Learning APL

https://xpqz.github.io/learnapl/intro.html

by todsacerdoti • 5 years ago

199 102 5 years ago

xpqz.github.io

queuebert2• 5 years ago

Don't flame me, but I think APL is the perfect language to introduce coding to kids. I've had great success with it personally. The symbols are suprisingly intuitive to them, and APL's strengths match the domain of what kids typically do with computers at first.

rak15075• 5 years ago

Interesting! Aaron Hsu agrees. https://www.youtube.com/watch?v=kIItfQJEVdM

leephillips2• 5 years ago

I don’t think you’re wrong at all. I was not a kid at the time, but APL was the first language I learned. There is one drawback: it kind of spoils you. If you continue to do any programming you’ll probably have to use other (inferior) languages, and you’ll always miss APL.

blowski• 5 years ago

That’s why I recommend starting with GW-BASIC like I did.

vanderZwan4• 5 years ago

it kind of spoils you

I think in some way anyone's first language will spoil them for whatever that language is best suited for. My "first" language was Z80 assembly, because I wanted to make games for my calculator (I ended up writing just one[0], also "first" because technically my first was TI-BASIC of course). I've missed feeling like I directly was speaking "to the metal" ever since. Don't get me wrong, I'm super-happy with the high-level languages I've used since, but whenever I have to wrangle efficient code out of JavaScript there's some (probably misplaced) nostalgia for when I didn't have to worry about whether or not the JIT will kick in or not.

[0] https://www.ticalc.org/archives/files/fileinfo/328/32817.htm...

jazzyjackson• 5 years ago

It may also appeal to children who desire to read and write in a secret code language (seems like a phase some of us have, anyway!)

bryanrasmussen• 5 years ago

so maybe the first language should be brainfuck.

vanderZwan4• 5 years ago

Well, the Snap! people seem to agree, since it's available as a library there, as I discovered just a few hours ago:

https://twitter.com/nathrune/status/1423703525612212232

That actually sounds like quite a powerful addition to Snap!

shaunxcode2• 5 years ago

that is beautiful!

ZachWick• 5 years ago

APL was originally created by Iverson as a notation used to teach mathematics and only became a programming language able to be executed by a computer around the time that (or shortly after) he published his book _A Programming Language_ [0] in 1962.

Even throwing away the “executable by a computer” part of APL and only considering it as a notation, APL can be powerful. Iverson gave a lecture, which was published as _Notation as a Tool of Thought_ [1] that contains a good discussion of what exactly makes a given notation “good”.

[0] https://web.archive.org/web/20141027152546/http://www.softwa...

[1] https://dl.acm.org/doi/pdf/10.1145/358896.358899

DaedPsyker• 5 years ago

Can I ask, why has APL suddenly become the flavour of the month? This isn't a downer on the language, I'm just genuinely curious.

mlochbaum5• 5 years ago

It happens like once a year, it's not that sudden. That said, Learning APL looks like an outstanding resource when other introductions to Dyalog are outdated, incomplete, or just low-quality, so I think it deserves to be highlighted even if everyone's otherwise tired of hearing about APL.

porker• 5 years ago

The Array Language podcast which started a couple of months ago and was featured on HN has helped.

Banana699• 5 years ago

APL, forth and prolog are routinely rediscovered over and over again. They are like a cat standing in the path of a door : refusing the mainstream and refusing obscurity.

Maybe lisp was in that club too but clojure made it mainstream again, by my horribly inaccurate, unscientific and probably wrong reckoning.

vanderZwan4• 5 years ago

Clojure is in a kind of "niche mainstream" position. The others are "just" niche, I guess

rbanffy2• 5 years ago

I love how Forth taught me to build the language up instead if the program down.

Zababa4• 5 years ago

I've been reading hacker news for around two years and it's one of the recurrent topics I think. It comes in waves so we may be in a peak of some sorts, but I think it'll always come and go.

civilized5• 5 years ago

Here's what I don't get about APL. Why is it so important that everything be a single symbol? Why not give the symbols more descriptive names and let people use those names, instead of being forced to work with only the symbols? Heck, it wouldn't be much work to make a text editor that lets you switch back and forth.

