the specific case I had in mind was near this
(let ([unless? odd?])
(for/sum ([x '(1 2 3 4 5)]
#:unless (uless? x))
x)) ;6
but as long as one is defaulting #:unless
PLUS #unless is common to all the for's across some arbitrary scope,
ideally one could elide it from each of the for's within that scope.
your current-unless looks a good candidate for another place.
On Saturday, August 4, 2018 at 3:47:42 PM UTC-4, Greg Hendershott wrote:
>
> Do you mean that in something like this:
>
> (for/sum ([x '(1 2 3 4 5)]
> #:unless (odd? x))
> x) ;6
>
> You have many occurrences of the `(odd? x)` and you'd like to define
> that in one place that you can vary?
>
> If so:
>
> First, if they're in the same local scope, you could use `let`:
>
> (let ([unless? odd?])
> (for/sum ([x '(1 2 3 4 5)]
> #:unless (uless? x))
> x)) ;6
>
> Or (same difference) if they're in the same function definition, you
> could use a function parameter:
>
> (define (f unless?)
> (for/sum ([x '(1 2 3 4 5)]
> #:unless (unless? x))
> x))
> (f odd?) ;6
>
> Or -- and maybe this is what you're asking? -- you could define a
> Racket parameter:
>
> (define current-unless? (make-parameter (const #f)))
>
> (define (f)
> (for/sum ([x '(1 2 3 4 5)]
> #:unless ((current-unless?) x))
> x))
>
> (parameterize ([current-unless? odd?])
> (f)) ;6
>
> I think this is the best you can do; the `for` macros don't provide a
> parameter to inject an #:unless clause.
>
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