Functional Visitor

1
Functional Visitor

• Initially proposed by Shriram Krishnamurthi _at_
  Brown University

• Motivation Example
• Container checking -- Make sure each Container
  is not overloaded

Weight
2

• Motivation Example
• Typical Object-oriented Implementation

abstract class Item int total() return
getWeight().getValue() class Simple extends
Item class Container extends Item Vector
contains int total() int t0
//for each element of the Vector, call total(),
then sum // toghether to t return
t void check() if(getCapacity()lt
this.total()) System.out.println(oveerlo
aded) //for each element of the Vector,
if instanceof(Container) // call check() on
it.
3

• Motivation Example
• How about Adaptive implementation (by DJ)

• DifferenceVisitorDJ
• // stack handling
• public void start() stack new Stack()
• public void before(Container o)
• stack.push(sV.getReturnValue())
• public void after(Container o)
• difference ((Integer) sV.getReturnValue()).
  intValue() -
• ((Integer) stack.pop()).intValue()
• public Object getReturnValue() return new
  Integer(difference)

• Using side effect, lose the power of recursive
  computation and combinability provided by
  recursive function

4

• Motivation Example
• But we do want the structure-shy feature of
  Adaptive programming, i.e. DJ.
• Why not let the Visitor have access to the return
  value of its sub traversals, so that we can
  simulate the functional programming style in
  Visitor?
• It implies we have to all Visitor methods having
  return value.

5

• Interface of Functional Visitor
• Three classes
• FuntionVisitor, FuntionalOGS, Subtraversal
• In a subclass of FunctionVisitor, you can define
  method like
• Object around(SomeClass h,Subtraversal st)
• Object combine(Object values)
• The semantics (informal) of the around method is
  as follows when the traversal reaches an object
  whose type matches SomeClass , the return value
  of the traversal on this object is determined by
  the return value of this around method. The
  return value is available to its parent object
  for further combination. The Subtraversal class
  is provided by the run time library and the
  object will be passed to the around method
  automatically at run time.

6

• Interface of Functional Visitor
• In the body of the Object around(SomeClass
  h,Subtraversal st), you can control how the sub
  traversals will going and access the return
  values of the sub traversals, by one of the
  following four ways
• Object apply(String label)
• Object applySuper()
• Object applyElements(String label)
• Object apply()
• In your around method, you have the complete
  control of the traversal
• If you don't define around method on a class,
  then the default combine semantics will be
  applied automatically to the array of Object
  which get from each of the edges from the class.
  The way you want to combine is specified as
  Object combine(Object values).

7

• Interface of Functional Visitor
• It is backward-compatible with the default DJ
  package, i.e., if you don't define any around or
  combine method in a visitor class, then it will
  behave just the same as an ordinary DJ visitor

8

• Container Example implemented by Function
  Visitor
• class Main
• static public void main(String args)
  throws Exception
• ClassGraph cgnew ClassGraph(true,false)
  
• Container a Container.parse(System.in)
  
• TraversalGraph allWeights new
  TraversalGraph(
• "from Container bypassing -gt
  ,tail, to Weight",cg)
• FunctionalOGS fogsnew
  FunctionalOGS(a,allWeights)
• fogs.traverse(new DifferenceVisitorDJ())
  

9

• Container Example implemented by Function
  Visitor
• class DifferenceVisitorDJ extends
  FunctionVisitor
• Object combine(Object values)
• int total0
• for(int i0 iltvalues.length i)
• if(valuesi!null)
• total ((Integer)valuesi).intVa
  lue()
• return new Integer(total)
• Object around(Weight w,Subtraversal st)
  
• return w.get_i()
• Object around(Container c,Subtraversal
  st)
• Integer total (Integer)st.apply("conte
  nts")
• if(total.intValue() gt
  c.get_capacity().get_i().intValue())
• System.out.println(" A Overloaded
  Container")

10

• Conclusion
• Functional Visitor combines the power of Adaptive
  Programming and functional programming
• Users have more flexibilities to control the
  traversal at run time
• Ironically, the functional style is implemented
  by side effect.
• You will use it in a more complex situation,
  i.e., to find unbound variables in a language
  struct.