src/test/run-pass/macros/type-macros-hlist.rs

// run-pass
use std::ops::*;

#[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
struct Nil;
 // empty HList
#[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
struct Cons<H, T: HList>(H, T);
 // cons cell of HList

 // trait to classify valid HLists
trait HList { }
impl HList for Nil { }
impl <H, T: HList> HList for Cons<H, T> { }

// term-level macro for HLists
macro_rules! hlist({  } => { Nil } ; { $ head : expr } => {
                   Cons ( $ head , Nil ) } ; {
                   $ head : expr , $ ( $ tail : expr ) , * } => {
                   Cons ( $ head , hlist ! ( $ ( $ tail ) , * ) ) } ;);

// type-level macro for HLists
macro_rules! HList({  } => { Nil } ; { $ head : ty } => {
                   Cons < $ head , Nil > } ; {
                   $ head : ty , $ ( $ tail : ty ) , * } => {
                   Cons < $ head , HList ! ( $ ( $ tail ) , * ) > } ;);

// nil case for HList append
impl <Ys: HList> Add<Ys> for Nil {
    type
    Output
    =
    Ys;

    fn add(self, rhs: Ys) -> Ys { rhs }
}

// cons case for HList append
impl <Rec: HList + Sized, X, Xs: HList, Ys: HList> Add<Ys> for Cons<X, Xs>
 where Xs: Add<Ys, Output = Rec> {
    type
    Output
    =
    Cons<X, Rec>;

    fn add(self, rhs: Ys) -> Cons<X, Rec> { Cons(self.0, self.1 + rhs) }
}

// type macro Expr allows us to expand the + operator appropriately
macro_rules! Expr({ ( $ ( $ LHS : tt ) + ) } => { Expr ! ( $ ( $ LHS ) + ) } ;
                  { HList ! [ $ ( $ LHS : tt ) * ] + $ ( $ RHS : tt ) + } => {
                  < Expr ! ( HList ! [ $ ( $ LHS ) * ] ) as Add < Expr ! (
                  $ ( $ RHS ) + ) >> :: Output } ; {
                  $ LHS : tt + $ ( $ RHS : tt ) + } => {
                  < Expr ! ( $ LHS ) as Add < Expr ! ( $ ( $ RHS ) + ) >> ::
                  Output } ; { $ LHS : ty } => { $ LHS } ;);

// test demonstrating term level `xs + ys` and type level `Expr!(Xs + Ys)`
fn main() {
    fn aux<Xs: HList, Ys: HList>(xs: Xs, ys: Ys) -> Expr!(Xs + Ys) where
     Xs: Add<Ys> {
        xs + ys
    }

    let xs: HList!(& str , bool , Vec < u64 >) =
        hlist!("foo" , false , vec ! [  ]);
    let ys: HList!(u64 , [ u8 ; 3 ] , (  )) =
        hlist!(0 , [ 0 , 1 , 2 ] , (  ));

    // demonstrate recursive expansion of Expr!
    let zs:
            Expr!((
                  HList ! [ & str ] + HList ! [ bool ] + HList ! [ Vec < u64 >
                  ] ) + ( HList ! [ u64 ] + HList ! [ [ u8 ; 3 ] , (  ) ] ) +
                  HList ! [  ]) = aux(xs, ys);
    assert_eq!(zs , hlist ! [
               "foo" , false , vec ! [  ] , 0 , [ 0 , 1 , 2 ] , (  ) ])
}