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Comment by compressedgas | Hacker News Reader

This only shows that it can do method type specialization to get rid of type dispatch.

Can you add a new multimethod clause to an existing if-then-else type dispatch method? For example add:

  foo(x::Baz) = x.foo

Or is harm still done because the method is not open for extension when written like that?

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You can't edit a function like that. If you add a new function definitoin for

    foo(x::Baz)

then you will never enter the method that has those if else statements. This is the benefit of multiple dispatch. You can't add more if else statements inside a method that is already written but you can add new methods.

Manual dispatch can be an antipattern hindering composability though. If package A does:

    function foo(x)
      # some lines of code
      y = x isa Int ? 2.0 : 3x
      # some other lines that do things with y
    end

Then package B would have to define foo(x::MyType), copy the contents from package A, change the `y = ...` bit, and make sure the rest stays in sync with package A.

If instead package A did this:

    foo_magic(x::Int) = 2.0
    foo_magic(x) = 3x

    function foo(x)
      # some lines of code
      y = foo_stuff(x)
      # some other lines that do things with y
    end

Your package B could just add an extra definition of foo_magic(x::MyType) instead.

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