The problem is that even though "some" and "filter" are lazy, "alt" is
still not, so calling "(alt (sub-function1 c) ...)" in the meta-meta-
function still evaluates (sub-function1 c), etc. It could be shown in
the REPL:
Clojure
user=> (defn alt [& functions]
(fn [tokens]
(some #(% tokens) functions)))
#'user/alt
user=> (defn sub-function1 [c] (println "1:" c) (fn [c] false))
#'user/sub-function1
user=> (defn sub-function2 [c] (println "2:" c) (fn [c] true))
#'user/sub-function2
user=> (defn sub-function3 [c] (println "3:" c) (fn [c] false))
#'user/sub-function3
user=> (defn a-meta-meta-function [c]
(alt (sub-function1 c) (sub-function2 c) (sub-function3 c)))
#'user/a-meta-meta-function
user=> ((a-meta-meta-function "CONTEXT") [:a :b :c])
1: CONTEXT
2: CONTEXT
3: CONTEXT
true
What I wish is for calling '((a-meta-meta-function "CONTEXT")
[:a :b :c])' to output:
1: CONTEXT
2: CONTEXT
...only, which means that the sub-functions are called only until a
sub-function's created function returns a true value. The root of the
problem is that in Clojure's rules (I think), the arguments of a
regular function are all evaluated before being plugged into the
function. That's why I'm wondering if a macro could solve my problem.
Thanks for your answers!
On Jan 11, 3:02 pm, Stuart Sierra <the.stuart.sie...@gmail.com> wrote:
> "some" is already lazy, so you may not need to change anything at
> all. You might also be able to use "filter", which will not do
> anything until you consume the output sequence.
> -Stuart Sierra
>
> On Jan 11, 4:44 pm, samppi <rbysam...@gmail.com> wrote:
>
> > Let's say I have a function, alt:
>
> > (defn alt [& functions]
> > (fn [tokens]
> > (some #(% tokens) functions)))
>
> > It creates a function from a bunch of sub-functions that accepts one
> > collection of tokens and figures out which sub-function returns a true
> > value when the tokens are plugged into it.
>
> > Is there a way to change alt—maybe to a macro—that lazily evaluates
> > the functions? This is so that with:
>
> > (defn a-meta-meta-function [c]
> > (alt (sub-function1 c) (sub-function2 c) (sub-function3 c)))
>
> > ...calling ((a-meta-meta-function foo) some-tokens) doesn't evaluate
> > (sub-function 2 c) and (sub-function3 c) until needed to receive some-
> > tokens.
--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Clojure" group.
To post to this group, send email to clojure@googlegroups.com
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
-~----------~----~----~----~------~----~------~--~---
