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Comment by StefanKarpinski | Hacker News Reader

Tip: the first move when trying to write a macro is doing `Meta.@dump` on examples of argument expressions you want your macro to consume and produce. Then write code that transforms the inputs to the outputs.

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Right but it wasn’t obvious to me (at the time) what a change to the source code would do to the ast, so it wasn’t easy to know all the cases to handle, especially with quasiquotation (I think double-backtick style programming was basically impossible).

For example, maybe you want to handle something that looks like:

  foo ~~> bar

In lisp syntax (and recall that is what the Julia ast is: everything is a head and then arguments) it might look like:

  (~~> foo bar)
  ; or
  (op ~~> foo bar)

But if you change to e.g.

  foo ~~> bar + 5

You might get

  (+ (~~> foo bar) 5)

  (~~> foo (progn (+ bar 5)))

I don’t remember what you got or which cases were tricky, only that I could never guess what the output of dump would be.

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