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From: ajk@wren.cs.rmit.OZ.AU (Alan Kent)
Newsgroups: comp.lang.eiffel
Subject: How can I do this in Eiffel?
Message-ID: <ajk.671419932@wren>
Date: 12 Apr 91 01:32:12 GMT
Sender: news@goanna.cs.rmit.oz.au
Distribution: comp
Lines: 46

Can someone tell me if I am missing something obvious? I cannot see how to
do this nicely in Eiffel.

I have a tree structure of nodes. These nodes are not all of exactly the
same class. To be precise, the tree is a parse tree for a language so you
have IF nodes, operator nodes, CASE nodes and so on. What I need to do
frequently is walk the tree and perform operations on nodes. For example,
count the number of variable declaration nodes, determine that maximum
stack depth needed by the function by examining expressions, determine
the number of nodes in the graph, determine the maximum depth of nesting
of loops and so on. I also wish to be able to add new nodes with different
structures at any time, so I cannot define a single superclass which can
walk the tree - each node type must have its own methods for walking down
into subnodes (eg: CASE has a list of alternatives, binary operators have
exactly two subnodes, who knows what I will want in the future).

Frankly, I dont not know how to do this nicely. I only want to write the
tree walking code once and I really want to be able to pass as a parameter
somehow the operation to be done. I can do this in C by passing a pointer to
a function. I could not see anything similar in Eiffel. I also got wondering
how you would do it in general in an OO language. I guess you could

(1) Pass a method name which is to be sent to each node in the tree
(2) Same as (1), but pass arguments to the method.

but this means that the node definition must contain the code I wish to do
on the tree nodes. For example, if I want to count the number of IF nodes
by using a "count_if_nodes" method, then the node superclass must contain
such a method. This has the disadvantage if I want to write some new little
function, I have to change my central class definition. Yuck. The alternative
I guess would be

(3) Pass a method name and an object to send the method to with the
    current node as a parameter.

This seems similar to the hetrogeneneous discussion going on. If I understand
what has been said, this sort of thing would be easy in a dynamically typed
language. For a statically typed language for this to be completely safe,
you need to be able to specify a function or method with types for parameters.
I guess this is possible in ANSI-C.

Anyway, any suggestions appreciated. Comments about nice ways of doing this
if you could design your own language features would also be of interest to me.

Dr Alan Kent
ajk@goanna.cs.rmit.OZ.AU