Why Concatenative Programming Matters
evincarofautumn.blogspot.com
evincarofautumn.blogspot.com
Yes, I know Christopher Diggins' Cat is statically typed. The problem isn't in compiler diagnostics, it's in readability.
In a stack language, an expression as simple as 'a b c d' is cryptic. If c consumes two arguments, in applicative terms it would be d(c(b, a)); if b consumes one argument and c only one, then it would be d(c(b(a))); if d consumes two arguments, b one, and c none, then it would be d(c, b(a)). And this ambiguity makes it very hard for me to read the code unless I'm intimate with all the individual words. You have to understand everything to understand anything.
Unfortunately, I suspect that concatenative languages tend to derive their power from this ambiguity, because the lack of specificity leaves considerable flexibility for what the words can do with the stack.
So my considered opinion for now - and this article doesn't really change that, it is mostly a tutorial - is that stack languages are best as a compiler target rather than a human source form.
However, giving Prog a concatenative basis lets me do all kinds of interesting things much more easily than if I were to work from lambda calculus. With the syntactic issues taken care of by sugar, it’s a net gain. I hope you’ll see your way to trying it out one day.
For what it's worth, my experience is that practice and good factoring can make concatenative languages much less cryptic. In Forth, the ideal size of a procedure is about a line, and procedures that take more than three arguments are considered a "code smell" that suggests a different factoring might be more appropriate. When you only care about the top few stack elements within any given definition there's a lot less to juggle mentally.
[1] http://en.cppreference.com/w/cpp/language/operator_precedenc...
Yes, but all of those operators are binary operators, instead of let-me-look-at-the-documentation-ary, lots of those precedence rules are drilled into people at a young age, and lots of developers compulsively wrap them in parentheses anyway, just to make their intent absolutely clear.
All of that is very different from the experience of a concatenative language.
That might be the ideal, but if you take a look at real-world Forth code, most of it is much longer than the ideal.
There are a couple of other things that help readability in Forth. One is thorough commenting, and the other is the use of intermediate variables.
Unfortunately, real-world Forth code often doesn't have much of either -- intermediate variables are often frowned upon as being "un-Forthish", and thorough commenting results in a program that is so verbose that (except for the embedded domain) you might as well have written your code in a more conventional language in the first place.
Idiomatic Factor does not (explicitly) use the stack.
Also, Factor does allow you to use local (named) variables and infix syntax when it makes more sense to do so. Concatenative languages do not need to be unreadable at all and for some things I think the concatenative code is actually more naturally readable.
"If c consumes two arguments, in applicative terms it would be d(c(b, a)); if b consumes one argument and c only one, then it would be d(c(b(a))); if d consumes two arguments, b one, and c none, then it would be d(c, b(a))."
'a b c d' in a concatenative language is akin to 'a | b | c | d' in a shell script (where a stack is being threaded from function to function) or 'd(c(b(a(STACK))))' in most languages where each function has a type 'Stack func (Stack s)'. It doesn't change depending on how many arguments a function takes off the stack - its always 'd(c(b(a(STACK))))'. Of course, since literals are essentially functions which push their value onto the stack, what you say is true if a, b and c are literals. I don't find this nearly as confusing as what it sounds like from your comment.
In code like '1 2 + print', all the words you see are functions. 1 pushes the value of 1 onto the stack and so on. This is interesting because you can easily factor out any part by replacing it with a new word that calls the replaced word:
:A 1 2 + print
:add2 2 + ;
:B a add2 print
This is also interesting: http://concatenative.org/wiki/view/Concatenative%20language
In a shell pipeline of 'a | b | c | d', I can see the dataflow clearly. 'c' never sees the output of 'a' directly; it must be mediated by 'b'. But for a stack, 'a b c d', I have no idea if c uses the value a or not, unless I know the definition of b and how many items c consumes.
When reading stack code, I find myself mentally inserting parentheses to see how far back each operation reaches; for higher-level operators, I find myself wanting to insert comments describing the shape of the code being implicitly constructed. I need to see the "tree" (though frequently a dag) shape.
This isn't different in complex infix expressions. There too, parentheses are often needed for clarity - and good programmers use them even when not necessary - and for very complicated expressions, I'll spread them out across multiple lines, with indentation to make the tree explicit, where performance or similar concerns prevents writing out the expression in a more long-hand fashion.
I've also been doing a lot of Max/MSP programming lately and really love working in its "passing messages along patch cables" system, but am frustrated by its lack of higher order objects and lack of aggregate data types (hell, it won't even let me create a list of lists).
What I really want is a Max/Pd-like language with richer set of data types (algebraic types with pattern matching would be a good start) which can be trivially and unambiguously converted between visual and textual representations. A concatenative language may make this a lot easier.
