I attempted to run that APL, and it didn't quite work as is on my system (Dyalog on macOS). As written, it seems to be an odd mixture of things that assume ⎕IO 0 and ⎕IO 1, in that with ⎕IO←1, the bymonth assignment only gets two columns instead of three, but with ⎕IO←0, it needs to be 0=22 and 1↓cal instead of 0=23 and 2↓cal.

Similarly, the lines assignment needed to be {⍵⊂⍨0=9|⍳≢⍵}⍤1⍉bymonth.

After those changes it worked OMM, but the month names were appended directly to the end of the first Saturday's date with no space (e.g. "1 2January"). So I changed ↑months to ↑(' ',¨months) in the last line to insert a space there (there's likely a better way to do that; I'm still pretty new at APL).

I added `-h` to make sure `cal` didn't highlight today's date, which can throw things off, and eliminated intermediate vars that were only used once; this was what I wound up with (works from a clean ws in Dyalog OMM):

    ⎕io ← 0
    lines ← {⍵⊂⍨0=9|⍳≢⍵}⍤1⍉{⍵⊂⍨0=22|⍳≢⍵}⍤1↑1↓⎕sh 'cal -hy'
    months ← ~∘' '¨,⍉⊃¨lines
    days ← ∊⍤1⍉{⍵/⍨(' '∨.≠¨⍵)∨∧\' '∧.=¨⍵}⍤1⍉¯1↓(23⍴1 0 0)⊂⍤1↑⊃,/,⌿2↓¨lines
    ⎕←53↑(¯2↓⍤1⊢days) , ↑(' ',¨months)\⍨∨/' 1 '⍷days

It (should be) purely ⎕IO←0. https://tio.run/##jZK7SgNBFIb7fYrTbQKbyyYINum1srBNM27G7OLemJ...

Potentially cal -y on macOS produces a different shape? What's ⍴↑cal?

Try doing cal←⎕SE.UCMD'Calendar 2021' and replace ↑2↓cal with 1↓cal - potentially that'll work?

⍴↑⎕sh 'cal -y' is 36 66. ⍴⎕SE.UCMD'Calendar 2021' is 33 66. Hm.

What system are you on? On Ubuntu, cal -y | wc -l also gives me 36.

I was just testing with TIO, it looks like the version of `cal` on there is different to the common one. The ⎕SE.UCMD'Calendar 2021' version should work cross platform (as it doesn't rely on anything external).

It's too bad APL never enjoyed ubiquity on Unix-like platforms. The world would have been a better place. Maybe if Stallman or some early GNU participant had championed APL we'd all be either using APL or constantly excusing ourselves. Like Emacs did for Lisp.

I'm decent with sed. I wouldn't call myself an expert, but know how to employ addressing and hold spaces, which are sadly aracana to most people who employ sed. I ended up learning sed because it was there, proved useful, and it fit into the command-line toolkit really well. (In order of importance.) APL would never slot into that toolkit as well as other tools, but it's value-add would more than compensate. Unfortunately, the value-add isn't enough to compensate for the fact that it's not there and in your face every time you're exploring a problem and its solution space.

Seems there is an alternative history where Stallman went with APL and not lisp:

https://en.wikipedia.org/wiki/APL_(programming_language)#GNU...

> (...) Richard Stallman, founder of the GNU Project, was an early adopter of APL, using it to write a text editor as a high school student in the summer of 1969.

And:

  Rho, rho, rho of X
  Always equals 1
  Rho is dimension, rho rho rank.
  APL is fun!

    Richard M. Stallman, 1969

https://www.gnu.org/software/apl/

But:

> GNU APL was written and is being maintained by Jürgen Sauermann.

and I believe it's fairly young when compared to emacs/emacs lisp.

> Maybe if Stallman or some early GNU participant had championed APL we'd all be either using APL or constantly excusing ourselves.

I think the major issue is that APL seems to require more than ASCII to work it.

AFAIU APL implementations very early on supported ASCII equivalents. But maybe it was too late to either disabuse people of the notion that it required non-ASCII, or the convention proved too strong to overcome.

there are ascii variants of APL like j,q,k etc

In the very halcyon days of Unix, lets just say early-80's, I met so many new-to-C guys coming away from APL screaming, with their hair on fire, that I, as an innocent young mangenue, was sure it would be one of those languages that would die a horrid yet rapid death and I could therefore completely ignore it.

Yet, here we are. I'm still amazed to see people using it, and even more amazed to see someone break it out for some toy play.

Still, not gonna learn it. I like my hair.

Yeah agreed, Dyalog APL started out on Unix but is now more focused on Windows, I wish I could integrate it into scripts and things but it's often difficult/impossible to do so.

(⊃0⍵0)⊃ ⊂{⍨⊂}

And people say Ruby is beautiful