Add Ability for Closure to Allow Argument But Never Use It

[therealgymmy]

I'm not sure how to phrase this properly, but basically I believe it'd be nice to see the following possible in rust.

A closure that takes in an argument, but never actually binds that argument to a name.

The reason being that in a loop, sometimes we don't actually need to know which iteration we are in. We just need to repeat a certain action for an arbitrary number of times.

For example,

// Imaginary syntax
each( [0, ..10], | :uint | {    // or maybe does not need |:uint| at all...
    doStuff(); // for 10 times

               // we don't actually need to use the element in the vector
               // Hence, really no need to assign the param a name,
               // and get a warning for not using it.
});

Essentially, argument wise, the closure will treat its argument the same way as the following C/C++ function.

int foo (uint32_t) {    // never use the param
   doStuff();
}

In C++, we could use the above function for something like this.

// Performance here is probably not nearly as good as hand-written loops,
// but that's general idea.
std::vector<uint32_t> num {1, 2, 3, 4, 5, 6};
std::for_each(num.begin(), num.end(), foo());

Alternatively, perhaps we could enhance the loop expression and allow it to take an argument to specify the number of iterations we want.

For example,

loop (until 10) {
    doStuff();
}

[catamorphism]

You can write a closure with an argument it doesn't use, so that the compiler doesn't warn about it:

|_x| { ...whatever... }

Names beginning with an underscore are exempt from unused-variable warnings.

You can also write a closure with no arguments:

do repeat(5) || { ...whatever... };

where repeat could look like:

fn repeat(i: uint, action: fn()) {
     if i > 0 {
         action();
         repeat(i - 1, action);
     }
}

Is either of those what you're looking for?

[therealgymmy]

Awesome!
I guess I need to dig into the language a bit more next time before making a request :-p

[catamorphism]

No problem! It's always ok to ask questions like this on IRC, too (see https://github.com/mozilla/rust/wiki/Note-development-policy#Communication ).