APL seems to be designed for people who place an extremely, extremely high weight on code golf-level terseness. IMHO, terseness isn't the be-all, end-all of comprehensibility. I'd rather just use the words "max" and "min" than learn redundant special symbols for the same that save me a couple characters.

mlochbaum5• 5 years ago

If you're used to it, I'd guess you can read [EDIT: was five; thought more] two to four symbols in the time you can read a short name. And you can use the higher-level pattern-recognition skills that would otherwise be applied to reading names to understand combinations of symbols. At least for me it's much easier to visually manipulate symbols in my head.

You've written as though using symbols instead of names comes at an extraordinary cost. Having used both I don't think this is the case (and it's the conventional languages that force you to use words; you're free to define reverse ← ⌽ in APL but usually not the other way around). If you want to read more about this perspective, you could check out my page "What is a primitive?" discussed on HN yesterday: https://news.ycombinator.com/item?id=28060423

valenterry• 5 years ago

I doubt that it can be read much faster. Think about newspaper: you don't read the symbols that make up words - you literally read the words themselves. Taht's why it is pettry esay to fllw txet wtih miexd up caharcetrs. As long as first and last latter stay the same.

One advantage is that long expressions might fit in one line, but that's about it.

I mainly use Scala, which allows to use symbols like ← ⌽ as well and quite a few libraries do that - but I found unless it's a very commonly used symbol, it's not helpful for productivity.

rak15075• 5 years ago

https://code.kx.com/q/ uses words for a lot of things. Instead of the APL ⌈/ you write 'max'. Instead of the APL ⌈\ you write 'maxs'. And so on.

Personally, I much prefer the APL for a few reasons. If I tell you that +/ is sum, you can reasonably guess what product is. (×/).

However, in q, if I tell you 'sum' is sum, can you guess what product is? Well, maybe. But it's prd. Would you have guessed that first try?

This seems like it might not be that important, and obviously to the q developers it isn't. But it is a consideration that logically leads to terse symbols.

If you had to write reduce(plus, arr) every time of course you'd rather have a word for 'sum', it's only because of the symbols that let you decompose larger problems into the smaller primitives without ending up with more verbose code than other languages.

throwaway675309• 5 years ago

With modern day IDEs, auto suggestions would likely lead most developers to easily discovering the exact syntax for "product".

Additionally, words that describe the inherent function makes reading the source A THOUSAND TIMES easier.

If A new developer sees the word max/min/sum, they can almost assuredly guess the meaning versus a random pictograph. And even after learning that ‰ means reduce (for example), it's just one more thing that I have to remember.

Jtsummers3• 5 years ago

The history of APL is helpful to consider. It, initially, was a notation for describing computer systems and programs, and its use as a programming language came later (though not greatly later).

If you want something APL-like but without the symbol heavy nature, the nearest is going to be languages like Haskell that similarly promote tacit programming but without quite as heavy a reliance on symbolic notations, or descendent languages of APL like q.

jodrellblank2• 5 years ago

It wouldn't take much work to make a text editor that lets you switch this:

    if (username == "alice") {
        results[5] = true;
    }

into this:

    if username valueequals STRINGaliceSTRING begin
        let results index 5 be true stop
    endif

because symbols are incomprehensible, words are much clearer, and you don't care about saving "a couple of characters" and you don't like codegolf. Would you use it? If not, why not? How do you know the amount of symbols you use is the perfect amount and not merely the amount you are habituated to?

