letsyntax.js

let let = macro {
  case { $mac syntax $id:ident $punc = $rhs:expr } => {
    var mac = #{ $mac };
    var id  = #{ $id };
    var val = #{ $val };
    var arg = #{ $($rhs) };
    var punc = #{ $punc };

    if (punc[0].token.type !== parser.Token.Punctuator ||
        punc[0].token.value !== '...') {
      throw new SyntaxError('Unexpected token: ' + punc[0].token.value +
                            ' (expected ...)');
    }

    if (id[0].token.value[0] !== '$') {
      throw new SyntaxError('Syntax identifiers must start with $: ' + 
                            id[0].token.value);
    }

    return res = [
      makeIdent('match', mac),
      makePunc('.'),
      makeIdent('patternEnv'),
      makeDelim('[]', [makeValue(id[0].token.value)]),
      makePunc('='),
      makeDelim('{}', [
        makeIdent('level'), makePunc(':'), makeValue(1), makePunc(','),
        makeIdent('match'), makePunc(':'), arg[0], makePunc('.'),
        makeIdent('map'), makeDelim('()', #{
          function(t) {
            return {
              level: 0,
              match: [t]
            };
          }
        })
      ])
    ];
  }
  case { $mac syntax $id:ident = $rhs:expr } => {
    var mac = #{ $mac };
    var id  = #{ $id };
    var val = #{ $val };
    var arg = #{ $($rhs) };

    if (id[0].token.value[0] !== '$') {
      throw new SyntaxError('Syntax identifiers must start with $: ' + 
                            id[0].token.value);
    }

    return res = [
      makeIdent('match', mac),
      makePunc('.'),
      makeIdent('patternEnv'),
      makeDelim('[]', [makeValue(id[0].token.value)]),
      makePunc('='),
      makeDelim('{}', [
        makeIdent('level'), makePunc(':'), makeValue(0), makePunc(','),
        makeIdent('match'), makePunc(':'), arg[0]
      ])
    ];
  }
  case { _ } => {
    return #{ let };
  }
}

macro $test {
  case { $mac () } => {
    let syntax $foo ... = [makeValue(12), makePunc('+'), makeValue(42)];
    return #{ $foo ...  }
  }
}

$test()