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Re: making a thunk out of a list
From: |
Neil Jerram |
Subject: |
Re: making a thunk out of a list |
Date: |
Tue, 18 Sep 2007 23:16:26 +0100 |
User-agent: |
Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (gnu/linux) |
"Marco Maggi" <address@hidden> writes:
> Ciao,
>
> maybe this is simple, but today I cannot find a solution;
> while iterating over the nodes of a graph I can accumulate
> in a list the sequence of function invocations upon each
> node (pseudo-code):
>
> (let ((result '()))
> ;; for each node in the iteration:
> (set! result (cons (list action-upon-node node)
> result))
> ;; at the end
> (reverse result))
>
> I do it in a method to memoize the iteration; now I
> would like to make a thunk out of the list, the following
> works:
>
> (lambda () (for-each primitive-eval result))
>
> but I would like a thunk that does not use FOR-EACH. I fail
> to see how to do it with a macro, and I cannot APPLY a
> LAMBDA, for example the following does not work:
>
> ((list lambda '() result))
>
> Ideas?
You've probably solved this by now, but I found your question curious,
so...
You have a list of nodes, and you have a procedure f that you want to
apply to each node. So that's just `(map f nodes)', isn't it?
But then you want to delay the f calls, and wrap the whole thing up
in a thunk. So:
(define (make-delayed-map-thunk f nodes)
(lambda ()
(map f nodes)))
I guess it can't really be this simple, so what am I missing?
Regards,
Neil
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