I've reformatted your code to Lisp standard style (2 spaces instead of
tabs, no parens on their own lines) for better readability.
(define strange-warning (lambda (sym options)
(let ((el (assq sym options)))
(if (pair? el)
(if (list? el)
"list"
(cdr el)) ; <== strange warning here
"found nothing"))))
While this code is hard to understand, the warning is incorrect. In
particular, if el => (a . b), then it is a pair but not a list, and the
(cdr el) branch will be taken correctly, returning b. This is a minor
compiler bug on at least two levels: no warning should be produced, and the
reference to "false" is incorrect. You should file a bug at bugs.callcc.org
.
I would say that discriminating between pairs and lists this way is a bad
idea: you should always use lists with a car that distinguishes the two
types of values you want to treat differently. In particular, pair? is
O(1) whereas list? is O(n) in the length of the list. There's such a thing
as carrying dynamic typing too far.
(define no-warning (lambda (sym options)
(let ((el (assq sym options)))
(if (list? el)
"list"
(if (pair? el)
(cdr el) ; <== no warning here
"found nothing")))))
No warning is to be expected. Again, if el => (a . b), it will fail the
list? test but pass the pair? test, and taking its cdr will produce b.
However, this code is *not* semantically identical with strange-warning.
If el is (), then it is a list but not a pair, and this procedure will
return "list", whereas strange-warning will return "found nothing".
(define also-no-warning (lambda (sym options)
(define el (assq sym options))
(if (pair? el)
(if (list? el)
"list"
(cdr el)) ; <== also no warning here
"found nothing")))
Again, no warning is to be expected. The fact that strange-warning
triggers a warning but also-no-warning does not is very mysterious.
On Sat, Jun 30, 2018 at 5:18 AM, Martin Schneeweis <mar...@schneeweis.at>
wrote:
> Hi,
>
> the attached file contains 3 procedures
>
> a) strange-warning
> b) no-warning
> c) also-no-warning
>
> (a) and (b) should be semantically identical - just the if's are
> rearranged.
>
> "csc strange-warning.scm" produces a type-warning for (a) but not for
> (b):
>
> ...call to `cdr', expected argument #1 of type `pair' but was given
> an argument of type `false'...
>
> (c) has the same if-arrangement as (a) - but the let-form is replaced
> with a define-form.
>
> If you are wondering: i reduced my original procedure to the minimum -
> the original version is a procedure that returns the value for a key in
> an association list using either "cdr" or "cadr" depending on the type
> of the assq-return-value.
>
> lg
> Martin
>
> ----
> chicken -version
> (c) 2008-2017, The CHICKEN Team
> (c) 2000-2007, Felix L. Winkelmann
> Version 4.13.0 (rev 68eeaaef)
> linux-unix-gnu-x86-64 [ 64bit manyargs dload ptables ]
> compiled 2017-12-11 on yves.more-magic.net (Linux)
>
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