Semantics of Edge Visitor Methods

1
Semantics of Edge Visitor Methods

• Karl and Pengcheng

2
Kinds of edge methods

• construction edges
• cbefore / cafter / caround
• repetition edges
• rbefore / rafter / raround
• strategy edges
• sbefore / safter / saround
• In the following focus is on before methods.

3
Instead of code on construction edges

• Pattern matching capability
• if traversal context object has property P, then
  execute code.

4
Edge methods

• 0 stars
• -gt Source, x, Target
• void cbefore_x(Source s, Target t)
• 1 star
• -gt Source, x,
• void cbefore_x(Source s, Object t)
• -gt Source, , Target
• void cbefore(Source s, String partName,
  Target t)
• -gt , x, Target
• void cbefore_x(Object s, Target t)

5
Edge methods

• 2 stars
• -gt Source, ,
• void cbefore(Source s, Object t)
• -gt , x,
• void cbefore_x(Object s,Object t)
• -gt , , Target
• void cbefore(Object s, String partName,
  Target t)
• 3 stars
• -gt , ,
• void cbefore(Object s, String partName, Object
  t)

6
ExampleAdvice on all construction edges

• void cbefore(Object s, String partName, Object
  t)
• // print a message whenever a construction edge
• // is traversed
• System.out.println(
• source s.getClass().getName()
• partname partname
• target t.getClass().getName())

7
Alternative design

• Add method to class Visitor to ask about the
  currently active construction edge.

8
Addition to Visitor

• has a method
• CEdge getCEdge()
• that returns the CEdge of the
• current or most recent construction edge being
  traversed. The motivation is to fill in the
  blanks about the edge being traversed. The
  interface of cbefore may not uniquely determine
  an edge. DJ now also supports edge patterns.

9
Example

• void cbefore(Source s)
• -gt Source, ,
• This s an example of a cbefore method on a
  construction edge that maps several edges.
  getCEdge() will give us the details of the
  current edge and the programmer may use
  conditional statements to make the behavior
  dependent on the details of the edge.

10
CEdge

• CEdge
• ltsourceNamegt String
• ltpartNamegt String
• lttargetNamegt String
• ltedgeKindgt String.
• // derived / public, protected, private

11
Visitor Methods forConstruction Edges

• void cbefore_x(Source s, Target t)
• -gt Source,x,Target
• void cbefore(Source s, Target t)
• -gt Source, , Target
• void cbefore_x(Source s)
• -gt Source, x,
• void cbefore(Source s)
• -gt Source, ,

All but the first method needs getCEdge to
get edge details.
12
Visitor Methods forConstruction Edges

• void cbefore_x(Target t)
• -gt ,x,Target
• void cbefore(Target t)
• -gt ,,Target
• void cbefore_x()
• -gt ,x,
• void cbefore() // all edges
• -gt ,,

13
Visitor Methods forStrategies

• void sbefore(Strategy s) // strategy
• is executed before the traversal corresponding to
  s is started. More precisely, if the current
  object graph node is currently not in the scope
  of s, but it will be after the next traversal
  step (going through an edge in the object graph),
  the code of the sbefore method will be executed
  before the next traversal step.
• void safter(Strategy s) // strategy
• is executed after the traversal corresponding to
  s is finished. More precisely, if s the current
  object graph node is currently in the scope of s,
  but it will not be after the next traversal step
  (going through an edge in the object graph), the
  code of the safter method will be executed before
  the next traversal step.

14
Class Dictionary

• Person Brothers Sisters Status.
• Status Single Married.
• Single .
• Married ltmarriedTogt Person.
• Brothers Person.
• Sisters Person.

15
Strategy

• from Person bypassing Person
• through Married
• bypassing Person
• through Person
• bypassing Person
• through Brothers, Sisters
• bypassing Person
• to Person

16
Strategy and Traversal States

• Person -gt Married bypassing Person
• Married -gt Person bypassing Person
• Person -gt Brothers bypassing Person
• Person -gt Sisters bypassing Person
• Brothers -gt Person bypassing Person
• Sisters -gt Person bypassing Person
• void before (Person h)
• if (previousSEdges()) print(h.getName())
  else
• if (previousSEdges(Married-gtPerson))
• print(spouseh.getName()) else
• if (previousSEdges(Brothers-gtPerson))
• print(brother-in-lawh.getName())else
• if (previousSEdges(Sisters-gtPerson))
• print(sister-in-lawh.getName())