"Why not give the symbols more descriptive names and let people use those names

For the same reason almost nobody wants {} to be BEGIN/END; the symbols are so ingrained, so well understood, so automatic and habitual that there's no benefit to trying to turn them back into words. Why not move towards doing the same for other common operations? Once you've used ⌊ as floor and ⌈ as ceiling, Math.Floor() feels like a drag. Once you're familiar with 4↑list to get the first 4 items, list[0..4] is a drag and it has more symbols! list.take(4) has more symbols and is no clearer. How do you stop a forever-expanding proliferation of symbols? I don't know, but APL seems to have done a surprisingly good job of general purpose computing in under ~80 symbols which has hardly grown in 70 years.

"APL seems to be designed for people who place an extremely, extremely high weight on code golf-level terseness."

Would you be surprised that this is valid Dyalog APL for a function to find the highest value in an array of positive integers?

    result←findMax data

    max←0
    :For i :In data
        :If i>max
            max←i
        :EndIf
    :EndFor

    result←max

then

          findMax 5 1 2 3 5 6 3 1
 
    6

Dyalog has keywords, classes, namespaces, methods, libraries, and they get laughed at because who wants :EndFor . It's a bit of a PR issue - if you head to APL it's largely because you like golf, because if that's not what you want you may as well use Python/etc. But once you get there even if APL remains too opaque to use for everything it becomes annoying to know a short way to express what you want that you are comfortable with and have to laboriously boilerplate it out in another language with many lines, and have those lines contain more symbols into the bargain. And we say that one of the hard problems is "naming things", symbols and tacit programming can help avoid putting a name to variables that only hold some intermediate state you don't actually care about but need for the next couple of lines.

rajandatta2• 5 years ago

The best way to think about this is that all of these variants exist:

APL uses custom glyphs

J uses symbols from an ASCII keyboard - its a big set of primitives

Q from Kx systems uses words.

The conceptual basis for the languages is very simple. As others have said - your experiences will guide where you feel most comfortable.

rajandatta2• 5 years ago

Commenter here ... meant to say 'The conceptual basis for languages is very similar'.

ronbrz• 5 years ago

Having been interested in APL, but never taking the plunge and learning it, I've seen this article "Notation as a tool of thought" (this is linked in the submitted article as well) thrown around: https://www.jsoftware.com/papers/tot.htm

The paper kicks off with a quote by George Boole: "That language is an instrument of human reason, and not merely a medium for the expression of thought, is a truth generally admitted." Having a terse language in itself can lend itself to thinking about problems and playing around with them in your head more effectively. I know when I'm thinking about an algorithm to be implemented in something like java there isn't room for all the boilerplate in my head. Stuff like "public static void main..." gets fuzzed out when thinking about a solution, to be replaced the idea of "place where main method is".

I like the idea of boiling down useful abstractions to the most simple symbols we can manipulate mentally, to be able to fit the most possible high level abstractions in our mind at the same time. This should make it possible to be able to play more with constructing solutions. Never feeling particularly locked down because you have to manipulate a lot of syntax and curly braces to implement or think about an idea. That you only have to switch around a few symbols for a change in semantic meaning.

Having never learned something like APL myself, I can't speak personally about the effectiveness of this idea.

jodrellblank2• 5 years ago

Something I didn't think of earlier is function trains, which annoyingly won't make much sense without knowing some APL - they're a pattern which takes more effort to explain than they take to start using. But still, simple APL expressions run right to left so the example below pushes 3 into ⊢ which echoes it unchanged, that goes into + which is a no-op here on a positive integer, and then the 3 goes into ⍳ which makes the first 3 numbers:

          ⍳+⊢ 3       ⍝ the first 3 numbers
    1 2 3

it behaves differently from the same thing in parens shown below, which triggers a function train pattern and slightly breaks the right-to-left execution by making the data able to feed into two functions, not just one. In the below code the 3 is pushed into ⊢ on the right which echoes it unchanged but it also jumps over and is pushed into ⍳ on the left to make the first three numbers 1 2 3, and these two results become (1 2 3 + 3) and the whole thing makes 4 5 6:

