Class for providing variable substitution in a command at runtime.

CaseClassMacros.scala

import scala.language.experimental.macros
import scala.reflect.macros.blackbox

/**
  * Macro bundle for case class macros.
  */
class CaseClassMacrosImpl(val c: blackbox.Context) {

  import c.universe._

  /**
    * Creates a new conversion class that provides evidence for converting a case class to a sequence of `String`.
    *
    * @tparam T the type of the case class
    * @return the conversion type
    */
  def fieldConverter[T: c.WeakTypeTag]: c.Expr[CaseClassToSeq[T]] = {
    val symbol = c.weakTypeOf[T].typeSymbol
    val traitSymbol = c.weakTypeOf[CaseClassToSeq[T]].typeSymbol
    val name = c.freshName(traitSymbol.name.toTypeName)
    val fields = fieldsOf(c.Expr[T](q"caseClass"))
    val result =
      q"""final class $name extends $traitSymbol[$symbol] {
          override def toSeq(caseClass: $symbol): Seq[String] = $fields
        }
        new $name
     """
    c.Expr[CaseClassToSeq[T]](result)
  }

  /**
    * Retrieves the fields of a case class as a sequence in the order that they were defined.
    *
    * @tparam T the type of the case class
    * @return the fields of the case class
    */
  def fieldsOf[T: c.WeakTypeTag](caseClass: c.Expr[T]): c.Expr[Seq[String]] = {
    val classType = caseClassOf[T]
    val getters = caseFieldGetters(classType)
    val result = withCachedExpr(caseClass) { term =>
      val fieldsAsStrings = getters.map(param => q"$term.$param.toString")
      q"Seq(..$fieldsAsStrings)"
    }
    c.Expr[Seq[String]](result)
  }

  /**
    * Determines the `Type` of the given case class or aborts if it is not a case class.
    *
    * @tparam T the case class type tag
    * @return the weak type of the case class
    */
  def caseClassOf[T: c.WeakTypeTag]: c.Type = {
    val classType = c.weakTypeOf[T]
    val symbol = classType.typeSymbol
    if (!symbol.isClass || !symbol.asClass.isCaseClass)
      c.abort(c.enclosingPosition, s"$symbol is not a case class")
    classType
  }

  /**
    * Finds the getters of each field that is also a parameter in the first parameter list of the primary constructor of
    * a case class.
    *
    * @param classType the type of the case class
    * @return the terms in the same order that they were declared
    */
  def caseFieldGetters(classType: c.Type): List[TermSymbol] = {
    classType.decls.sorted.filter(_.isTerm).map(_.asTerm).
      filter(term => term.isCaseAccessor && term.isGetter)
  }

  /**
    * Assigns the result of evaluating an expression to a value and then provides the term of that value to a
    * function that can then reuse that cached value it constructing another tree.
    *
    * This is part of good macro hygiene.
    *
    * @param expr  the expression to evaluate
    * @param other the function that uses the value
    * @tparam T the type of the expression
    * @return a block containing the value and result of the function
    */
  def withCachedExpr[T](expr: c.Expr[T])(other: (TermName) => c.Tree): c.Tree = {
    val resolvedTerm = TermName(c.freshName)
    val resolvedExpr = q"val $resolvedTerm = $expr"
    val exprs = List(resolvedExpr, other(resolvedTerm))
    q"..$exprs"
  }
}

/**
  * Macros for working with case classes.
  */
object CaseClassMacros {

  /**
    * Creates a sequence of the fields of a case class.
    *
    * The fields that are returned are the parameters in the first parameter list of the primary constructor.
    *
    * Each field is access via its getter and is converted into a `String` via its `toString` method.
    *
    * @param caseClass the case class instance
    * @tparam T the type of the case class
    * @return a sequence of the fields in the order that they were declared
    */
  def fieldsOf[T](caseClass: T): Seq[String] = macro CaseClassMacrosImpl.fieldsOf[T]
}

/**
  * Converts a case class to a sequence of `String`.
  *
  * This uses a technique known as
  * <a href="http://docs.scala-lang.org/overviews/macros/implicits#fundep-materialization">Fundep materialization</a>
  * and is useful when combined with Type Class Pattern and/or
  * <a href="http://docs.scala-lang.org/tutorials/FAQ/finding-implicits.html#context-bounds">Context Bounds</a>.
  */
object CaseClassToSeq {

