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Comment by kbp | Hacker News Reader

I remember finding it pretty revelatory the first time I saw LET defined as a macro like

    (defmacro let (bindings &body body)
      `((lambda ,(mapcar #'first bindings) ,@body)
        ,@(mapcar #'second bindings)))

Seeing it use MAPCAR like that just made something click for me for whatever reason.

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To add on, Racket in particular has some very slick syntax for writing these kinds of things:

  (define-syntax-rule
    (let ([id expr] ...) body ...)
    ((λ (id ...) body ...) expr ...))

I definitely liked this about Racket but I did also find the elipses to be a bit magic to begin with...

As with all things, the trick to chase the magic away is to implement them. I recently made a little term rewriting library using racket/match syntax but with the pattern/templates specifiable at runtime. Ellipses were a bit confusing to begin with but when you break list patterns down into cons pairs + a cons-like pattern binary combinator to represent ellipses, it started making sense. The unexpected thing for me was that I thought nested ellipses were going to be the tough part, but it turns out they actually just... work.

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