CODEWITHSUNDEEP
scalatemplatestypessbtscala-2.10
A T
A T

Reputation: 13846

Template types not working in Scala?

Here are the errors I received when running sbt run on this listing [3.5] from [1]:

import util.parsing.combinator.JavaTokenParsers

trait ArithParser extends JavaTokenParsers {
    type T
    def expr: Parser[T] = chainl1(term,   "+" ^^^ Add |  "-" ^^^ Sub)
    def term            = chainl1(factor, "*" ^^^ Mul |  "/" ^^^ Div)
    def factor          = floatingPointNumber  ^^ Num | "(" ~> expr <~ ")"

    def Add = (T,T)  => T
    def Sub = (T,T)  => T
    def Mul = (T,T)  => T
    def Div = (T,T)  => T
    def Num = String => T
}

trait DirectEvaluation {
    type T = Double
    val Add = (_: Double) + (_:Double)
    val Sub = (_: Double) - (_:Double)
    val Mul = (_: Double) * (_:Double)
    val Div = (_: Double) / (_:Double)
    val Num = (_: String).toDouble
}

trait ASTBuilding {
    type T = Expr
    sealed abstract class Expr

    case class Add(e1: Expr, e2: Expr) extends Expr
    case class Sub(e1: Expr, e2: Expr) extends Expr
    case class Mul(e1: Expr, e2: Expr) extends Expr
    case class Div(e1: Expr, e2: Expr) extends Expr
    case class Num(e: String) extends Expr
}

object Interpreter extends ArithParser with DirectEvaluation
object Compiler extends ArithParser with ASTBuilding

object Main extends ArithParser {
    def main(args: Array[String]) {
        val defArgs = if (args.isEmpty) Array("10.5 - 4*2") else args
        val parser: ArithParser = if(defArgs.head === "eval") {
                println("Interpreter!"); Interpreter
            } else {
                println("Compiler!"); Compiler
            }
        defArgs.tail foreach {arg =>
            println("Input: " + arg)
            println("Output: " + parseAll(parser.expr, arg))
        }
    }
}

[1] E. Labun, “Combinator Parsing in Scala,” Technische Hochschule Mittelhessen, 2012.

How do I fix these errors?

Upvotes: 0

Views: 92

Answers (1)

drexin
drexin

Reputation: 24403

The (T,T) => T is a type annotation, so you have to replace the = with a :.

def Add: (T,T) => T

That is also exactly how it is in the thesis you refer to.

The === also does not exist in the listing, it's just a ==.

And you have to call parseAll on the parser:

parser.parseAll(...)

It seems to me that you are missing the basics of scala programming, otherwise you could have easily fixed those errors yourself. My advice is, read a good scala book to get a basic understanding of the language before you start with advanced topics.

Upvotes: 1

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