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snippetrustMajor

Is there any way to create a type alias for multiple traits?

Submitted by: @import:stackoverflow-api··
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anyaliasformultiplewaytypetraitstherecreate

Problem

I have a generic function that prints the minimum of two items:

use std::fmt::Display;

fn print_min(a: &T, b: &T) {
    println!("min = {}", if a < b { a } else { b });
}


This works pretty well with anything that implements both the PartialOrd and Display traits:

print_min(&45, &46);
// min = 45
print_min(&"a", &"b");
// min = a


Having to put the PartialOrd + Display in the function definition is kind of ugly, especially if I want to have a whole bunch of functions that operate on this (implementing a binary search tree, for example), or if my bounds get more complex. My first inclination was to attempt to write a type alias:

type PartialDisplay = PartialOrd + Display;


but this gives me some fairly bizarre compiler errors:

error[E0393]: the type parameter Rhs must be explicitly specified
--> src/main.rs:7:23
|
7 | type PartialDisplay = PartialOrd + Display;
| ^^^^^^^^^^ missing reference to
Rhs
|
= note: because of the default
Self reference, type parameters must be specified on object types

error[E0225]: only auto traits can be used as additional traits in a trait object
--> src/main.rs:7:36
|
7 | type PartialDisplay = PartialOrd + Display;
| ^^^^^^^ non-auto additional trait


I'm guessing either my syntax is wrong or this just isn't possible yet. I'd like something like

type PartialDisplay = ???
fn print_min { /* ... */ }

Solution

PartialOrd and Display are traits. It has been discussed how to implement an alias but it was decided that it wasn't needed.

Instead, you can create a new trait with the traits you want as super traits and provide a blanket implementation:

use std::fmt::Display;

trait PartialDisplay: PartialOrd + Display {}
impl PartialDisplay for T {}

fn print_min(a: &T, b: &T) {
    println!("min = {}", if a < b { a } else { b });
}

fn main() {
    print_min(&45, &46);
    print_min(&"aa", &"bb");
}

Code Snippets

use std::fmt::Display;

trait PartialDisplay: PartialOrd + Display {}
impl<T: PartialOrd + Display> PartialDisplay for T {}

fn print_min<T: PartialDisplay>(a: &T, b: &T) {
    println!("min = {}", if a < b { a } else { b });
}

fn main() {
    print_min(&45, &46);
    print_min(&"aa", &"bb");
}

Context

Stack Overflow Q#26070559, score: 80

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