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CHAPTER 8 PARSERS BECAUSE BNF IS NOT JUST FOR ACADEMICS ANYMORE
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Next, we define factor, which is either a number or parentheses around a sumExpr. Because sumExpr, prodExpr, and factor reference each other and, thus, are recursive, we must define the type of at least one of the three vals so the type inferencer can do its work.
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lazy val factor: Parser[Double] = floatingPointNumber ^^ (_.toDouble) | "(" ~> sumExpr <~ ")"
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We convert floatingPointNumber into a Double by passing a function that converts a String to a Double. Next, we use ~> and <~ to discard the parentheses around sumExpr. Calc mixes in RunParser, so we have to define the abstract RootType type and the root method.
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type RootType = Double def root = sumExpr }
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That s it. We ve defined the conversions from the Strings and List to Doubles. Let s see how well it works.
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scala> Calc.run("1")
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res0: Calc.ParseResult[Calc.RootType] = [1.2] parsed: 1.0
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scala> Calc.run("1 + 1")
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res1: Calc.ParseResult[Calc.RootType] = [1.6] parsed: 2.0
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scala> Calc.run("1 + 1 / 17")
res2: Calc.ParseResult[Calc.RootType] = [1.11] parsed: 1.0588235294117647
scala> Calc.run("(1 + 1) / 17")
res3: Calc.ParseResult[Calc.RootType] = [1.13] parsed: 0.11764705882352941
CHAPTER 8 PARSERS BECAUSE BNF IS NOT JUST FOR ACADEMICS ANYMORE
In this section, we ve converted from BNF description of the grammar to a running application using Scala s parser combinator library. The example, a four-function calculator, was very simple. In the next section, we re going to tackle something more complex. We re going to parse JSON.
JSON Parsing
JSON, or JavaScript Object Notation, is a common and lightweight mechanism for exchanging data over the Internet. JSON is a subset of JavaScript and can be parsed by the fast, stable, efficient JavaScript parsers that are built into browsers. JSON is as readable as XML, more compact than XML and corresponds to the types built into most dynamic languages: strings, numbers, Booleans, arrays, and dictionaries or hash maps. In this section, we re going to follow the ECMAScript spec to build our Parser: we re going to copy and paste the spec into our Scala code and literally code our Parser to the spec as we go. The only new concept in this Parser is the regular-expression Parser. The RegexParsers trait has an implicit conversion from a regular expression to a Parser[String]. If the input matches the regular expression, the matching string will be returned by the Parser. We use this in the definition of spaces: """\s*""".r.5 The triple quotes contain a String that does not support any form of escaping, so \s is the literal \s, rather than escaping s, and the regular expression \s stands for space. The .r at the end of the String converts the String into a regular expression. The code in Listing 8-2 creates a JSON Parser.
Listing 8-2. ECMAScript Spec -> JSON Parser import scala.util.parsing.combinator._ object JSON extends RegexParsers with RunParser { // translation from ECMAScript spec // http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf /* Whitespace */ lazy val spaces: Parser[String] = """\s*""".r
5. See the Java RegEx Pattern documentation: http://java.sun.com/javase/6/docs/api/java/util/ regex/Pattern.html.
CHAPTER 8 PARSERS BECAUSE BNF IS NOT JUST FOR ACADEMICS ANYMORE
/* Source characters (any valid unicode character) */ lazy val sourceCharacter: Parser[Char] = elem("Source Character", c => true) /* HexDigit :: one of 0 1 2 3 4 5 6 7 8 9 a b c d e f A B */ lazy val hexDigit: Parser[Char] = elem("Hex Digit", c => ((c >= '0' && (c >= 'a' && (c >= 'A' && /* 7.8 Literals Syntax Literal :: NullLiteral BooleanLiteral NumericLiteral StringLiteral */ lazy val literal: Parser[Any] = nullLiteral | booleanLiteral | numericLiteral | stringLiteral /* 7.8.1 Null Literals Syntax NullLiteral :: null */ lazy val nullLiteral: Parser[Any] = spaces ~ "null" ~ spaces ^^^ None /* 7.8.2 Boolean Literals Syntax BooleanLiteral :: true
C D E F
c <= '9') || c <= 'f') || c <= 'F')))
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