17
Strategy and Traversal States

• Person -gt Married bypassing Person
• Married -gt Person bypassing Person
• Person -gt Brothers bypassing Person
• Person -gt Sisters bypassing Person
• Brothers -gt Person bypassing Person
• Sisters -gt Person bypassing Person
• void before (Person h)
• if (nextSEdges(Person-gtMarried))
  print(h.getName()) else
• if (nextSEdges(Person-gtBrothers,Sisters))
• print(spouseh.getName()) else
• if (previousSEdges(Brothers-gtPerson))
• print(brother-in-lawh.getName())else
• if (previousSEdges(Sisters-gtPerson))
• print(sister-in-lawh.getName())

18
Giving access to pathsin visitor objects

• Object graph path OPath
• Class graph path CPath
• Strategy graph path SPath
• methods available for looking at path and
  retrieving objects.
• For OPath retrieve objects by name WandVisitor
  simulation.

19
Traversal Context Object

• Traversal passes this to visitor.

20
Programming with Strategies

• Strategies are abstractions of class graphs and
  it is useful to program directly with those
  abstractions. Therefore we extend DJ in two ways
  by allowing
• to check whether the traversal is currently in
  the scope of a subtraversal
• if (SEdges(sg)) (e.g., inside a cbefore
  method) checks whether the currently active
  strategy graph edges are contained in the
  strategy sg. Notice that e.g. during the
  traversal of an object graph edge, multiple
  strategy graph edges may be active.
• edge methods on strategies

21
Programming with Strategies

• to check whether the traversal is currently in
  the scope of a subtraversal
• We want to talk about the state of the traversal
  in before methods. This allows us to distinguish
  how we arrived at an object graph node.
• edge methods on strategies
• Strategies are built out of simpler strategies.
  We want to attach before and after code to the
  simpler strategies.

22
Programming with Strategies

• At a given point during the traversal a set of
  strategy graph edges is active. I think of it as
  the strategy edges corresponding to traversal
  graph edges on which a token travels. But there
  is a more direct explanation the set of strategy
  graph edges that are needed for the expansion
  into object paths.
• The following example illustrates the concept of
  active strategy graph edges. Basically when
  several tokens are distributed in different
  copies of the class graph inside the traversal
  graph, several edges are active.

23
Short-cut
Active strategy graph edges A -gt B for A-gtx1X
strategy A -gt B B -gt C
Object graph
Object graph
Traversal graph
a1A
A
start set
x1X
B
x
0..1
b
b
x2X
X
X
B
x
x
c1C
c
b
B
c2C
finish set
C
Used for token set and currently active object
c3C
24
Short-cut
Active strategy graph edges A -gt B for x1X-gtB
strategy A -gt B B -gt C
Object graph
Traversal graph
A
A
start set
x1X
B
x
0..1
b
b
x2X
X
X
B
x
x
c1C
c
b
B
c2C
finish set
C
c3C
Used for token set and currently active object
25
Short-cut
Active strategy graph edges A-gtB B-gtC for
B-gtx2X
strategy A -gt B B -gt C
Object graph
Traversal graph
A
A
start set
x1X
B
x
0..1
b
b
x2X
X
X
B
x
x
c1C
c
b
B
c2C
finish set
C
c3C
Used for token set and currently active object
26
Short-cut
Active strategy graph edges B-gtC for x2X-gtc1C
strategy A -gt B B -gt C
Object graph
Traversal graph
A
A
start set
x1X
B
x
0..1
b
b
x2X
X
X
B
x
x
c1C
c
b
B
c2C
finish set
C
c3C
Used for token set and currently active object
27
Short-cut
strategy A -gt B B -gt C
Object graph
Traversal graph
A
A
start set
x1X
B
x
0..1
b
b
x2X
X
X
B
x
x
c1C
c
b
B
c2C
finish set
C
c3C
Used for token set and currently active object
28
Short-cut
Active strategy graph edges A-gtB for A-gtx1X
strategy A -gt B B -gt C
Object graph
Traversal graph
A
After going back to x1X
A
start set
x1X
B
x
0..1
b
b
x2X
X
X
B
x
x
c1C
c
b
B
c2C
finish set
C
Used for token set and currently active object
c3C
29
Short-cut
Active strategy graph edges none for A
strategy A -gt B B -gt C
Object graph
Traversal graph
A
After going back to A
A
start set
x1X
B
x
0..1
b
b
x2X
X
X
B
x
x
c1C
c
b
B
c2C
finish set
C
Used for token set and currently active object
c3C
30
SEdges

• SEdges Vector(SEdge).
• SEdge
• ltsourceNamegt String
• lttargetNamegt String.

