Declarative macro to apply a macro to each item?

help
#1

I am aware about order that macros are expanded.
Is it possible to write a generic macro which apply macro, lets say generate, to items generated by macro primitives?

for_each!( generate, primitives );

where for_each, generate, primitives are 3 macroses.

Playground

#2

Why not use repetition?

macro_rules! generate {
    ( $($Type:ident),+ ) => {
       $( let _x: $Type; )+
    };
}

fn main() {
    generate!(i8, i16, i32, i64, i128, u8, u16, u32, u64, u128, f32, f64);
}

Repetition is a generic technique for macros to do for_each stuff.

3 Likes
#3

You might be looking for something called the Callback Pattern. This is where you modify the inner macros (i.e. generate and primitives`) to take the name of a callback macro which will be invoked on each item inside them.

It's a little different from your original example, but could you adapt something like this to your use case?

macro_rules! apply {
    ($callback:ident, $($args:tt),*) => {
        $(
            $callback!($args);
        )*
    }
}

macro_rules! for_each_primitive {
    ($callback:ident) => {
        apply!($callback, i8, i16, i32, i64, i128, u8, u16, u32, u64, u128, f32, f64);
    };
}

macro_rules! generate {
    ( $Type:ident ) => {
        let x: $Type; /* some code */
    };
}

fn main() {
    for_each_primitive!(generate);
}

(playground)

I wrote a worked example for this sort of thing a while back:

7 Likes
#4

Thank you. Problem of the solution it does not allow to move the list into a macro and reuse it.

#5

Thanks, that really solve the original problem. But in my case I have extra arguments applied to each type and attempt to use prefix in apply! does not work and it looks like a problem without solution.

Here:

macro_rules! apply
{
  ( $Callback:ident @PREFIX( $( $Prefix : tt )* ) @ARGS( $( $Arg:tt ),* ) ) =>
  {
    $( $Callback!( @PREFIX( $( $Prefix )* ) @ARGS( $Arg ) ); )* /* ! meta-variable `Prefix` repeats 0 times, but `Arg` repeats 12 times */
    // $( $Callback!( @PREFIX() @ARGS( $Arg ) ); )* /* <- works if prefix is not paseed */
  }
}

Is that solvable?

Payground

#6

Yes. Have a close look at the @ARGS( $Arg ) and compare it to @PREFIX( $( $Prefix )* ). You've also wrapped the entire Callback!() call in a level of repetition, but $Arg and $Prefix are unrelated.

macro_rules! apply
{
    (
        $Callback:ident
        @PREFIX( $( $Prefix : tt )* )
        @ARGS( $( $Arg:tt ),* )
    ) =>
    {
        $Callback!(
            @PREFIX( $( $Prefix )* )
            @ARGS( $( $Arg )* )
        );
  }
}

Making those two tweaks gave me the following output:

i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64
2 Likes
#7

Thank you. It runs, but that solution change a bit semantics.
Output should be

i8
hello!
i16
hello!
...

But it is

i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64
#8

It would not be possible to repeat different times in a single repetition.

But it can be possible to repeat a single token in a single repetition.

1 Like