Parser Combinator ^^ with Optional Values

Question

I'm trying to modify an example from DSLs in Action.

Originally, this code was used to parse items followed by account.

  lazy val order: Parser[Order] = 
    items ~ account_spec ^^ {
      case i ~ a => Order(i, a)
    }

The following text could be parsed with the above parser:

(100 IBM shares to buy at max 45) for account "A1234
------------- item -------------  ------ account ----

But, I'd like to add FOO and an optional not values to the parser:

  lazy val order: Parser[Order] = 
    items <~ "FOO" ~> ("not"?) ~ account_spec ^^ {
      case i ~ n ~ a => println(n); Order(i, a)
    }

FOO must follow account and, optionally, not will follow.

Example:

(100 IBM shares to buy at max 45) FOO not for account "A1234
---------- item ------------------ --- --- ------ account ----

However, the above code gives me this compile-time error:

[WARNING] ....\OrderDsl.scala:19: error: constructor cannot be instantiated to 
expected type;
[WARNING]  found   : ch8.trading.semantic.dsl.OrderDsl.~[a,b]
[WARNING]  required: ch8.trading.semantic.dsl.AST.Items
[WARNING]       case i ~ n ~ a => println(n); Order(i, a)
[WARNING]                  ^

How can I modify the case statement to support parsing an optional "not" value?

senia · Accepted Answer · 2014-01-09 18:32:19Z

3

a <~ "FOO" ~> b means "ignore results of parsers "FOO" and b and return the result of a".

Rewrite your method like this:

lazy val order: Parser[Order] = 
  items ~ opt("FOO") ~ opt("not") ~ account_spec ^^ {
    case i ~ _ ~ n ~ a => println(n); Order(i, a)
  }

General mnemonic for ~> and <~: you can ignore from start to operator or from operator to end, but not the middle part of expression.

edited Jan 9, 2014 at 18:32

answered Jan 9, 2014 at 17:24

senia

38.1k6 gold badges93 silver badges134 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Kevin Meredith Over a year ago

Thanks, @senia. Your answer cleared up my mis-use of ~> and <~ together.

senia Over a year ago

@KevinMeredith: Actually you could use items ~ (opt("FOO") ~> opt("not") ~ account_spec) ^^ { case i ~ (n ~ a) => ...}.

Kevin Meredith Over a year ago

If we added case i ~ foo ~ n ..., is it true that foo could only equal None and Some("FOO")? My pattern match on foo match { case None => ... case Some("FOO") is telling me that the match is non-exhaustive.

senia Over a year ago

@KevinMeredith: yes, but compiler knows only the type of foo. You should use foo.isDefined.

Collectives™ on Stack Overflow

Parser Combinator ^^ with Optional Values

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related