31
Class Dictionary

• Person Brothers Sisters Status.
• Status Single Married.
• Single .
• Married ltmarriedTogt Person.
• Brothers Person.
• Sisters Person.

32
Strategy

• from Person bypassing Person
• through Married
• bypassing Person
• through Person
• bypassing Person
• through Brothers, Sisters
• bypassing Person
• to Person

33
Strategy and Traversal States

• Person -gt Married bypassing Person
• Married -gt Person bypassing Person
• Person -gt Brothers bypassing Person
• Person -gt Sisters bypassing Person
• Brothers -gt Person bypassing Person
• Sisters -gt Person bypassing Person
• void before (Person h)
• if (previousSEdges()) print(h.getName())
  else
• if (previousSEdges(Married-gtPerson))
• print(spouseh.getName()) else
• if (previousSEdges(Brothers-gtPerson))
• print(brother-in-lawh.getName())else
• if (previousSEdges(Sisters-gtPerson))
• print(sister-in-lawh.getName())

34
Strategy and Traversal States

• Person -gt Married bypassing Person
• Married -gt Person bypassing Person
• Person -gt Brothers bypassing Person
• Person -gt Sisters bypassing Person
• Brothers -gt Person bypassing Person
• Sisters -gt Person bypassing Person
• void before (Person h)
• if (nextSEdges(Person-gtMarried))
  print(h.getName()) else
• if (nextSEdges(Person-gtBrothers,Sisters))
• print(spouseh.getName()) else
• if (previousSEdges(Brothers-gtPerson))
• print(brother-in-lawh.getName())else
• if (previousSEdges(Sisters-gtPerson))
• print(sister-in-lawh.getName())

35
Programming with Strategies

• At a given point during the traversal a set of
  strategy graph edges is active. The set of active
  strategy graph edges changes as we move through
  the object graph. When we are at a node in the
  object graph, we can talk about the next and
  previous set of strategy graph edges.
• boolean nextSEdges(Strategy s)
• boolean nextSEdges(String str)
• boolean previousSEdges(Strategy s)
• boolean previousSEdges(String str)
• The meaning is is the set of strategy graph
  edges contained in the strategy?
• nextSEdges(Person-gtBrothers,Sisters)
• previousSEdges(Brothers-gtPerson)

36
Programming with Strategies

• When we are at an edge in the object graph, we
  can talk about the current set of strategy graph
  edges.
• boolean SEdges(Strategy s)
• boolean SEdges(String str)
• boolean SEdges(TraversalGraph tg)
• The meaning is is the set of strategy graph
  edges contained in the strategy?

37
Visitor Methods forStrategy Edges

• void sbefore(Source s, Target t) // strategy
• is executed before the traversal corresponding to
  s-gtt is started. More precisely, if s-gtt is
  currently not in the set of active strategy
  edges, but it will be after the next traversal
  step (going through an edge in the object graph),
  the code of the sbefore method will be executed
  before the next traversal step.
• void safter(Source s, Target t) // strategy
• is executed after the traversal corresponding to
  s-gtt is finished. More precisely, if s-gtt is
  currently in the set of active strategy edges,
  but it will not be after the next traversal step
  (going through an edge in the object graph), the
  code of the safter method will be executed before
  the next traversal step.

38
Be more general

• Allow code attachment not only to strategy edges
  but to strategies.

39
Visitor Methods forStrategies

• void sbefore(Strategy s) // strategy
• is executed before the traversal corresponding to
  s is started. More precisely, if the current
  object graph node is currently not in the scope
  of s, but it will be after the next traversal
  step (going through an edge in the object graph),
  the code of the sbefore method will be executed
  before the next traversal step.
• void safter(Strategy s) // strategy
• is executed after the traversal corresponding to
  s is finished. More precisely, if s the current
  object graph node is currently in the scope of s,
  but it will not be after the next traversal step
  (going through an edge in the object graph), the
  code of the safter method will be executed before
  the next traversal step.

