macro / backquote notation

[filfil]

Hi,

I've found an expression like this

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`(and ,.(first (nreverse ris)))

that is seems, evaluating, the same of

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`(and ,@(first (nreverse ris)))

I've never seen or read about [,.]: is that anything different with [,@]?

Thanks

filfil

[Goheeca]

Yes, comma-dot operates destructively with the form which is unquoted by that. Look at this SBCL session:

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* (defvar *a* (list 'a 'b 'c))

*A*
* `(1 2 3 ,@*a* 4 5 6)

(1 2 3 A B C 4 5 6)
* *a*

(A B C)
* `(1 2 3 ,.*a* 4 5 6)

(1 2 3 A B C 4 5 6)
* *a*

(A B C 4 5 6)
*

[filfil]

Thank you very much!

[findinglisp]

filfil, was the usage you found in performance critical code, or just a macro expander? If the latter, it seems like a false economy. Better to use ,@ and avoid the possibility of bugs caused by destructively modifying the following form, IMO. Generally, if your macro expander runs incrementally slower, you simply won't notice or care.

[filfil]

It's a macroexpander.Thank you for the warning!