rpn.pegjs

{
  function buildBinaryExpression(op, left, right) {
    return {
      type: 'BinaryExpression',
      operator: op,
      left: left,
      right: right
    }
  }
}

Expression
  = head:Integer tails:(ExpressionRest)* {      
      return tails.reduce(function(prev, cur){        
        return buildBinaryExpression(cur.operator, prev, cur.right);
      }, head);
    }

ExpressionRest
  = _ f:Expression _ op:Operator {
    return {
      operator: op,
      right: f
    }
  }

Operator
  = "+" 
  / "-"
  / "*"
  / "/"

Integer "integer"
  = [0-9]+ {
    return parseInt(text(), 10); 
  }

_ "whitespace"
  = " "*