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POScala Parser Combinators tricks for recursive bnf?
 

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  1. POScala Parser Combinators tricks for recursive bnf?
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    <p>Im trying to match this syntax:</p> <pre><code>pgm ::= exprs exprs ::= expr [; exprs] expr ::= ID | expr . [0-9]+ </code></pre> <p>My scala packrat parser combinator looks like this:</p> <pre><code>import scala.util.parsing.combinator.PackratParsers import scala.util.parsing.combinator.syntactical._ object Dotter extends StandardTokenParsers with PackratParsers { lexical.delimiters ++= List(".",";") def pgm = repsep(expr,";") def expr :Parser[Any]= ident | expr~"."~num def num = numericLit def parse(input: String) = phrase(pgm)(new PackratReader(new lexical.Scanner(input))) match { case Success(result, _) =&gt; println("Success!"); Some(result) case n @ _ =&gt; println(n);println("bla"); None } def main(args: Array[String]) { val prg = "x.1.2.3;" + "y.4.1.1;" + "z;" + "n.1.10.30" parse(prg); } } </code></pre> <p>But this doesnt work. Either it "matches greedy" and tells me:</p> <pre><code>[1.2] failure: end of input expected x.1.2.3;y.4.1.1;z;n.1.10.30 </code></pre> <p>or if I change the <code>|</code> to a <code>|||</code> I get a stackoverflow:</p> <pre><code>Exception in thread "main" java.lang.StackOverflowError at java.lang.Character.isLetter(Unknown Source) at java.lang.Character.isLetter(Unknown Source) at scala.util.parsing.combinator.lexical.Lexical$$anonfun$letter$1.apply(Lexical.scala:32) at scala.util.parsing.combinator.lexical.Lexical$$anonfun$letter$1.apply(Lexical.scala:32) ... </code></pre> <p>I kindoff understand why I get the errors; what can I do to parse a syntax like the above? It doesnt seem that esoteric to me</p> <p>EDIT: Based on the paper referenced in <a href="http://scala-programming-language.1934581.n4.nabble.com/Packrat-parser-guidance-td1956908.html" rel="noreferrer">http://scala-programming-language.1934581.n4.nabble.com/Packrat-parser-guidance-td1956908.html</a> I found out that my program didnt actually use the new packrat parser.</p> <p>Ie. change <code>Parser[Any]</code> to <code>PackratParser[Any]</code> and use <code>lazy val</code> instead of <code>def</code></p> <p>I rewrote the above to this:</p> <pre><code>import scala.util.parsing.combinator.PackratParsers import scala.util.parsing.combinator.syntactical._ object Dotter extends StandardTokenParsers with PackratParsers { lexical.delimiters ++= List(".",";") lazy val pgm : PackratParser[Any] = repsep(expr,";") lazy val expr :PackratParser[Any]= expr~"."~num | ident lazy val num = numericLit def parse(input: String) = phrase(pgm)(new PackratReader(new lexical.Scanner(input))) match { case Success(result, _) =&gt; println("Success!"); Some(result) case n @ _ =&gt; println(n);println("bla"); None } def main(args: Array[String]) { val prg = "x.1.2.3 ;" + "y.4.1.1;" + "z;" + "n.1.10.30" parse(prg); } } </code></pre>
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