Re: Threading Macro

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On 07/31/2013 10:22 AM, [email protected] wrote:
> Hi Joseph,
> 
>> I have recently come a cross a number of references to Clojure's 
>> threading macros, -> and ->>. These macros have the form: ((or ->
>> ->>) x form ...). The value of x is spliced in as the first or
>> last argument, respectively, of form with its result being
>> likewise spliced into the next form and so on. From the examples
>> I have seen, these macros can be useful, but instead of
>> replicating these macros exactly in bigloo, I decided to create a
>> generalized version called T> where _ is used to indicate where
>> values are to be spliced in. T> has the form: (T> x form1 ...) In
>> form1 and subsequent forms the _ character is used to indicate 
>> where x should be spliced.
>> 
>> Examples:
>> 
>> (T> 4 (+ _ 3) (* 3 _)) => 21
>> 
>> (T> '( 1 2 3 4) (map odd? _) (map (lambda (x) (* x 2)) _))  => (2
>> 6)
>> 
>> The definition of T> is attached. I would be interested in any
>> comments or suggestions you may have.
> I don't understand your example. I would have replaced MAP with
> FILTER, as in:
> 
> (T> '( 1 2 3 4) (filter odd? _) (map (lambda (x) (* x 2)) _))
> 
> What are the concrete use cases they have in Clojure for this
> macro? It vaguely reminds me of the monadic threads as one may find
> in Ocaml.
> 

- From what I understand, the purpose of the threading macros in clojure
is to increase readability of code, as to simulate an imperative code.
In clojure, for instance

(+ (- (* 3 4) 20) 10)

becomes

(-> (* 3 4)
    (- ,, 20)
    (+ ,, 10))


- -- 
Eduardo Belani
"The urge to save humanity is almost always a false front for the urge
to rule."
Mencken, H.L.
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