Function combine_language::expression_parser

pub fn expression_parser<O, P, F, T>(term: P, op: O, f: F) -> Expression<O, P, F> where O: Parser<Output=(T, Assoc)>, P: Parser<Input=O::Input>, F: Fn(P::Output, T, P::Output) -> P::Output

Constructs an expression parser out of a term parser, an operator parser and a function which combines a binary expression to new expressions.

 use self::Expr::*;
 #[derive(PartialEq, Debug)]
 enum Expr {
      Id(String),
      Op(Box<Expr>, &'static str, Box<Expr>)
 }
 fn op(l: Expr, o: &'static str, r: Expr) -> Expr {
     Op(Box::new(l), o, Box::new(r))
 }
 fn id(s: &str) -> Expr {
     Id(String::from(s))
 }
 let op_parser = string("+").or(string("*"))
     .map(|op| {
         let prec = match op {
             "+" => 6,
             "*" => 7,
             _ => unreachable!()
         };
         (op, Assoc { precedence: prec, fixity: Fixity::Left })
     })
     .skip(spaces());
 let term = many(letter())
     .map(Id)
     .skip(spaces());
 let mut parser = expression_parser(term, op_parser, op);
 let result = parser.parse("a + b * c + d");
 assert_eq!(result, Ok((op(op(id("a"), "+", op(id("b"), "*", id("c"))), "+", id("d")), "")));