Created
December 9, 2013 11:35
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Compute: A very simple language for addition/substraction.
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import scala.util.parsing.combinator._ | |
import scala.util.matching._ | |
object Compute { | |
sealed trait Expression | |
object Expression { | |
case class Literal(value: Int) extends Expression | |
case class Apply(operator: Operator, lhs: Expression, rhs: Expression) extends Expression | |
sealed trait Operator | |
object Operator { | |
case object Minus extends Operator | |
case object Plus extends Operator | |
} | |
} | |
import Expression._ | |
import Operator._ | |
object Parser extends RegexParsers { | |
def parse(input: String): Either[String, Expression] = parseAll(apply, input) match { | |
case Success(expression, _) => Right(expression) | |
case NoSuccess(msg, input) => Left(s"${input.pos}: $msg") | |
} | |
private val operators = Map(Minus -> "-", Plus -> "+") | |
private def apply: Parser[Expression] = (literal ~ (operator ~ literal).*).map { | |
case x ~ Nil => x | |
case lhs ~ ((op ~ rhs) :: xs) => xs.foldLeft(Apply(op, lhs, rhs)) { case (e, (op ~ rhs)) => Apply(op, e, rhs) } | |
} | |
private def literal: Parser[Literal] = ("\\d+"r).map(_.toInt).map(Literal(_)) | |
private def operator: Parser[Operator] = operators.map { case (op, sym) => literal(sym).map { _ => op } }.reduce(_ | _) | |
} | |
object Interpreter { | |
def eval: Expression => Int = _ match { | |
case Apply(op, lhs, rhs) => operators(op)(eval(lhs), eval(rhs)) | |
case Literal(value) => value | |
} | |
private val operators = Map[Operator, (Int, Int) => Int](Minus -> (_ - _), Plus -> (_ + _)) | |
} | |
def apply(expression: String): Either[String, Int] = Parser.parse(expression).right.map(Interpreter.eval) | |
} |
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