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CHAPTER 8 PARSERS BECAUSE BNF IS NOT JUST FOR ACADEMICS ANYMORE
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/* EscapeCharacter :: SingleEscapeCharacter DecimalDigit x u */ lazy val escapeCharacter: Parser[Unit] = (singleEscapeCharacter | decimalDigit | 'x' | 'u') ^^^ () /* HexEscapeSequence :: x HexDigit HexDigit */ lazy val hexEscapeSequence: Parser[Char] = 'x' ~> hexDigit ~ hexDigit ^^ {case d1 ~ d2 => Integer.parseInt(d1.toString + d2, 16).toChar} /* UnicodeEscapeSequence :: u HexDigit HexDigit HexDigit HexDigit */ lazy val unicodeEscapeSequence = 'u' ~> hexDigit ~ hexDigit ~ hexDigit ~ hexDigit ^^ {case d1 ~ d2 ~ d3 ~ d4 => Integer.parseInt(d1.toString + d2 + d3 + d4, 16).toChar} /* ArrayLiteral : [ Elision(opt) ] [ ElementList ] [ ElementList , Elision(opt) ] */ lazy val arrayLiteral: Parser[List[Any]] = spaces ~ '[' ~ spaces ~> elementList <~ spaces ~ opt(elision) ~ spaces ~ ']' ~ spaces /* ElementList : Elision(opt) AssignmentExpression ElementList , Elision(opt) AssignmentExpression */
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CHAPTER 8 PARSERS BECAUSE BNF IS NOT JUST FOR ACADEMICS ANYMORE
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lazy val elementList: Parser[List[Any]] = repsep(spaces ~> jsonObject, elision) /* Elision : , Elision , */ lazy val elision: Parser[Unit] = rep1(spaces ~ ',' ~ spaces) ^^^ () /* ObjectLiteral : { } { PropertyNameAndValueList } */ lazy val objectLiteral: Parser[Map[String, Any]] = spaces ~ '{' ~ spaces ~ '}' ~ spaces ^^^ Map[String, Any]() | spaces ~ '{' ~ spaces ~> propertyNameAndValueList <~ spaces ~'}' ~ spaces ^^ (vl => Map(vl :_*)) /* PropertyNameAndValueList : PropertyName : AssignmentExpression PropertyNameAndValueList , PropertyName : AssignmentExpression */ lazy val propertyNameAndValueList:Parser[List[(String, Any)]] = rep1sep((spaces ~> propertyName) ~ (spaces ~ ':' ~ spaces ~> jsonObject) ^^ { case n ~ v => (n, v)}, spaces ~ ',' ~ spaces) /* PropertyName : Identifier StringLiteral NumericLiteral */ lazy val propertyName: Parser[String] = stringLiteral | numericLiteral ^^ (_.longValue.toString) | identifier
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/* IdentifierName :: IdentifierStart IdentifierName IdentifierPart */ lazy val identifier: Parser[String] = identifierStart | identifier ~ identifierPart ^^ { case a ~ b => a+b } /* IdentifierStart :: UnicodeLetter $ _ */ lazy val identifierStart: Parser[String] = '$' ^^^ "$" | '_' ^^^ "_" | unicodeLetter ^^ (_.toString) /* IdentifierPart :: IdentifierStart UnicodeDigit */ lazy val identifierPart: Parser[String] = identifierStart | unicodeDigit ^^ (_.toString) lazy val unicodeLetter = elem("Letter", Character.isLetter) lazy val unicodeDigit = elem("Letter", Character.isDigit) lazy val jsonObject: Parser[Any] = objectLiteral | arrayLiteral | literal type RootType = Any def root = jsonObject }
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So, that s it. Let s see how the Parser does with various JSON input:
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scala> JSON.run("1")
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res0: JSON.ParseResult[JSON.RootType] = [1.2] parsed: 1.0
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scala> JSON.run("'Elwood Eats Mice'")
res1: JSON.ParseResult[JSON.RootType] = [1.19] parsed: Elwood Eats Mice
scala> JSON.run("'Elwood \u0021 Eats Mice'")
res2: JSON.ParseResult[JSON.RootType] = [1.21] parsed: Elwood ! Eats Mice
scala> JSON.run("[1,2,3,]")
res3: JSON.ParseResult[JSON.RootType] = [1.9] parsed: List(1.0, 2.0, 3.0)
scala> JSON.run("{a:true, 'hello':33}")
res4: parsed: Map(a -> true, hello -> 33.0)
Great. We ve converted the ECMAScript spec into a running Parser. Let s see what we can layer on top of the JSON Parser. Let s build some code that will parse Twitter APIs.
Twitter JSON Parsing
JSON is great for storing and transmitting dynamically typed information, but Scala is a statically typed language. It would be great to be able to convert the dynamic stuff parsed from JSON data into statically typed Scala. As an example, we re going to use some of the
CHAPTER 8 PARSERS BECAUSE BNF IS NOT JUST FOR ACADEMICS ANYMORE
Twitter APIs that return Twitter s public timeline.6 We ll request the data in JSON format, parse it, and then run the parsed data through a conversion to create Scala instances that represent the Twitter data.
Utilities
In order to use Scala s for comprehension to safely and easily extract data from the result of the JSON parsing, we re going to write a helper trait, SafeMap, which does type-safe testing and casting. SafeMap has an is method that takes an implicit parameter, man, which is a Manifest representing the type T. The Scala compiler correctly builds the Manifest and passes it to the method.7 We use the Manifest to get the class for T and do testing. First we have the code in Listing 8-3 and then a dissection.
Listing 8-3. Type-Safe Map Utility trait SafeMap { import scala.reflect._ class SafeCast(in: Any) { def is[T](implicit man: Manifest[T]): Option[T] = { // special case for Boolean val cls = if (man.toString == "boolean") classOf[java.lang.Boolean] else man.erasure in match { case null => None case t: AnyRef if cls.isAssignableFrom(t.getClass) => Some(t.asInstanceOf[T]) case _ => None } } }
6. http://apiwiki.twitter.com/REST+API+Documentation. Alex Payne, co-author with Dean Wampler of the fine book Programming Scala (O Reilly, forthcoming), maintains Twitter s APIs. 7. Manifest is an experimental feature introduced in Scala 2.7.2, but it s so very useful that I m including it in the book.
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