Re: Does this 'and' construct make sense?

[Urs Liska]

Am 06.03.2015 10:57, schrieb Richard Shann:
> On Fri, 2015-03-06 at 08:25 +0100, Urs Liska wrote:
> > I find
> >
> > %% switch on debugging.
> > #(if (and #t (defined? 'set-debug-cell-accesses!))
> >     (set-debug-cell-accesses! 5000))
> >
> > (found in init.ly)
> >
> > strange.
> > Wouldn't it be better ot simply remove the 'and' construct?
> > #t evaluates to #t always, isn't it?
> The terminology used in the manual is "#t evaluates to true". (And,
> indeed #f is the only thing that evaluates to false). I think I then
> found it tricky to find where in the manual this concept (true/false)
> was explained, but came to understand it applies to the semantics of
> "if" and "while".

The point in this is that if an expression evaluates to "something" this 
"something" is considered #t with regard to conditionals but the return 
value is "something".

So

#(if (get-something-that-returns-an-empty-or-non-empty-list)
     (do-something)
     (do-nothing))
#(let
  ((result (get-something-that-returns-an-empty-or-non-empty-list)))
    (if result
        (display result)))

In the second case result can be passed to "if" but holds the result of 
the procedure that can then be used.
This can also be made fruitful in 'and' or 'or' constructs that will 
hold a result in the end - instead of only the boolean result of the 
condition.

> > Or is this one more Scheme subtlety I don't get?
> so, I don't think so - the commonest cause is where someone starts with
> (if (and Debug (defined? 'xxx))
>
> and then decides not to have a separate Debug flag and replaces it in
> bulk with #t
>
> Richard

OK, I think this means that could be removed.

Best
Urs

>


--
Urs Liska
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