          (⍳+⊢) 3    ⍝ 3 plus the first 3 numbers

    4 5 6

It's not the parens which make the difference, assigning it to a variable to make a named function also does it:

          f ← ⍳+⊢
          f 3
    4 5 6

Point being, there's something semantically meaningful happening but it has no symbol to turn into words. It's not the proximity of the functions being bunched together even, it's roughly an odd number of functions separated from their argument(s) but with some edge cases. The text editor would have to be parsing APL to detect it, and I don't know how it would describe what's happening in useful words. "iota plus self_right" might still let you recognise three things together, but does "enclose bind grade_up index self_right" look like a function train? Does "It's easier to spot the pattern with symbols, it's easier to have the pattern exist at all in a symbols based language.

When the Dyalog IDE can show you:

          +,-,×,÷

    ┌─┼───┐      
    + , ┌─┼───┐  
        - , ┌─┼─┐
            × , ÷

to explain how the train is being interpreted, what does the text version of that say?

max_• 5 years ago

You might not believe this. But reading the symbols (if u know what they mean) is actually easier that reading words.

For instance, | 3 2 1 0 is read as "reverse" the array "3 2 1 0". So | 3 2 1 0 gives you 0 1 2 3.

It certainly seems easier to read than "reverse 3 2 1 0". – which assumes your a fluent english speaker.

Zababa4• 5 years ago

This link explains it well I think http://nsl.com/papers/denial.html

civilized5• 5 years ago

In a lot of these APL discussions I see an obsession with terse implementations of simple standard library-type functionality. Who on Earth has to literally write a loop in the middle of their source code to find the max of a bunch of numbers? I would just call the appropriate standard library function, especially in languages like Python that have excellent general-purpose data structures and standard libraries for them.

yiyus5• 5 years ago

It's not about terseness, it's about making easy to see patterns and what is going on. You probably learnt the distributive property in school:

    a*(b+c) = a*b + a*c

Let's make that "more readable":

    equals(multiply(a, sum(b, c)), sum(multiply(a, b), multiply(a, c)))

It's not about the terseness, I just don't see a clear pattern using the "readable" notation. But it is obvious when using symbols.

You can find another interesting example at: https://wjmn.github.io/posts/j-can-look-like-apl/

seppel• 5 years ago

But it is the switching from infix operators to prefix operator that is making the second example less readable (and the implicit precedence of the operators). Writing it as

    (a times (b plus c)) equals ((a times b) plus (a times c))

would be a fair comparison, which is much more readble than

    =(*(a, +(b, c)), +(*(a, b), *(a, c)))

civilized5• 5 years ago

I like the idea that APL could provide this advantage but I just haven't seen any compelling examples. And that's my experience with a lot of APL stuff - beautiful theory that just doesn't quite seem to deliver on the promises.

It feels like the Zen of APL is "symbols are a freaking great idea, let's do more of those!" when intellectual communities that are heavy on symbolic manipulation are generally quite conservative in the introduction of new symbols... let alone the dozens that APL introduces.

To each his own and all, but APL might have to content itself with a small group of devoted fans.

082349872349872• 5 years ago

Try using a mid-1960's i/o device: a cardpunch, a tape punch, a mechanical tty at what, 110 baud? (or less)

skruger2• 5 years ago

Author here. I wrote this because I thought APL deserved an up to date introductory text. It's basically the notes I kept when learning it myself. Stoked to see it on HN.

sxp• 5 years ago

Thanks for the writeup. I'm still not a fan of APL due to its non-ASCII characters, but I like the link to https://beyondloom.com/blog/denial.html that you provided.

moistly2• 5 years ago

I note that on my iOS iPad some of the symbols render as a empty boxes, e.g. in the Fizzbuzz example the left arrows point to an empty box. I have no idea if that is in fact the correct symbol—it could well be! (Also, I have no idea if this could be corrected.)

Jtsummers3• 5 years ago

https://mastering.dyalog.com/User-Defined-Functions.html

See 5.3.5 for what that bit of code represents. The box is, in fact, being displayed correctly.

moonchild2• 5 years ago

Heyo, I wrote a small critique[0] of it on lobsters. Nice stuff! Glad to see we finally have an alternative to mastering dyalog apl.