  /**
    * Provides an implicit conversion from a case class to a sequence of `String`.
    *
    * This should be used to provide evidence in Context Bounds.
    *
    * @tparam T the type of the case class
    * @return the conversion type
    */
  implicit def materializeCaseClassToSeq[T]: CaseClassToSeq[T] = macro CaseClassMacrosImpl.fieldConverter[T]
}

/**
  * Represents a conversion from a case class to a sequence of `String`.
  *
  * @tparam T the type of the case class
  */
trait CaseClassToSeq[T] {

  /**
    * Converts the instance to a sequence of `String`.
    *
    * @param caseClass the case class instance
    * @return the case class as a sequence
    */
  def toSeq(caseClass: T): Seq[String]
}

command.conf

copy {
  # This is the command for copying a file.
  # If using the Vagrant VM the settings can be obtained from ssh-config.
  command = ["""C:\Program Files (x86)\PuTTY\pscp.exe""", "-P", "2222",
    "-i", ".vagrant/machines/default/virtualbox/private_key.ppk", "-batch",
    "-hostkey", ${agent.test.remote.hostkey},
    "-l", "vagrant", "$source", "127.0.0.1:/home/vagrant/$target"]
  # Variables in the command.
  variables {
    # The local path of the file to be copied.
    source = "$source"
    # The target path of the file on the remote.
    target = "$target"
  }
}

CopyCommand.scala

/**
  * A command for copying files from the local machine to a remote machine.
  *
  * @param command   the command to be executed
  * @param variables the variable names in the command
  */
case class CopyCommand(override val command: Seq[String], variables: CopyVariables)
  extends SafeVariableCommand(command, variables) {
}

/**
  * The variables for copying.
  *
  * @param source the source variable
  * @param target the target variable
  */
case class CopyVariables(source: String, target: String)

Example.scala

val configs = ConfigFactory.load("command")
val copyCmd = configs.get[CopyCommand]("copy").value
val variables = CopyVariables("""C:\Users\me\test.txt""", "~/test.txt")
val cmd = copyCmd.substitute(variables)
cmd !

VariableCommand.scala

import scala.annotation.tailrec

/**
  * This represents a command which contains named variables to be substituted at runtime.
  *
  * The command is a sequence of strings where each string is a command or argument. Each command or argument may
  * contain zero, one or more variables.
  *
  * Variable substitution works by replacing a variable name with another value. There is no special escaping since the
  * caller is free to choose whatever name of the variable that is guaranteed to be unambiguous.
  *
  * Determining where variables are and what they need to be replaced with is done when the command is created.
  *
  * This is a generic implementation so the number of variables permitted is not fixed. However this means that the
  * caller is responsible for providing the correct number of values for substitution.
  *
  * Initialisation is worst case the number of commands multiplied by the number of variable names. However this is
  * intended to be created once and used multiple times so that cost is amortised. Substitution is linear time,
  * proportional to the number of commands and arguments plus the number of variables. (At least this is the idea)
  *
  * This is similar to Scala's string interpolation and [[StringContext]] however it works for variable strings as
  * opposed to string literals.
  *
  * @tparam T type of variables that are accepted
  */
trait VariableCommand[T] {

  /**
    * The command that contains variables.
    */
  val command: Seq[String]

  /**
    * The names of each variable to be replaced.
    */
  val names: T

  implicit def toSeq(variables: T): Seq[String]

  private type VariableSubstitution = (Seq[String]) => String

  private val namesList: List[String] = names.toList

  private val substitutions: Seq[VariableSubstitution] = createSubstitutions

  /**
    * Substitutes the variable names in the command with the values.
    *
    * The order of the values must match the order of the variable names.
    *
    * @param values the values to use as substitutions
    * @return the command with the substituted values
    * @throws IllegalArgumentException if the number of values does not match the number of variable names
    */
  def substitute(values: T): Seq[String] = substitutions.map(build => build(values))

  private def createSubstitutions: Seq[VariableSubstitution] = command.map(createSubstitution)

