IF, WHEN or SOME

[Stephen Feyrer]

Hi there,

I have been trying to shake this thought for a while now.  Essentially, my thought was if you can return a function why not decision component of an IF, WHEN or SOME statement?  That would give you a re-usable named choice.

Then you could write:

(celebration: do-something do-something-else)

This would be equivalent to writing:

(def success [apples bananas pears])

(defn celebration: [x y] (if (empty? success) x y))

(celebration: (do-something do-something-else))

I'm reasonably certain of the foolishness of this thought but occasionally, I have doubts.

Maybe I'm barking up the wrong tree or possibly I've seen something like this before and forgotten about it.  Perhaps, this is just taking things too far...  Either way, it's deferring the choice until it's needed.  In the right hands it could make for more readable code.

For completeness sake, to define the first form above you'd use:

(defc celebration: (if (empty? success)))

A more usable example might look like:

(def nums [1 2 3 4 5 6 7 8])

(defc even-nums: (some (even? nums)))

I guess this makes the real question, is it a good thing to be able to defer choice like this?

Btw, defc would be like def-choice but other options might be deft - def-test or defp - def-predicate.

--

Kind regards

Stephen

[Timothy Baldridge]

I talked a bit about this in my video on Boolean Blindness: https://www.youtube.com/watch?v=K1LaaJMscCc

Might be worth a watch. 

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[Stephen Feyrer]

Hi Tim,

Thank you.

--

Kind regards

Stephen.

[Stephen Feyrer]

Hi,

I have just been playing around with this idea and I got something.

user=> (def some-numbers ‘(2 4 6 8))  #This is my value to test later.

#’user/some-numbers

user=> (def evens? (partial (when (apply = (map even? some-numbers)))))

#’user/evens?

user=> (evens? (println “one”))

one

NullPointerException   user/eval239 (NO_SOURCE_FILE:74)

user=> 

What is my mistake?

Thanks.

[Orestis Markou]

In addition, when is a macro, so I don’t think you can call partial on it :)

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[Orestis Markou]

evens? is not a macro, therefore when you do (evens? (println “one”)), the println will be evaluated first, and its return value, nil, gets passed into the evens function. 

20 Σεπ 2018, 5:44 μμ, ο χρήστης «Stephen Feyrer <stephen...@gmail.com>» έγραψε:

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[Alan Thompson]

`println` always returns `nil`.  So it prints "one", returns `nil`, and you try to execute the form:

(nil)  => NullPointerException

user=> (evens? (println “one”))

one

NullPointerException   user/eval239 (NO_SOURCE_FILE:74)

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(Robert Firth)

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[Alan Thompson]

Should have been:    (evens? nil)

[Paul Rutledge]

I haven't used them much myself but the idea of "reified control flow" is called a continuation. Some other lisps implement continuations, Clojure does not. I'm sure there's plenty of material out there about why Clojure/Rich did not provide continuations.

[Carlo Zancanaro]

Hey Stephen,

On Fri, Sep 21 2018, Stephen Feyrer wrote:
> user=> (def some-numbers ‘(2 4 6 8)) #This is my value to test
> later.
> #’user/some-numbers

At this point we have some-numbers = '(2 4 6 8)

> user=> (def evens? (partial (when (apply = (map even?
> some-numbers)))))
> #’user/evens?

Let's work through this:

(map even? '(2 4 6 8)) = '(true true true true)
(apply = '(true true true true)) = true
(when true) = nil
(partial nil) = nil

This means that we have evens? = nil

> user=> (evens? (println “one”))
> one
> NullPointerException user/eval239 (NO_SOURCE_FILE:74)

So now we have:

(println "one") = nil (with the side effect of printing "one")
(nil nil) throws NullPointerException

Clojure is attempting to call nil as a function, which throws a
NullPointerException.

I hope that helps! I'm not sure what you're actually trying to do,
so it's hard for me to give advice on how to do it.

Carlo