4 ms·

My Lisp is rusty and was never that great to begin with, but: Why is mcd a macro? And isn’t it missing a gensym?

• tmtvl 2y ago
  To answer in reverse order: it doesn't need a gensym because it isn't introducing a new variable. The only variable being used is `dir`, and that is passed in. As for why it's a macro rather than a function, I don't know, but it's possible that one of the features (probably variable substitution) is important. (frobnicate (mcd (foo bar))) Gets expanded to: (frobnicate (fg (mkdir -p (foo bar)) (cd (foo bar)))) At compile-time. I personally would have used a function declared inline, though.
  • wk_end 2y ago
    But my intuition is that it needs to (well, should) introduce a new variable in order to store the evaluation of dir, otherwise it gets evaluated twice. If (foo bar) has no side-effects, that’s just a performance loss, but if it does it’s probably behaving in unexpected ways.
    • tmtvl 2y ago
      Yeah, that is the strange thing, but binding it to a gensymmed variable would make it interchangeable with an inline function. That said, maybe `fg` is a macro which does some rewriting of its body, that could explain the decision.