Den tor. 13. aug. 2020 kl. 21.55 skrev Kevin Forchione <[email protected]>:
> Hi guys,
>
> Pollen makes use of something like this:
>
> (require syntax/parse/define)
>
> (define-simple-macro (#%top . x) 'x)
>
>
> and it comes in handy in some of my projects. But I’m wondering if there’s
> an equivalent for redirecting an application that hasn’t been defined?
>
> In other words, if foo is not a procedure then (foo 1 2 3) would redirect
> to (do-this foo 1 2 3) where do-this is defined?
>
> Any help is appreciated.
>
Hi Kevin,
In Bracket [1] used your idea to produce s-expression, if the first argument
of #%app isn't a function:
; In the BRACKET language an application of
; a non-function evaluates to an expression.
(define-syntax (sym-app stx)
(syntax-case stx ()
[(_ op arg ...)
(quasisyntax/loc stx
(let ([o op])
(if (procedure? o)
#,(syntax/loc stx (#%app o arg ...))
(if (holdable? o)
(cons o '(arg ...))
(cons o (list arg ...))))))]))
There is an additional wrinkle - if a function is "holdable" it "holds"
(postpones) the
evaluation of its arguments. This can simply we be removed, if you don't
need it.
More details here:
https://github.com/soegaard/bracket/blob/master/bracket/bracket.rkt#L80
/Jens Axel
https://racket-stories.com
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