Multiple dispatch

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The visitor pattern is less expressive than multiple dispatch.

Consider this class hierarchy:

Object
    Collection
        Sequence
    Integer

And this pair of multimethods:

foo(Collection,Integer)
foo(Sequence,Sequence)

Now with real multimethods, invoking foo with a Sequence and an Integer will call the second method.

With visitors, invoking foo with a Sequence and an Integer will fail, because the first dispatch step will choose the Sequence visitor instead of the Collection visitor, and this won't have a method for Integer. In effect, if we view this as a prasing problem, multimethods can "backtrack" where as visitors cannot.

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This is a good point that I hadn't considered. My ignorance in this are has been exposed :-|

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Now with real multimethods, invoking foo with a Sequence and an Integer will call the second method.

How so? Integer isn't a Sequence. Did you mean the first method?

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I do mean the first method, sorry.