The "Weaver" challenge looks like it has potential for being won in APL with Amend (@).

@ngn "At".

@Adám is that its official name?

@ngn Yes‌​.

@Adám I'm so close to beating Jelly...

@Adám uh, "Authentication Required"

@ngn Sorry, change that 7 to a 6.

@Adám ah, while APL\360 has limited support for arrays, it looks like '' serves as the (1-dimensional) empty array for both numbers and characters

@EriktheOutgolfer No, '' is the empty character array. Zilde (⍬) was added later for convenience. You had to use (0⍴0).

empty array jokes starting in 3... 2... 1...

@ngn it's not very funny...

@EriktheOutgolfer But, you were allowed to catenate empty arrays of different prototypes.

@ngn More like: Empty array jokes starting in IV, III, II, I,

@EriktheOutgolfer of course, nobody's told any yet :) <- empty array joke

@Adám why Roman?

@Adám and it looks like the right argument's prototype is used for the result

@ngn — That's outrageous! Down on the corner, they are only $3 each. — So why don't you buy there? — They're sold out. — Madam, when we're sold out, they're only $2 each!

@Adám marketing against APLers 101

@Adám :D thumb up

@EriktheOutgolfer Yes, Dyalog APL is different from all other APLs in that regard.

@ngn !

@Adám well, yeah... I had a distorted understanding of APL arrays at the time. Too hard to fix without a rewrite.

@ngn I guess that has to do with versioning of vintage APL variants

@EriktheOutgolfer hm... or the lack of 0 in Roman numerals

@EriktheOutgolfer This may have your interest.

> one vendor chooses the left, and all the others the right

hm, maybe Dyadic Systems?

@EriktheOutgolfer How did you guess?

@Adám well, because that's what exhibits such behavior

@EriktheOutgolfer Not because I just told you?

@Adám well, I had actually tried before...

@Adám done:

      ∇R←II N;I
[1]   I←⍳N
[2]   R←0⍴0
[3]   R←I,R
[4]   I←¯1↓I
[5]   →3×1≤⍴I∇
      R N

@EriktheOutgolfer Yuck, no stinking loops!

@EriktheOutgolfer I mean, good job.

@Adám hehehe

well, unfortunately that's the 60s :P

@EriktheOutgolfer No, you're supposed to solve it without using any looping.

.oO( any loop can be replaced with a reduce, if you're ngn enough )

@Ven (no nested arrays)

@Ven APL\360 did not support reductions with user defined functions.

and I think that reductions are a form of looping anyway

@EriktheOutgolfer in that spirit + is a form of looping too

@ngn hm, I'm not sure what Adám means by "no looping", since I doubt it would be possible without /

@Adám can we use ,\ ?

@ngn APL\360 doesn't have that...

@EriktheOutgolfer huh... what about ∘.< ?

@ngn yes it does have that

< is a scalar function

however, ,\ would obviously result in a nested array, which is just too much for ye olde APL\360

@EriktheOutgolfer a very roundabout way: (,(⍳N)∘.≥⍳N)/,N N⍴⍳N

@ngn sorry, but that's Dyalog, not APL\360 :P

@EriktheOutgolfer This is basic APL stuff. Maybe stranding was missing? N N -> (N,N)

@ngn ah, thought that was a train...

@ngn Nicely done!

@Adám I'm sure there must be a shorter way

@ngn Not code-golf.

@Adám uh...except that / does not exist as a function in APL\360

@EriktheOutgolfer Doesn't the expression work? It was an operator which took a Boolean array as operand.

@Adám oh it was‽ I thought it didn't exist at all, that's why I used filthy → there

@EriktheOutgolfer @ngn Now do ∊⍳¨N

@Adám N is a vector?

@Adám ⍳N

@ngn Yes.

huh, N is apparently a vector

(yes, I saw that coming...)

@Adám do we have A~B ?

