On 07/08/2015 22:43, Alexander D. Knauth wrote:
On Aug 7, 2015, at 12:27 PM, Michael Titke <michael.tied...@o2online.de> wrote:
I always suspected this to be a logical joke of forcing people to write their
own quantifiers and to not let them fall for predicate logic.
The best thing about a logic scheme declaring everything but really everything
to be true when it is not false are expressions like the following:
(when (cond (#f 'true)) 'what-is-true?) => 'what-is-true?
The confusing thing about your example, I think, is that cond returns #<void>
if none of the clauses match, when it would be slightly less bug-prone if it raised
an error in that case.
Right!
Yes, even unspecified is not false but it could have been. But that was not
specified. ;-)
Do you mean #<void>? I think it's great that #<void> is a truthy value,
because it allows things like:
Yes, I meant #<void>. That's even better then /unspecified/ even if it
doesn't alway capture the concept.
(and
(some-predicate ...)
(some-effect-returning-void ...)
(some-predicate ...)
(some-effect-returning-void ...)
...)
Or possibly one of those effects could return void on success, but false on a
failure, which would cause the `and` expression to stop and return false,
signaling a failure. That's one nice use of that, at least.
As a matter of style I prefer the form /(and predicate1 ... predicate-n
side-effect)/ or I know about the return values of a row of side-effects.
Also the simpler the true/false separation is, the better, in my opinion.
Additionally it's a good idea to keep it on a level where you can
distinguish it from error reporting.
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