Rust: using traits/typeclasses to implement a generic numeric function

Question

I have a function which works to make a linked list of integers:

enum List<T> { Cons(T, ~List<T>), End }

fn range(start: int, end: int) -> ~List<int> {
    if start >= end { ~End }
    else { ~Cons(start, range(start+1, end)) }
}

However, I want to make a range of any numeric type, including uints, doubles and the like. But this, for example, doesn't work:

fn range<T: ord>(start: T, end: T) -> ~List<T> {
    if start >= end { ~End }
    else { ~Cons(start, range(start+1, end)) }
}

which produces:

> rustc list.rs
list.rs:3:12: 3:15 error: use of undeclared type name `ord`
list.rs:3 fn range<T: ord>(start: T, end: T) -> ~List<T> {
                      ^~~
error: aborting due to previous error

How can I make a generic function in rust which restricts itself to be callable by "numeric" types? Without having to specifically write the interface myself? I had assumed that there were a number of standard-library traits (such as those listed in section 6.2.1.1 of the manual like eq, ord, etc, though now I'm wondering if those are proper "traits" at all) that I could use when declaring generic functions?

score 5 · Accepted Answer · 2013-06-14 01:02:29Z

5

In the current master, there is a trait named 'Num' which serves as a general trait for all numeric types. Work has been done recently to unify many of the common math functions to work on this trait rather than u8, f32, etc , specifically.

See https://github.com/mozilla/rust/blob/master/src/libstd/num/num.rs#L26 for the aforementioned Num trait.

Hope this helps!

edited Jun 14, 2013 at 1:02

answered Jun 13, 2013 at 17:12

user1898811

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Comments

MFlamer · Accepted Answer · 2013-06-10 05:43:24Z

4

The traits are usually uppercase. In this case it is Ord. See if that helps.

answered Jun 10, 2013 at 5:43

MFlamer

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1 Comment

Eric Holk Over a year ago

You'll also need a trait to let you add numbers. Unless it's changed since I last saw it, I think you want the Num trait. You might also need to call from_int to get your 1 literal into the correct type.

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