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1 point by NickSmith 3878 days ago | link | parent

Thanks Kenny, that hit the Mark :)

Looking at mac dsb I realise just how much I have to learn :(

4 points by kennytilton 3878 days ago | link

heh-heh, don't feel bad, i was not kidding when I said it is one of the craziest macros I ever wrote, and I have written over five hundred. Might be the craziest, actually. Macros are normally hard because we are writing code to write code, bouncing back and forth between the mindset of the expanding code and that of the expanded code we are after as we go from backquote to unquote and back.

But in this case with the keyword args the expanded code then had to include code to continue the binding process, because the expander code cannot guess which keywords had been supplied by the caller. Hence the rather sick stop mid-expansion to produce the kvs binding with remaining runtime args paired into an assoc before continuing on to produce the other keyword bindings, each necessitating a runtime lookup of the keyword before deciding if any default should be used, which default might itself still be code!

Man, I now I am confused. Good thing I wrote the macro before I wrote this comment. :)

Needless to say, the macro was evolved: first just getting required params to work. Trivial, but still. Then optionals, with defaults, including computed defaults. I had a bug here, recall, forgetting nil supplied in an optional position means nil, not the default. Then keyword args.

A bit of a giveaway, btw, is my apology above for leaving behind some print statements in the expanding code. We debug macros just like we debug any other code.

Another tip is that I might have broken out that little state machine in the beginning as a separate function to parse the params like:

  (a b c &o (d 42) (e (+ c 1)) &k x) 
...into an assoc:

   '((reqs a b c)(opts (d 42)(e (+ c 1))(keys x))
ie, Divide and conquer: get that working separately, then tackle the rest. I did not do that because of insufficient lookahead (did not realize what I got myself into). :)