  private def createSubstitution(arg: String): VariableSubstitution = {
    // Recursively go through each position in the argument and check to see if there is a variable at that position.
    // If there is then accumulate the part between where the last match ended and where the current match starts,
    // followed by the index of the variable (to be used for substitution with the values later). Then continue
    // searching from where the variable ends.
    // Once we reach the end then accumulate any remaining characters and reverse (since we accumulated by prepending to
    // the list) and then convert it to a substitution rule.
    @tailrec
    def loop(currentPos: Int, lastMatchEnd: Int, accum: List[Either[String, Int]]): VariableSubstitution = {
      if (currentPos < arg.length) {
        checkVariable(currentPos, arg) match {
          case Some(result) =>
            val (nextPos, varIndex) = result
            val nextAccum = if (currentPos > lastMatchEnd) {
              val beforeVar = arg.substring(lastMatchEnd, currentPos)
              Right(varIndex) :: Left(beforeVar) :: accum
            } else {
              Right(varIndex) :: accum
            }
            loop(nextPos, nextPos, nextAccum)
          case None         => loop(currentPos + 1, lastMatchEnd, accum)
        }
      } else {
        val finalAccum = if (lastMatchEnd < arg.length) Left(arg.substring(lastMatchEnd)) :: accum else accum
        mergeChoices(finalAccum.reverse)
      }
    }
    loop(0, 0, Nil)
  }

  private def checkVariable(startPos: Int, arg: String): Option[(Int, Int)] = {
    // Recursively check each position from the start until we find:
    // - a complete match, or
    // - there are no more names left that could be a match, or
    // - we have gone past the end of the argument
    // If a match is found then return both the position after the match (which becomes the next position to search
    // from) and also the index of the variable (this is used for substitution later on).
    @tailrec
    def loop(currentPos: Int, depth: Int, possibleNames: List[(String, Int)]): Option[(Int, Int)] = {
      // If the depth is the length of the term then we have already matched every character.
      possibleNames.find(_._1.length == depth) match {
        case Some(term) => Some(currentPos, term._2)
        case None       =>
          if (possibleNames.isEmpty || currentPos >= arg.length) None
          else {
            val c = arg.charAt(currentPos)
            val matching = possibleNames.filter(_._1.charAt(depth) == c)
            loop(currentPos + 1, depth + 1, matching)
          }
      }
    }
    loop(startPos, 0, namesList.zipWithIndex)
  }

  private def mergeChoices(choices: List[Either[String, Int]]): VariableSubstitution = {
    // Now we have all the "choices" where a choice is either the part of the argument to be copied verbatim (the
    // left) or the index of the variable to be substituted (the right).
    // Create a function which, given the variable values (that have the same indicies as the names), will apply each
    // choice to build up the resulting argument.
    @tailrec
    def loop(choices: List[Either[String, Int]], builder: StringBuilder)(values: Seq[String]): String = {
      choices match {
        case head :: tail =>
          head match {
            case Left(str)    => builder.append(str)
            case Right(index) => builder.append(values(index))
          }
          loop(tail, builder)(values)
        case Nil          => builder.toString
      }
    }
    // This actually starts the loop. We need to ensure that each function creates its own builder.
    def start(values: Seq[String]): String = loop(choices, new StringBuilder)(values)
    // Slight optimisation (both now and for substitutions) for when there are no substitutions.
    // Be careful with comparing the length of the list since its worst case complexity is the length of the list.
    if (choices.lengthCompare(1) == 0 && choices.head.isLeft) _ => choices.head.left.get
    else start _
  }

}

/**
  * An extension to [[VariableCommand]] which does not do any compile time checking of variables.
  */
class UnsafeVariableCommand(val command: Seq[String], val names: String*) extends VariableCommand[Seq[String]] {
  override implicit def toSeq(variables: Seq[String]): Seq[String] = {
    require(variables.length == names.length, s"The number of variables (${variables.length}) must be equal to the " +
      s"number of variable names (${names.length})")
    variables
  }
}

/**
  * An extension to [[VariableCommand]] which provides compile time checking of variables.
  *
  * @tparam T the type of variables
  */
class SafeVariableCommand[T: CaseClassToSeq](val command: Seq[String], val names: T) extends VariableCommand[T] {
  // Be careful with initialisation order in here.
  // The VariableCommand constructor needs to use toSeq so the fields it uses need to be initialised lazily.

  // Conjure up the implicit converter from the context bounds.
  private lazy val converter = implicitly[CaseClassToSeq[T]]

  protected lazy val nameSeq = converter.toSeq(names)

  override implicit def toSeq(variables: T): Seq[String] = converter.toSeq(variables)
}

I haven't tested this at any great length at all so if there are any bugs or improvements then please feel free to comment.

Modified to include a type safe version and an example of how it can be used.

The safe version is truly safe now. No need to implement a toSeq method. Macros to the rescue!