Build all pairs of elements (quadratic set) in declarative macro

Question

I have a list of identifier and I want to invoke a macro for each pair of identifiers from that list. For example, if I have a, b and c, I would like to generate this:

println!("{} <-> {}", a, a);
println!("{} <-> {}", a, b);
println!("{} <-> {}", a, c);
println!("{} <-> {}", b, a);
println!("{} <-> {}", b, b);
println!("{} <-> {}", b, c);
println!("{} <-> {}", c, a);
println!("{} <-> {}", c, b);
println!("{} <-> {}", c, c);

Of course, this is a dummy example. In my real code, the identifiers are types and I want to generate impl blocks or something like that.

My goal is to list each identifier only once. In my real code, I have around 12 identifier and don't want to manually write down all 12² = 144 pairs. So I thought that a macro might help me. I know that it can be solved with the all powerful procedural macros, but I hoped that it's also possible with declarative macros (macro_rules!).

I tried what I thought was the intuitive way to handle this (two nested "loops") (Playground):

macro_rules! print_all_pairs {
    ($($x:ident)*) => {
        $(
            $(
                println!("{} <-> {}", $x, $x);  // `$x, $x` feels awkward... 
            )*
        )*
    }
}

let a = 'a';
let b = 'b';
let c = 'c';

print_all_pairs!(a b c);

However, this results in this error:

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
 --> src/main.rs:4:14
  |
4 |               $(
  |  ______________^
5 | |                 println!("{} <-> {}", $x, $x);
6 | |             )*
  | |_____________^

I guess it makes kind of sense, so I tried something else (Playground):

macro_rules! print_all_pairs {
    ($($x:ident)*) => {
        print_all_pairs!(@inner $($x)*; $($x)*);
    };
    (@inner $($x:ident)*; $($y:ident)*) => {
        $(
            $(
                println!("{} <-> {}", $x, $y);
            )*
        )*
    };
}

But this results in the same error as above!

Is this possible with declarative macros at all?

There is definitely a short-coming in the expressivity of declarative macros. But I've hit the same problem with `quote` in proc macros. When you have two repeating variables, they are always interpolated pair-wise, and it's quite tricky to interpolate over all permutations. — Peter Hall, Feb 06 '19 at 11:46

score 8 · Accepted Answer · answered Feb 06 '19 at 11:41

Is this possible with declarative macros at all?

Yes.

But (to the best of my knowledge) we have to iterate through the list via head/tail recursion once instead of using the built-in $( ... )* mechanism everywhere. This means that the list length is limited by the recursion depth of macro expansion. That's not a problem for "only" 12 items, though.

In the code below, I separated the "for all pairs" functionality from the printing-code by passing a macro name to the for_all_pairs macro. (Playground).

// The macro that expands into all pairs
macro_rules! for_all_pairs {
    ($mac:ident: $($x:ident)*) => {
        // Duplicate the list
        for_all_pairs!(@inner $mac: $($x)*; $($x)*);
    };

    // The end of iteration: we exhausted the list
    (@inner $mac:ident: ; $($x:ident)*) => {};

    // The head/tail recursion: pick the first element of the first list
    // and recursively do it for the tail.
    (@inner $mac:ident: $head:ident $($tail:ident)*; $($x:ident)*) => {
        $(
            $mac!($head $x);
        )*
        for_all_pairs!(@inner $mac: $($tail)*; $($x)*);
    };
}

// What you actually want to do for each pair
macro_rules! print_pair {
    ($a:ident $b:ident) => {
        println!("{} <-> {}", $a, $b);
    }
}

// Test code
let a = 'a';
let b = 'b';
let c = 'c';

for_all_pairs!(print_pair: a b c);

This code prints:

a <-> a
a <-> b
a <-> c
b <-> a
b <-> b
b <-> c
c <-> a
c <-> b
c <-> c

Build all pairs of elements (quadratic set) in declarative macro

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