Scala example source code file (Binders.scala)

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Java - Scala tags/keywords

list, mappable, mappable, mapper, mapper, nameelement, nameelement, option, scope, string, t, t, todo, underbinder

The Scala Binders.scala source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2006-2011, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala.util.parsing.ast

import scala.collection.mutable.Map

//DISCLAIMER: this code is highly experimental!

  // TODO: avoid clashes when substituting
  // TODO: check binders in the same scope are distinct

/** <p>
 *    This trait provides the core Scrap-Your-Boilerplate abstractions as
 *    well as implementations for common datatypes.
 *  </p>
 *  <p>
 *    Based on Ralph Laemmel's <a target="_top"
 *    href="http://homepages.cwi.nl/~ralf/publications.html">SYB papers</a>.
 *  </p>
 *
 * @author Adriaan Moors 
 */
trait Mappable {
  trait Mapper { def apply[T <% Mappable[T]](x: T): T } /* TODO: having type `Forall T. T => T' is too strict: 
  sometimes we want to allow `Forall T >: precision. T => T' for some type `precision', so that, 
  beneath a certain threshold, we have some leeway.
  concretely: to use gmap for substitution, we simply require that ast nodes are mapped to ast nodes, 
  we can't require that the type is preserved precisely: a Name may map to e.g., a MethodCall 
  */

  trait Mappable[T] {
    // one-layer traversal
    def gmap(f: Mapper): T
    //  everywhere f x = f (gmapT (everywhere f) x)
    def everywhere(f: Mapper)(implicit c: T => Mappable[T]): T =
      f(gmap(new Mapper { def apply[T <% Mappable[T]](x: T): T = x.everywhere(f)}))
  }

  implicit def StringIsMappable(s: String): Mappable[String] =
    new Mappable[String] {
      def gmap(f: Mapper): String = f(s) 
    }

  implicit def ListIsMappable[t <% Mappable[t]](xs: List[t]): Mappable[List[t]] =
    new Mappable[List[t]] {
      def gmap(f: Mapper): List[t] = (for (x <- xs) yield f(x)).toList
    }

  implicit def OptionIsMappable[t <% Mappable[t]](xs: Option[t]): Mappable[Option[t]] =
    new Mappable[Option[t]] {
      def gmap(f: Mapper): Option[t] = (for (x <- xs) yield f(x))
    }
}

/** <p>
 *    This component provides functionality for enforcing variable binding
 *    during parse-time.
 *  </p>
 *  <p>
 *   When parsing simple languages, like Featherweight Scala, these parser
 *   combinators will fully enforce the binding discipline. When names are
 *   allowed to be left unqualified, these mechanisms would have to be
 *   complemented by an extra phase that resolves names that couldn't be
 *   resolved using the naive binding rules. (Maybe some machinery to
 *   model `implicit' binders (e.g., `this' and imported qualifiers) 
 *   and selection on a binder will suffice?)
 * </p>
 *
 * @author Adriaan Moors 
 */
trait Binders extends AbstractSyntax with Mappable {
  /** A `Scope' keeps track of one or more syntactic elements that represent bound names.
   * The elements it contains share the same scope and must all be distinct (wrt. ==)
   *
   * A `NameElement' `n' in the AST that is conceptually bound by a `Scope' `s', is replaced by a 
   * `BoundElement(n, s)'. (For example, in `val x:Int=x+1', the first `x' is modelled by a
   * Scope `s' that contains `x' and the second `x' is represented by a `BoundElement(`x', s)')
   * The term (`x+1') in scope of the Scope becomes an `UnderBinder(s, `x+1').
   *
   * A `NameElement' `n' is bound by a `Scope' `s' if it is wrapped as a `BoundElement(`n', s)', and 
   * `s' has a binder element that is semantically equal (`equals' or `==') to `n'.
   *
   * A `Scope' is represented textually by its list of binder elements, followed by the scope's `id'.
   * For example: `[x, y]!1' represents the scope with `id' `1' and binder elements `x' and `y'.
   * (`id' is solely used for this textual representation.)
   */
  class Scope[binderType <: NameElement] extends Iterable[binderType]{
    private val substitution: Map[binderType, Element] = 
      new scala.collection.mutable.LinkedHashMap[binderType, Element] // a LinkedHashMap is ordered by insertion order -- important!
    
