PARSERS BECAUSE BNF IS NOT JUST FOR ACADEMICS ANYMORE in Font

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CHAPTER 8 PARSERS BECAUSE BNF IS NOT JUST FOR ACADEMICS ANYMORE
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Yes, that definitely looks better. But, there s still a question of what these Parsers return when we pass a Seq[Char] into them. Or put another way, we want to get a Boolean true or false when we pass our input into them. So, let s define the return type of our expression:
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def p3: Parser[Boolean] = ('t' ~ 'r' ~ 'u' ~ 'e') | ('f' ~ 'a' ~ 'l' ~ 's' ~ 'e')
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That s what we want, but the compiler complains that it doesn t know how to convert the combined Parser into a Boolean. So, let s add a little bit of code to tell the Parser how to convert its result into a Boolean.
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def p3: Parser[Boolean] = ('t' ~ 'r' ~ 'u' ~ 'e' ^^^ true) | ('f' ~ 'a' ~ 'l' ~ 's' ~ 'e' ^^^ false)
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That works. The ^^^ method on Parser says, If we match the input, return this constant. We ve built a function that will match true or false and return the appropriate Boolean value if either pattern of characters is matched. But we can also use the characters that are part of the pattern to create the value returned when the input is applied to the function using the ^^ method. We ll define positiveDigit and digit Parsers:2
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def positiveDigit = elem('1') | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' def digit = positiveDigit | '0'
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In positiveDigit, we needed to specify elem('1') as the first part of the expression because '1' | '2' is a legal expression, so the implicit conversion of '1' to elem('1') does not take place. Note that we combined the positiveDigit Parser with elem('0') into a Parser that accepts all digits. Let s make this into a Parser that converts the digits into a Long:
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def long1: Parser[Long] = positiveDigit ~ rep(digit) ^^ { case (first: Char) ~ (rest: List[Char]) => (first :: rest).mkString.toLong }
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We create a Parser that matches a positiveDigit and then zero or more digits using rep(digit). If application of the predicate (positiveDigit ~ rep(digit)) succeeds, then we convert to a Long by applying the conversion function: case (first: Char) ~ (rest: List[Char]) => (first :: rest).mkString.toLong. The ^^ method on Parser causes the conversion function to be applied if the predicate succeeds. In this example, I was explicit about the types, but the type inferencer will get it right.
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2. The type inferencer will infer the correct return type, Parser[Char].
CHAPTER 8 PARSERS BECAUSE BNF IS NOT JUST FOR ACADEMICS ANYMORE
Let s tighten up the example a little by only accepting rest if it s fewer than 18 digits so we don t overflow the Long:
lazy val long2: Parser[Long] = positiveDigit ~ rep(digit) ^ { case first ~ rest if rest.length < 18 => (first :: rest).mkString.toLong }
In this case, we ve used the ^ method to connect the predicate to the conversion. In order for the Parser to succeed, we need to satisfy the predicate, and the partial function passed to ^ must be defined for the result of the predicate. In this case, the partial function will be satisfied if the length of rest is fewer than 18 characters. We ve also changed from a method to a lazy val. This is because the method does not do the parsing; rather, the method combines smaller Parsers into a single Parser. This building of the Parser need only happen once, and the resulting Parser can be used over and over, even simultaneously on multiple threads. With the basics under our belt, let s put our parser mojo to use.
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