40
Follow DJ rule

• Wherever a strategy is used, a string may be
  used.
• void sbefore(String str) // strategy
• void safter(String str) // strategy

41
Programming with strategies

• check whether currently in scope of subtraversal
• // may be used in before, cbefore, rbefore,
  sbefore
• // sg a substrategy of current strategy
• if (SEdges(sg))
• // currently in traversal determined by
  strategy sg
• // tg a subgraph of current traversal graph
• if (SEdges(tg))
• // currently in traversal determined by
  tg

42
Traversing an edge From C1 to C2
o2C2
o1C1
declared type of o2 is C3gtC2
e
go down e iff C1 lt.C C3 gt C2 when we go down
e, we execute the before method of all
cbefore methods that match the edge e in the
order the cbefore methods are listed in the
visitor.
43
Similarity

• When we traverse an edge in the object graph,
  several visitor methods (for edges) will be
  executed.
• the same visitor may have several cbefore methods
  whose expansion includes the same object graph
  edge.
• When we traverse a node in the object graph,
  several visitor methods (for nodes) may be
  executed.
• when an object graph node method is executed we
  also execute the methods of the super classes.

44
Executing cbefore_v(F f, W w)?

• Assuming that only one edge matches
• Object graph edge labeled v
• o1SubClass(F) -gt o2SubClass(W)
• Is F a superclass of class(o1)?
• Has F a construction edge labeled w to W?
• Execute method.

45
Finding the edge methods

• Object graph edge labeled v
• o1class(o1) -gt o2class(o2)
• Is there any edge visitor cbefore_v for a
  superclass of class(o1)?
• Execute method.
• Use a hash table.

46
before_v(F f, W w)?

• The second argument (W w) is uniquely determined
  by the class graph and is only given so the
  programmer can use the interface of the target
  node.
• Semantic check done by DJ Is W correct? Executed
  each time the before method is called?

47
Difference to DemeterJ for edge methods

• DJ only allows edge methods on construction edges
  and repetition edges while DemeterJ also allows
  edge methods on inheritance edges.
• Motivation In DJ, we want the behavior of a
  program to be invariant under flattening of the
  class graph.

48
Basket
Weight
Weight1
Fruit
Weight2
Weight1
Orange
Apple
B
Orange
49
The following needs to be updated.
50
Cycle checking example

• Growth plan consists of two steps
• Depth-first traversal visitor
• Cycle checking visitor

51
DFS tree
1
6
2
3
7
4
5
52
Visitors for graph algorithms

• StrategyType1
• Graph -gt Adjacency
• Adjacency -gt intermedVertex
• Vertex -gt Adjacency

53
Visitors for graph algorithms

• GenericDFT
• (StrategyType1 s)
• s.intermed
• change (_at_
  this.doSomething()
• next() _at_)

54
Visitors for graph algorithms

• void doSomething()
• //s.intermediate
• s.target
• boolean mark
• init (_at_ mark false _at_)
• change (_at_ if (!mark)
• marktrue next() _at_)

55
Visitors for graph algorithms

• GenericCycleCheck
• Stack stack
• init (_at_ stacknew Stack() _at_)
• s.target
• change (_at_
• if (stack.include(this)
• print(cycle found)
• else stack.push(this)
• next() stack.pop() _at_)
• modifies GenericDFT

56
Print vertices

• GenericPrintingDFT extends GenericDFT
• s.interm
• doSomething() this.print()

57
Visitors for graph algorithms

• s1
• Graph -gt Adjacency
• Adjacency -gt intermedVertex
• through -gt,neighbors,
• Vertex -gt Adjacency
• through lt- ,source,

58
Use

• s1 // previous view graph
• Graph
• dft() is GenericDFT(s1)
• dftPrint() is GenericPrintingDFT(s1)
• cycleCheck() is GenericCycleCheck(s1)
  

59
Finding connected components
2
3
1
2
3
1
1
3 connected components
60
Using strategy edge

• ConnectivityVisitor
• int compNumber
• init (_at_ compNumber0 _at_)
• s.source
• change (_at_ next() print(compNumber)

61
Using strategy edge

• ConnectivityVisitor
• s.source-gts.target
• s.target.newConnComp()
• s.target
• newConnComp() compNumber1
• modifies GenericDFT

62
Components of AP

• class graphs (C)
• strategies (S)
• structure-shy functionality (F)
• Main issue how do we specify and compose C, S
  and F, to maximize reuse of any of them when the
  others change.