    /** Returns a unique number identifying this Scope (only used for representation purposes).
     */
    val id: Int = _Binder.genId
    
    /** Returns the binders in this scope.
     * For a typical let-binding, this is just the variable name. For an argument list to a method body,
     * there is one binder per formal argument.
     */
    def iterator = substitution.keysIterator
    
    /** Return the `i'th binder in this scope.*/
    def apply(i: Int): binderType = this.iterator.toList(i)
    
    /** Returns true if this container has a binder equal (==) to `b'
     */
    def binds(b: binderType): Boolean = substitution.contains(b)
    
    def indexFor(b: binderType): Option[Int] = {
      val iter = this.iterator.zipWithIndex
      for ((that, count) <- iter) {
        if (that.name == b.name) // TODO: why do name equals and structural equals differ?
          return Some(count + 1)
        else
          Console.println(that+"!="+b)
      }
      
      None
    }

    /** Adds a new binder.
     * (e.g. the variable name in a local variable declaration) 
     *
     * @param b a new binder that is distinct from the existing binders in this scope, 
     *           and shares their conceptual scope. canAddBinder(b)` must hold.`
     * @return `binds(b)` and `getElementFor(b) eq b` will hold.
     */
    def addBinder(b: binderType) { substitution += Pair(b, b) }

    /** `canAddElement' indicates whether `b' may be added to this scope.
     *
     * TODO: strengthen this condition so that no binders may be added after this scope has been 
     *       linked to its `UnderBinder' (i.e., while parsing, BoundElements may be added to the Scope
     *       associated to the UnderBinder, but after that, no changes are allowed, except for substitution)?
     *
     * @return true if `b' had not been added yet 
     */
    def canAddBinder(b: binderType): Boolean = !binds(b)

    /** ``Replaces'' the bound occurrences of a contained binder by their new value.
     * The bound occurrences of `b' are not actually replaced; the scope keeps track
     * of a substitution that maps every binder to its current value. Since a `BoundElement' is
     * a proxy for the element it is bound to by its binder, `substitute' may thus be thought of
     * as replacing all the bound occurrences of the given binder `b' by their new value `value'.
     *
     * @param b    the binder whose bound occurrences should be given a new value. `binds(b)` must hold.
     * @param value the new value for the bound occurrences of `b'
     * @return `getElementFor(b) eq value` will hold.
     */
    def substitute(b: binderType, value: Element): Unit = substitution(b) = value
    
    /** Returns the current value for the bound occurrences of `b'.
     *
     * @param b the contained binder whose current value should be returned `binds(b)` must hold.
     */
    def getElementFor(b: binderType): Element = substitution(b)    

    override def toString: String =  this.iterator.toList.mkString("[",", ","]")+"!"+id // TODO show substitution?

    /** Returns a list of strings that represent the binder elements, each tagged with this scope's id.*/
    def bindersToString: List[String] = (for(b <- this.iterator) yield b+"!"+id).toList
    
    /** Return a new inheriting scope that won't check whether binding is respected until the scope is left (so as to support forward references) **/
    def allowForwardRef: Scope[binderType] = this // TODO

    /** Return a nested scope -- binders entered into it won't be visible in this scope, but if this scope allows forward references, 
      * the binding in the returned scope also does, and thus the check that all variables are bound is deferred until this scope is left  **/
    def nested: Scope[binderType] = this // TODO
    
    def onEnter() {}
    def onLeft() {}
  }

  
  trait BindingSensitive {
    // would like to specify this as one method:
    // def alpha_==[t <: NameElement](other: BoundElement[t]): Boolean
    // def alpha_==[bt <: binderType, st <: elementT](other: UnderBinder[bt, st]): Boolean 
  }