@ngn Nope, but you can easily write that.

oh, this is so ugly I shouldn't even post it

(,N∘.≥⍳⌈/N)/,((⍴N),⌈/N)⍴⍳⌈/N

@ngn lol just make a tradfn and use local variables

@ngn you can also assign in-place, but I like doing things more orderly :)

(because that totally wasn't my approach)

@ngn Precisely! Your solution is identical to the "official" answer.

the stone age of APL...

@Adám @ngn now do ∪V for a vector V

@EriktheOutgolfer (∨⌿<\∨\V∘.=V)/V

@EriktheOutgolfer ((V⍳V)=⍳⍴V)/V

@Adám did you copy-paste that comment from SO? :P nice

@Adám wait... it had dyadic iota?

@EriktheOutgolfer It is a classic idiom.

@ngn Yes, that's original.

@EriktheOutgolfer I think all the FinnAPL idioms work on APL\360. (Though you have to be explicit about ⎕IO. System functions were not invented yet.)

@Adám @ngn next task: X∩Y for two vectors X and Y (and no, ∩ doesn't exist in APL\360)

@EriktheOutgolfer (X∊Y)/X

@Adám hm, looks like you realized that quickly

@EriktheOutgolfer I grew up with APL*PLUS/PC which was not much more than APL\360. The "PLUS" was system functions and / and \ could take positive integers, not just Booleans. I think that was basically it.

@Adám uh, doesn't \ exist in APL\360? and you can't make tradops and dops, right?

@EriktheOutgolfer \ exists, but only as scan with scalar functions and as expand with Boolean left operand.

@EriktheOutgolfer No user defined operators, and no d-anything.

@Adám What's the d-anythings?

@Adám but I tried +\1 2 3 (with Shift+/, not the \ key directly) and it didn't work

@Pavel dops and dfns.

What's a dop?

@Pavel A user defined operator using the {…} style.

Ah

@EriktheOutgolfer That should work.

@Adám but it's a syntax error

      -?1 2 3
      syntax error
      -? 1 2 3
      )

  +\1 2 3
  SYNTAX ERROR
  +\ 1 2 3
  ^

more like that really

@EriktheOutgolfer Right, Morten seems to recall Scan being a later addition, and indeed, I can't find it in the reference manual.

If I want to do (⍳≢⍵) with scan, is that (≢¨,\⍵)? Can I compose ≢ and , somehow?

@FrownyFrog No, because the , is what creates the prefixes and only when the entire prefix is ready are you ready measure its length.

@FrownyFrog Can you use +\=⍨ instead?

⍞←+/=⍨'abcde'

@Adám 5

⍞←+\=⍨'abcde'

@Adám 1 2 3 4 5

wow cool

Of course, , is applied several times

I get it now

@DevelopingDeveloper Welcome. Interested in learning APL?

@Adám Hey @Adám I am! I have tried to sign-up for your classes a few times and got stuck with a meeting at work during that time and was unable to make it. Do you have any upcoming classes scheduled? I seem to have lost the link which I found to see the class schedule times

@DevelopingDeveloper You don't need to sign up, but if you want reminders, then you can. When a class is scheduled, the sign up page is here. You can read transcripts of past classes here. I usually have classes every Wednesday at 17:30 UTC, but after almost 40 hours of lessons, I've run low on stuff to teach. Any ideas?

@Adám Awesome, thank you for sending me this. Let me look through some of the transcripts and see what I can pick up from those and go from there

@DevelopingDeveloper Feel free to ask anything anytime. Do you have an APL interpreter installed?

@Adám No sir I do not, the small amount of APL I have done has been using TIO

@DevelopingDeveloper OK, while I recommend getting a free personal license you can also use TryAPL for an interactive session (REPL) more akin to the offline experience, or use the chat bot here for single stand-alone lines:

⎕←⍳2 3

@Adám
┌───┬───┬───┐
│1 1│1 2│1 3│
├───┼───┼───┤
│2 1│2 2│2 3│
└───┴───┴───┘