63
Components of AP
C
S
Each component should hard-code as few
assumptions as possible on the concrete form of
the others.
Should not refer to each other directly
F
64
Components of AP
C
S
mixed connections S-gtC C-gtF
Clt-gtC Flt-gtF
F
65
Composition of visitors, strategies and class
graphs
Generic visitors
Strategy types
gv1 gv2
st1
st1
st1
s1
Class graphs
s1 instantiation of st1 for CG1
gv1
CG1
CG2
gv2(gv1)(s1 CG1)
s1
gv2
instantiated behavior
66
Component coupling in Demeter/Java

• C-F coupling
• class dictionaries contain visitor classes
• visitor classes refer to class dictionaries
• Problem
• change of cd necessary for change in visitor
• visitors not reusable

67
Decoupling of C and F

• use generic visitors parameterized by strategy
  objects, use generic name change.
• change abstractly denotes all traversing methods
  over different class graphs. How the visitor
  participates is expressed in code associated with
  change.

68
Component coupling in Demeter/Java

• C-S coupling
• strategies refer to task-important classes also
  used in class dictionaries
• strategies are expressed in terms of
  implementation class dictionaries! That is a
  problem

69
Improvements for the C-S coupling

• Use strategy types. In an instantiated program we
  have strategies as objects.
• Strategies are instantiated from strategy types.

70
Component coupling in Demeter/Java

• F-F coupling
• visitor classes refer to methods defined in other
  visitor classes
• visitor classes refer to methods such as
  get-capacity.
• Problem
• visitor classes directly refer to each other
  (less reusability)

71
Improvements for F-F coupling

• parameterizing generic visitors with method
  valued variables
• counting, summing, collecting etc.
• equip generic visitors with input and return
  ports.

72
Structure of a generic visitor
genericModVisitor(Strategy s, Method m1,,mk)
//variables global for traversal init()
return() start() final() s.source
//variables that temp. extend host object
init() change() host.m1()
next.return next() host.localMethod1()
localMethod1()(_at_ _at_) // end of visitor
73
next

• next.return
• return method in next visitor is invoked
• next visitor is bound by modifies statement
• next()
• invokes the next implementation for the method
  denoted by change (either next visiting task
  attached to traversal denoted by change or the
  remainder of the traversal)

74
Composition of visitors, strategies and class
graphs
Generic visitors
Strategy types
gv1 gv2
st1
st1
st1
s1
Class graphs
s1 instantiation of ST1 for CG1
gv1
CG1
CG2
gv2(gv1)(s1 CG1)
s1
gv2
instantiated behavior
75
Component coupling in Demeter/Java

• Coupling between components is pretty low (thanks
  to strategies) but can be improved further.
• Tension between
• convenience for programmer
• having to define many name mappings can be
  perceived as inconvenient
• reduced coupling between components

76
Improvements

• Parameterize adaptive programs (S,F compositions)
  with class graphs.
• Refer to class-valued and relation-valued
  variables when programs are written.
• When programs are used with a class graph,
  specify mapping of variables to specific class
  graph elements.

77
Components of AP
C1
C2
S
C interface
Each component should hard-code as few
assumptions as possible on the concrete form of
the others.
Should not refer to each other directly
F
78
Improvements

• Parameterize visitor classes with strategies to
  reduce S-F coupling.
• Refer to class-valued and relation-valued
  variables when visitors are written. Also refer
  to parts of strategy s s.source, s.target
  (class-valued variables parameterized by
  strategy).
• Define strategy kinds and write visitors in terms
  of those.

79
Improvements

• Strategy kind
• from Begin via Middle to End
• Strategy
• from A via B via C to D
• Mapping
• from Begin via Middle to End
• from A via B via C to D

80
Components of AP
C1
C2
S1
C interface
S interface
Should not refer to each other directly
F
S2
81
Reduce coupling between visitors

• Visitor has a signature
• provided
• required