  /** A `BoundElement' is bound in a certain scope `scope', which keeps track of the actual element that 
   * `el' stands for.                                                               
   *
   * A `BoundElement' is represented textually by its bound element, followed by its scope's `id'.
   * For example: `x@1' represents the variable `x' that is bound in the scope with `id' `1'.
   *
   * @note `scope.binds(el)` holds before and after.
   */
  case class BoundElement[boundElement <: NameElement](el: boundElement, scope: Scope[boundElement]) extends NameElement with Proxy with BindingSensitive { 
    /** Returns the element this `BoundElement' stands for. 
     * The `Proxy' trait ensures `equals', `hashCode' and `toString' are forwarded to 
     * the result of this method.
     */
    def self: Element = scope.getElementFor(el)
    
    def name = self.asInstanceOf[NameElement].name // TODO: this is only safe when substituted to a NameElement, which certainly isn't required -- I want dynamic inheritance! :)

    // decorate element's representation with the id of the scope it's bound in
    override def toString: String =  super.toString+"@"+scope.id
    
    def alpha_==[t <: NameElement](other: BoundElement[t]): Boolean = scope.indexFor(el) == other.scope.indexFor(other.el)
  }  
  
  /** A variable that escaped its scope (i.e., a free variable) -- we don't deal very well with these yet
   */
  class UnboundElement[N <: NameElement](private val el: N) extends NameElement {
    def name = el.name+"@??"
  }
  
  // this is useless, as Element is a supertype of BoundElement --> the coercion will never be inferred
  // if we knew a more specific type for the element that the bound element represents, this could make sense
  // implicit def BoundElementProxy[t <: NameElement](e: BoundElement[t]): Element = e.self
                                                                     
  /** Represents an element with variables that are bound in a certain scope. 
   */
  class UnderBinder[binderType  <: NameElement, elementT <% Mappable[elementT]](val scope: Scope[binderType], private[Binders] val element: elementT) extends Element with BindingSensitive {
    override def toString: String = "(" + scope.toString + ") in { "+element.toString+" }"
      
    /** Alpha-equivalence -- TODO
     * Returns true if the `element' of the `other' `UnderBinder' is equal to this `element' up to alpha-conversion.
     *
     * That is, regular equality is used for all elements but `BoundElement's: such an element is 
     * equal to a `BoundElement' in `other' if their binders are equal. Binders are equal if they
     * are at the same index in their respective scope. 
     *
     * Example: UnderBinder([x, y]!1, x@1) alpha_== UnderBinder([a, b]!2, a@2)
     *          ! (UnderBinder([x, y]!1, y@1) alpha_== UnderBinder([a, b]!2, a@2))
     */
    /*def alpha_==[bt <: binderType, st <: elementT](other: UnderBinder[bt, st]): Boolean = {
       var result = true
       
       // TODO: generic zip or gmap2
       element.gmap2(other.element, new Mapper2 {
         def apply[s  <% Mappable[s], t  <% Mappable[t]](x :{s, t}): {s, t} = x match {
           case {be1: BoundElement[_], be2: BoundElement[_]} => result == result && be1.alpha_==(be2) // monadic gmap (cheating using state directly)
           case {ub1: UnderBinder[_, _], ub2: UnderBinder[_, _]} => result == result && be1.alpha_==(be2)        
           case {a, b} => result == result && a.equals(b)
         }; x
       })
    }*/
    
    def cloneElementWithSubst(subst: scala.collection.immutable.Map[NameElement, NameElement]) = element.gmap(new Mapper { def apply[t <% Mappable[t]](x :t): t = x match{
      case substable: NameElement if subst.contains(substable) => subst.get(substable).asInstanceOf[t] // TODO: wrong... substitution is not (necessarily) the identity function	
         //Console.println("substed: "+substable+"-> "+subst.get(substable)+")");
      case x => x // Console.println("subst: "+x+"(keys: "+subst.keys+")");x
    }})
    
    // TODO
    def cloneElementNoBoundElements = element.gmap(new Mapper { def apply[t <% Mappable[t]](x :t): t = x match{ 
      case BoundElement(el, _) => new UnboundElement(el).asInstanceOf[t] // TODO: precision stuff
      case x => x
    }})

    def extract: elementT = cloneElementNoBoundElements
    def extract(subst: scala.collection.immutable.Map[NameElement, NameElement]): elementT = cloneElementWithSubst(subst)
    
    /** Get a string representation of element, normally we don't allow direct access to element, but just getting a string representation is ok*/
    def elementToString: String = element.toString
  }
  
