Right, of course! It seems almost obvious now :P
Thanks Kevin
On Tuesday, March 7, 2017 at 12:42:08 AM UTC+1, red...@gmail.com wrote:
>
> On 03/06/2017 03:28 PM, juan.facorro wrote:
> > While I was debugging some code I found something related to the *loop
> > *macro that I found curious. I'm probably missing something but here it
> > goes.
> >
> > The expression generated by the *loop* macro includes a wrapping *let
> > *when there is any de-structuring in the bindings.
> >
> > (require '[clojure.walk :as walk])
> > (walk/macroexpand-all '(loop [[x & xs] xs a 3]))
>
> The init value of `a` could rely on `x` or `xs`, so those bindings need
> to be established before the loop, which is what the outer loop does.
> For example checkout `(walk/macroexpand-all '(loop [[x & xs] xs a x]))`.
>
> >
> > Which returns:
> >
> >
> > (let*[G__1249xsvec__1250G__1249x(clojure.core/nthvec__12500nil)
> > xs(clojure.core/nthnextvec__12501) a3] (loop*[G__1249G__1249a a]
> > (let*[vec__1251G__1249x(clojure.core/nthvec__12510nil)
> > xs(clojure.core/nthnextvec__12511) aa])))
> >
> >
> > Since all bindings get re-defined in the inner *let*, why is the outer
> > *let* even generated? Is there a reason for not having the following
> > generated instead?
> >
> >
> > (loop* [G__1249 xs a a]
> > (let* [vec__1251 G__1249
> > x (clojure.core/nth vec__1251 0 nil)
> > xs (clojure.core/nthnext vec__1251 1)
> > a a]))
> >
> >
> > The change in the loop macro is pretty simple and it seems to work (at
> > least with the tests available in GitHub's clojure/clojure).
> >
> >
> > (defmacro loop
> > "Evaluates the exprs in a lexical context in which the symbols in
> > the binding-forms are bound to their respective init-exprs or parts
> > therein. Acts as a recur target."
> > {:added "1.0", :special-form true, :forms '[(loop [bindings*]
> exprs*)]}
> > [bindings & body]
> > (assert-args
> > (vector? bindings) "a vector for its binding"
> > (even? (count bindings)) "an even number of forms in binding
> vector")
> > (let [db (destructure bindings)]
> > (if (= db bindings)
> > `(loop* ~bindings ~@body)
> > (let [vs (take-nth 2 (drop 1 bindings))
> > bs (take-nth 2 bindings)
> > gs (map (fn [b] (if (symbol? b) b (gensym))) bs)]
> > `(loop* ~(vec (interleave gs vs))
> > (let ~(vec (interleave bs gs))
> > ~@body))))))
> >
> >
> > But maybe this hasn't been changed because of the importance of the
> > *loop* macro, or maybe the impact in compile- and run-time is
> > insignificant to justify the change.
> >
> > Anyways... I thought I'd put the question out there.
> >
> >
> > Cheers!
> >
> > Juan
> >
> >
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> And what is good, Phaedrus,
> And what is not good—
> Need we ask anyone to tell us these things?
>
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