Deciphering Scala Errors When Returning Functions From Functions

muke Published at Dev

muke

I'm trying to build a basic combinator parser in Scala and when trying to compile the most basic parser with it (the parser of the letter 'A') I get three syntax errors from scalac which I don't understand. I'm looking at my code and can't figure out which parts of the syntax are incorrect, even when comparing with examples online. Could someone please explain what parts of my Scala code are wrong here?

Code:

import lexer
import scala.Option

object Main {
  def main(args: Array[String]) {
    val Lexer = new lexer.Lexer
    val tokens = Lexer.lex(args(0))
    val parseA = satsify(t => t.key == "A")
    println(parseA(tokens))
  }

  def satsify(predicate: Token => Bool): List(Token) => Option[(Token, List(Token))] = {
    tl: List(Token) => match tl { 
        case tl.isEmpty => None 
        case predicate(tl(0)) => Some(tl(0), tl.tail)
        case _ => None
    }
  }
}

Errors:

combParser.scala:2: error: '.' expected but ';' found.
import scala.Option
^
combParser.scala:12: error: '=' expected but '(' found.
  def satsify(predicate: Token => Bool): List(Token) => Option[(Token, List(Token))] = {
                                             ^
combParser.scala:19: error: illegal start of simple expression
}
^
three errors found

Mario Galic

There are quite a few syntax errors, mainly

type constructor takes a type parameters using square brackets, thus List[Token] instead of List(Token)
guards inside pattern matching cannot be used directly, thus case v if predicate(tl(0)) => instead of case predicate(tl(0)) =>

Taking above into account try

  def satsify(predicate: Token => Boolean): List[Token] => Option[(Token, List[Token])] = {
    (tl: List[Token]) => tl match {
      case Nil => None
      case head :: tail if predicate(head) => Some(head, tail)
      case _ => None
    }
  }

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