Re: Why is this macro misbehaving?

"jmckitrick" <j_mckitrick@xxxxxxxxx> writes:

I'd like to take a string like this: "1/2/3"
and bind it like this:

(let ((item "1") (arg1 "2") (arg2 "3"))
body)

Here's the macro:

(defmacro with-url ((&rest vars) seq &body body)
(let ((subseqs (split-sequence #\/ seq :remove-empty-subseqs t)))
`(let ,(loop
for var in vars
for subseq in subseqs collect
`(,var ,subseq))
,@body)))

The splitting must be done at runtime, not macroexpansion time.

(with-url (item arg1 arg2) (request-unhandled-part request)
(format t "arg1: ~A~%" arg1))

In this call, your macro gets the list (REQUEST-UNHANDLED-PART
REQUEST) as the SEQ argument. Trying to use split-sequence on it will
not work.

You need to move the splitting work into runtime; that's when the
value (rather than the form that produces the value) is available.

At that point, though, there's very little to be gained over simply
using DESTRUCTURING-BIND.

Zach
.

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