0. https://lobste.rs/s/stk8h0/learning_apl#c_g0p3y1

alanbernstein• 5 years ago

Thanks for this. I don't know when I will make the time to learn new languages, but I think I've decided that when I do, one of the array programming languages is where I'll focus. This page seems like as good a place to start as any.

The xkcd "light saber" thing made lisp seem appealing to me a few years ago, but after a brief intro to the array languages, now I think they're much more deserving of that comparison.

arendtio• 5 years ago

To me it looks like one of the steepest learning curves I know: uncommon syntax, special characters, custom keyboard layout required, different dialects, ...

And in the end, I wonder if it is more readable than regex which is sometimes considered 'write-only', because reading it can be so time consuming. I know you wrote about the readability topic, but I don't mean 'can I read it', but rather 'How much effort takes it to read it'.

rak15075• 5 years ago

I'd say there are two learning curves involved. The first is knowing what every symbol does. This sounds like it will be quite hard, but honestly probably takes less than a day. APL really doesn't have a lot of stuff to remember. It just looks a bit weird.

The second learning curve however is the much harder part, learning how to write APL 'idiomatically'. This is mostly true in any language though, especially one in a different paradigm.

leephillips2• 5 years ago

APL, among its other advantages, does have a steep learning curve: you can learn it very quickly.

foxfluff4• 5 years ago

As someone who spent a few weekends learning APL: no, the learning curve is not that steep.

The syntax is very simple, there's nothing scary about special characters if you can get over your fear of them, custom keyboard layout is absolutely not required (I've never used a custom layout for APL or Japanese or Cyrillic or Greek or any other non-latin characters that I type on a keyboard). Dialects are a thing but Dyalog is such an obvious choice if you want to learn APL that I wouldn't even worry about the rest. (K is worth looking into for its own sake but I don't consider it a dialect of APL; J might be a dialect worth exploring but so far it didn't impress me much and I found it much harder to read than APL).

One more thing: I find that reading APL is easier than writing APL.

giardini• 5 years ago

arendtio says>"To me it looks like one of the steepest learning curves I know: uncommon syntax, special characters, custom keyboard layout required, different dialects, ..."<

so, while a considerably smaller language, it's like learning Chinese (for someone who uses a Western alphabet)?

tigershark• 5 years ago

Maybe I’m missing something, but at the bottom there are these two statements:

  7×3 ⍝ Dyadic × is multiplication
  ×¯7 ⍝ Monadic × is 'direction', or signum (the sign, in our case). Thanks.

And looking at the output I can see:

  21
  ¯1

Is it a typo or really -7 is becoming -1 for some esoteric reason that escapes my comprehension?

leprechaun1066• 5 years ago

It's the sign of the number. -7 is negative so you get -1. 7 gives you 1 and 0 gives 0.

yiyus5• 5 years ago

It's not a typo. Monadic × will return 1 if the right argument is positive, ¯1 (minus one) if it is negative, and zero if it is zero.

nudpiedo• 5 years ago

isn't there any way to download the whole thing as PDF rather than just each section?

jboy552• 5 years ago

I had given myself a project of writing my interview phone screen coding question in as many languages as I could, more as a lark, but also in 'prep' for a candidate being 'funny' when I say 'use whatever language you are comfortable in'.

I did APL, which was fun, and easy. However in researching the language I came across some IBM accounting source code written in APL. It really clued me into how the language got such a following in HR/Finance applications. You got an array of everyones hours per day. Weekly total hours, pay, tax, medicare, etc, were all broken out in little calculative statements based on the array of hours. So rather than long strings of APL characters, there was just massive amounts of english and these little digraphs or trigraphs of APL for the calculative values.

valbaca• 5 years ago

I had given myself a project of writing my interview phone screen coding question in as many languages as I could, more as a lark, but also in 'prep' for a candidate being 'funny' when I say 'use whatever language you are comfortable in'.