  //SYB type class instances
  implicit def UnderBinderIsMappable[bt <: NameElement <% Mappable[bt], st <% Mappable[st]](ub: UnderBinder[bt, st]): Mappable[UnderBinder[bt, st]] = 
    new Mappable[UnderBinder[bt, st]] {
      def gmap(f: Mapper): UnderBinder[bt, st] = UnderBinder(f(ub.scope), f(ub.element))
    }  
  
  implicit def ScopeIsMappable[bt <: NameElement <% Mappable[bt]](scope: Scope[bt]): Mappable[Scope[bt]] = 
    new Mappable[Scope[bt]] {
      def gmap(f: Mapper): Scope[bt] = { val newScope = new Scope[bt]()
        for(b <- scope) newScope.addBinder(f(b))
        newScope
      }
    }  
  
  implicit def NameElementIsMappable(self: NameElement): Mappable[NameElement] = new Mappable[NameElement] {
    def gmap(f: Mapper): NameElement = self match {
      case BoundElement(el, scope) => BoundElement(f(el), f(scope))
      case _ => UserNameElementIsMappable(self).gmap(f)
    }
  }
  
  def UserNameElementIsMappable[t <: NameElement](self: t): Mappable[t]  

  object UnderBinder {
    def apply[binderType <: NameElement, elementT <% Mappable[elementT]](scope: Scope[binderType], element: elementT) = new UnderBinder(scope, element) 
    def unit[bt <: NameElement, elementT <% Mappable[elementT]](x: elementT) = UnderBinder(new Scope[bt](), x)
  }
  
  /** If a list of `UnderBinder's all have the same scope, they can be turned in to an UnderBinder 
   * containing a list of the elements in the original `UnderBinder'.
   *
   * The name `sequence' comes from the fact that this method's type is equal to the type of monadic sequence.
   *
   * @note `!orig.isEmpty` implies `orig.forall(ub => ub.scope eq orig(0).scope)`
   * 
   */
  def sequence[bt <: NameElement, st <% Mappable[st]](orig: List[UnderBinder[bt, st]]): UnderBinder[bt, List[st]] = 
    if(orig.isEmpty) UnderBinder.unit(Nil) 
    else UnderBinder(orig(0).scope, orig.map(_.element))

  // couldn't come up with a better name...
  def unsequence[bt <: NameElement, st <% Mappable[st]](orig: UnderBinder[bt, List[st]]): List[UnderBinder[bt, st]] = 
    orig.element.map(sc => UnderBinder(orig.scope, sc))

  /** An environment that maps a `NameElement' to the scope in which it is bound.
   * This can be used to model scoping during parsing.
   *
   * (This class is similar to Burak's ECOOP paper on pattern matching, except that we use `=='
   *  instead of `eq', thus types can't be unified in general)
   *
   * TODO: more documentation
   */
  abstract class BinderEnv {
    def apply[A <: NameElement](v: A): Option[Scope[A]]
    def extend[a <: NameElement](v : a, x : Scope[a]) = new BinderEnv { 
      def apply[b <: NameElement](w : b): Option[Scope[b]] =  
        if(w == v) Some(x.asInstanceOf[Scope[b]]) 
        else BinderEnv.this.apply(w) 
    }
  }

  object EmptyBinderEnv extends BinderEnv {
    def apply[A <: NameElement](v: A): Option[Scope[A]] = None
  }

  /** Returns a given result, but executes the supplied closure before returning.
   * (The effect of this closure does not influence the returned value.)
   *
   * TODO: move this to some utility object higher in the scala hierarchy?
   *
   * @param result the result to be returned
   * @param block  code to be executed, purely for its side-effects
   */
  trait ReturnAndDo[T]{
    def andDo(block: => Unit): T
  } // gotta love Smalltalk syntax :-)

  def return_[T](result: T): ReturnAndDo[T] =
    new ReturnAndDo[T] {
      val r = result
      def andDo(block: => Unit): T = {block; r}
    }

  private object _Binder {
    private var currentId = 0
    private[Binders] def genId = return_(currentId) andDo {currentId=currentId+1}
  }
}