do-notation.sjs

syntax seq = function (ctx) {
  let monad = ctx.next().value;
  let bodyCtx = ctx.next().value.inner(); 

  function next() {
    let result
    let n = bodyCtx.next()
    if(n.done) return #``
    let stx = n.value
    if (stx.isIdentifier()) { 
      // eat `:<-`
      bodyCtx.next();
      bodyCtx.next();
      
      let r = bodyCtx.next('expr').value;
      return #`${r}.chain(x => { let ${stx} = x; let ret = ${next()}; return ret; })`;
    } else if (stx.isKeyword('return')) {
      let r = bodyCtx.next('expr').value;
      return #`${monad}.of(${r})`;
    } else {
      throw new Error('unknown syntax: ' + stx);
    }
  }
  return next()
}

//let x = null;

seq Array {
  x <- [1,2,3]
  y <- [2,4,6]
  z <- [3,6,9]
  return x + y + z
}