I would honestly love a candidate to use some esoteric/legacy/wacky/niche language to spice it up.

If they can walk me through it that would be a Strong Hire in my book (assuming it actually works when I test it later).

fasquoika• 5 years ago

It really clued me into how the language got such a following in HR/Finance applications. You got an array of everyones hours per day. Weekly total hours, pay, tax, medicare, etc, were all broken out in little calculative statements based on the array of hours.

Yeah I guess APL is kind of 'spreadsheets as a Real Programming Language', isn't it?

NhanH• 5 years ago

Do you still have a link to said source code?

jboy552• 5 years ago

I wish I did, I've searched for it a few times, but that was perhaps 10 years or more ago.

klelatti• 5 years ago

I can remember still attending an IBM planning meeting in 1984 when APL was being touted as 'the future'. Cue lots of eye rolling from the sales teams who were not entirely convinced.

Many years later I came across it again in a financial modelling application where it worked well. Only problem was that there were (I think) only three people in the company who understood it. So it didn't last long as the risks / costs were just too great. Which I guess would be a typical scenario for many APL applications.

threatofrain• 5 years ago

What is the most popular and optimistic APL'ish community today?

chrispsn• 5 years ago

The APL Orchard and The K Tree.

https://chat.stackexchange.com/rooms/52405/the-apl-orchard

https://chat.stackexchange.com/rooms/90748/the-k-tree

joeman1000• 5 years ago

This is really cool. I’m glad I didn’t get too far into learning it now, I can use this guide happily! Thank you for your effort in producing it.

emmanueloga_• 5 years ago

Here's a silly idea... could APL be to [programming language] what emmet.io is to HTML and CSS? :-)

I imagine writing an APL expression that expands to, say, 100 lines of C++ :-p

rak15075• 5 years ago

Cool idea, https://github.com/phantomics/april does this a bit, would definitely like to see something similar in other languages too.

haolez• 5 years ago

I like APL's symbols, since the code becomes easier to read. But I also like the tacit stuff from J. What should I do? :)

jph00• 5 years ago

Dyalog supports tacit programming, much like J https://aplwiki.com/wiki/Tacit_programming

moonchild2• 5 years ago

Not so well. There are no capped forks and, more importantly, no gerunds.

cjv• 5 years ago

How does APL compare to J? Which one is more enriching to learn?

skruger2• 5 years ago

I asked myself the same -- should I start with APL, J or K? I concluded that APL is the "original" from which the others draw inspiration, and so it felt most natural to start there. Note that wherever you decide to start, the experience you gain carries over to the others.

pabs32• 5 years ago

I wonder how many other programming languages there are that are still mainly proprietary. It seems like most languages these days have their primary implementation as open source.

razetime2• 5 years ago

APL's ASCII based relation, J is opensource(GPL, I think). [BQN](https://mlochbaum.github.io/BQN/) is a newer rationalized entry which is completely FOSS.

K is another array language which has a proprietary implementation. Tcl is a relatively popular proprietary language.

pabs32• 5 years ago

Thanks for the info/links.

Wikipedia says Tcl is an open source language, did you mean a different Tcl?

https://en.wikipedia.org/wiki/Tcl

rbanffy2• 5 years ago

IIRC, one key selling point of K was that it’s runtime fits inside an x86’s L3 cache.

ano88888• 5 years ago

my brain hurts just by looking at the sample code. Pass for now.

yewenjie• 5 years ago

Is there a modern APL like language with the same level of terseness but with open source implementation?

grzm• 5 years ago

You might want to take a look at J, also by Iverson (along with Hui)

https://en.wikipedia.org/wiki/J_(programming_language)

https://www.jsoftware.com/#/

userbinator• 5 years ago

https://en.wikipedia.org/wiki/J_(programming_language) is probably the most well-known of them.

Learning APL

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