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5 points by akkartik 569 days ago | link | parent

"In `acond`, `aif` and `awhen`, `%test` or `%t` gets replaced with the 'test' form. `%then` gets replaced by the 'then' form, and `%else` by the 'else' form."

    (aif 9 (+ 9 %else) (+ 10 %test))
    ; => (if 9 (+ 9 (+ 10 9)) (+ 10 9))
"If nested you can access the previous level by doubling the first letter of the symbol. For example, '%ttest' would get you the previous [containing?] 'test' form, while '%eeelse' would get you the 'else' form 2 levels up. In the `aand` and `aor` macros you can reference arguments by using a symbol of form `<star><num>` where 'num' is the 1-index of the argument. Previous levels are accessed by doubling the `<star>` character. So the second 'test' form of an `aand` can by accessed with `<star>2` and the third argument of the previous [containing?] `aand` would be `<star><star>3`." [Working around the crappy pseudomarkdown here.]

    (aand (+ 30 20) *1)
    ; => (and (+ 30 20) (+ 30 20))

    (aand 1 2 "third" (aand 33 **3))
    ; => (and 1 2 "third" (and 33 "third"))
Utterly bonkers, and I love the hack :)

3 points by akkartik 567 days ago | link

Staring at these examples again, another thought occurs to me: do these macros cause multiple evaluation, or are the equivalences above just loose? Can anybody tell? I can't tell just from skimming the implementation.

If they're doing multiple evaluations they're a lot less useful than Paul Graham's original anaphoric macros even if they seem superficially more powerful/expressive.


3 points by i4cu 567 days ago | link

> do these macros cause multiple evaluation

from the read-me:

"Note that as of right now the symbols are replaced by copying in the expression it references, not by binding to a common variable. Hence not suitable for using with expressions that cause side-effects or involve a lot of computation. That will be changed soon."

I presume, by reading this, these expressions will be evaluated each time they are triggered within the operation, so the answer is - yes they will. But, as the read-me also suggests, this is a "WORK IN PROGRESS" and the author has stated intentions to assign variables, which would solve the issue.


2 points by hjek 568 days ago | link

If you include * whitespace * between words and stars, they are not parsed as an emphasized section.