Exercise 2.76.  As a large system with generic operations evolves, new 
types of data objects or new operations may be needed. For each of the 
three strategies -- generic operations with explicit dispatch, 
data-directed style, and message-passing-style -- describe the changes 
that must be made to a system in order to add new types or new 
operations. Which organization would be most appropriate for a system in 
which new types must often be added? Which would be most appropriate for 
a system in which new operations must often be added?

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In explicit dispatch, adding types requires changing all the operations 
to handle the new types.  Adding operations doesn't require any extra 
work apart from actually handling the dispatch.

In the data-directed style, anything which is added needs to be 
registered in the dispatch table, but this doesn't require changes to 
any other existing code.  Of course code still has to be written for 
each desired combination of operation and data type.

In the message-passing style, adding a new type might require changing 
existing code to use a new constructor, and adding any new operation 
will require adding support for that operation to every existing type.

If new types are frequently added, the data-directed style might be 
best.  Explicit dispatch in a generic constructor could also be combined 
with a message-passing style.

If new operations are often added, they have to be implemented for each 
type, which can be done with explicit dispatching or data-directed style 
without needing to touch existing code.

In practice I think future edits are hard to predict and unlikely to be 
a good descriminant between such styles.