 }
};
assert_eq!(result, 2);

Capturing variables

Unfortunately, you can't refer to variables of the scope around the spez! {} macro:

let a = 1;
let result = spez! {
    for x = 1;
    match i32 {
        println!("{}", a); // ERROR
    }
};

In a generic function

As mentioned above, the macro is of not much use in generic context, as the specialization is resolved based on the bounds rather than on the actual type in the instantiation of the generic function:

fn f<T: Debug>(v: T) -> &'static str {
    spez! {
        for v;
        match i32 -> &'static str {
            ":)"
        }
        match<T: Debug> T -> &'static str {
            ":("
        }
        match<T> T -> &'static str {
            ":(("
        }
    }
}
assert_eq!(f(0i32), ":(");

In a macro

This is a demonstration of a macro that prints the Debug output of a value, but falls back to "<object of type ...>" if it doesn't implement Debug.

macro_rules! debug {
    ($e:expr) => {
        spez! {
            for x = $e;
            match<T: Debug> T {
                println!("{:?}", x);
            }
            match<T> T {
                println!("<object of type {}>", std::any::type_name::<T>());
            }
        }
    }
}
debug!(123);
debug!(NoDebugType);

Dependencies

~1.5MB
~38K SLoC