> I don't fully understand the TreeModel example, what would your tool do with all the men/displayBlock/childrenBlock ... - methods?

Ah, I should've explained what TreeModel is; it is a Spec UI widget to display tree-like structures:

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So the way this works is that I give the TreeModel some root object (DCFsm) and then blocks that describes some behavior.

So in childrenBlock: [ :parent | ... ] the block is expected to return child elements for the parent.

In displayBlock: you pass a block that returns the label that you will see.. e.g. [ :anObject | anObject name , ' (' , anObject class name , ')' ]

iconBlock: returns the icon for the object and so on and so forth.

So the way I have implemented it right now is something like

~~~~~~~~~~~~~~~~~

childrenFor: anObject

| col name |

name := anObject class asString asSymbol.

col := OrderedCollection new.

name = #DMMindMap

ifTrue: [ ^ col ].

(anObject isKindOf: DCDiagram)

ifTrue: [ col addAll: anObject elements ].

(anObject isKindOf: DCNamedElement)

ifTrue: [ 

name = #BormParticipant

ifTrue: [ col addAll: anObject nodes ].

name = #BormActivity | (name = #BormState)

ifTrue: [ col addAll: anObject outgoing ].

name = #BormActivity

ifTrue: [ col addAll: anObject sent ].

name = #BormCommunication

ifTrue: [ col addAll: anObject dataFlows ].

name = #BormCommunication

ifTrue: [ 

anObject hasConstraint

ifTrue: [ col add: anObject constraint ] ].

name = #BormTransition

ifTrue: [ 

anObject hasConstraint

ifTrue: [ col add: anObject constraint ] ] ].

^ col

~~~~~~~~~~~~~~~~~

Which is unreadable, unmaintainable and nonextensible. All the #Borm* symbols actually refer to classes of a completely independent package and repository which doesn't even need to be loaded in the system.

I think what Peter means is: the object oriented way to handle different types is by dispatching on each type (~ double dispatch, ~ visiting). You could need this for different aspects of your application. In the end, the same core mechanism will be implemented multiple times. Sadly, minor semantics/structural differences will make it hard to abstract this out.

Yes, double-dispatch seems like the apt solution, however it would lead to NxM number of classes (as mentioned before). But the more I think about it the more I feel that I will not get any better than that. 

Peter