Re: Is this a bug in while-let or do I missunderstand it?

[arthur miller]

>+@code{while-let} replaces a pattern in which a binding is established

>+outside the @code{while}-loop, tested as part of the condition of

>+@code{while} and subsequently changed inside the loop using the same

>+_expression_ that it was originally bound to:

>+

>+@example

>+(let ((result (do-computation)))

>+  (while result

>+    (do-stuff-with result)

>+    (setq result (do-computation))))

>+@end example

>+

>+Using @code{while-let}, this can be written more succinctly as:

>+

>+@example

>+(while-let ((result (do-computation)))

>+  (do-stuff-with result))

>+@end example

>+

>+One crucial difference here is the fact that in the first code example,

>+@code{result} is scoped outside the @code{wile} loop, whereas in the

>+second example, its scope is confined to the @code{while-let} loop.  As

>+a result, changing the value of @code{result} inside the loop has no

>+effect on the subsequent iteration.

>+@end defmac

The scope of the let-binding is the same in both. The crucial

difference is that in the first example, the user have control over

when and how 'result' is evaluated. The user can for example do:

(let ((result (do-computation)))

  (while result

    (do-stuff-with result)

    (setq result (do-some-other-computation))))

Whereas in the other example, the code is automatically generated

to pass in the original condition calculation, and the user can not

interfere with the computation of the condition.