It's an applicative language created especially for audio DSP. It seems to bridge the gap you describe by providing dataflow combinators and a graphical representation for programs.
[1]http://en.wikipedia.org/wiki/Stack-oriented_programming_lang...
http://thinking-forth.sourceforge.net/
Also see https://news.ycombinator.com/item?id=3389286 http://news.ycombinator.com/item?id=2378062 http://news.ycombinator.com/item?id=2884131 http://news.ycombinator.com/item?id=2585750
In my work, I found that the lack of syntactic marking for arguments of function calls is a devastating drawback. Coming back to my own function definitions a week after writing them, I spent minutes unpuzzling the code. I could use every trick and write the function in the most elegant way, using combinators in the style of Factor, and that made it worse. But writing it out with local bindings also didn't help. As soon as you write a nested expression, you start to lose the sense of which values are being passed where.
gcd n1 n2 n2 0 = if n1 else n2 n1 n2 mod gcd
gcd zero if else "mod" plus swap gcd
factorial 1 interval product
I was designing a Forth-like functional language with generic functions, and for a long time I felt that the lack of markings didn't matter. I thought that one could understand the code "well enough" without knowing exactly where the arguments were going. There is something to this argument, and there's a lot we can do to make code say what it does. But sometimes the author just had to implement something, and technicalities arose, and when you go to read it, the code doesn't "read" and you have to bring your own intelligence to figure out what it does -- it won't tell you. In these cases, explicit argument marking is indispensable.
In Forth-like languages, simple things are wonderful and elegant. There is no other shorter way to write than point-free. And, writing with combinators is satisfying, and it can make the "essence of the function" more evident. But unless you severely constrain your style to ultra short definitions, avoid nested expressions, and use local variables profusely, you're constructing puzzles for yourself.
It's almost like the concatenative style is too optimal. You have to back up a step in beauty in order to get a practical tool.
Makes me wonder if trying to do what you want in that model is a recipe for disaster; instead, maybe you have to let the model tell you what you can do: if it's simple you get to do it and if it's not you don't. Chuck would see that as an advantage, but most programmers wouldn't.
Edit: the article I linked to talks about that at length. His conclusion is that if you're going to build systems in Forth, you need the freedom to shrink or morph the problem into something that can be solved simply in Forth. If you don't have that freedom (e.g. because people are feeding you inflexible requirements), it probably won't work...
The FP paper is very effective in making people who are hard core imperative programmers want to try FP. But the CP article here doesn't really IMHO elicit the same desire.
You should try writing another one with some practical examples.
The answer seems to be that concatenative languages give you the power of functional languages using a low-level nearly absent syntax (like Lisp). I guess that's useful as way to demystify the compilation process (like how assembler and CS demystify how the CPU actually gets work done, as contrasted against magic like Ruby), and matters if you want to understand or write a VM, or reason about a program in a completely unbiased way that (for better or for worse) has no cognate to human language (but may be an elegant non-linguistic mechanical language).
For general purpose programming, I prefer a little syntactic sugar. Humans seem to have an instinct for language, and it is helpful to leverage that instinct when writing and ituitively grokking programs, if not for constructing formal proofs.
with that said, you can do "concatenative" programming in any language supporting a functional style (python, ruby, clojure, haskell, javascript), and some of these languages (clojure, haskell) encourage or force functional style. clojure and haskell have different opinions about how this should be done, but they are opinions, not gospel, and each has their use for different problem domains and personalities.
the author makes some assertions that a "concatenative" language, one that encourages a concatenative style, are better than functional languages. i don't think these opinions were adequately backed up at all - i'm not just skeptical, i sort of think he's wrong. both clojure and haskell strongly encourage composition over application. these languages already have concatenative opinions. i don't really see the fuss here, i don't see the need to introduce more metaphors into an ecosystem which is already unfriendly to noobs. what we need is a language as noob friendly as python which has functional opinions. ruby got noobs using lambdas without realizing it, but ruby also has some weird opinions about magic which the functional community frowns upon. there's a clear niche here.
[1] (edit for gruseom) but we can try to learn from those who are successful, especially those who failed a few times first. Martin Odersky has written all about his failures leading to Scala, and Rich Hickey talks about what he thinks made Clojure successful all the time.
That's exactly right. But then you go on to do just that ("from this we learn...") in a bogus way. We don't know what drives these things. All we have are competing arguments from personal preference.
And I would love to see a functional language as powerful and carefully designed as Haskell, without all of that language’s minor annoyances and barriers to entry. Even though the Haskell culture is great, it’s unfortunate (to paraphrase SPJ) that they picked the scary term “monad” instead of “warm fuzzy friendly thing”.
If it weren't for it sitting on the JVM and having access to all those Java libraries, Clojure would have been dead in the water.
Cat - Christopher Diggins http://www.cat-language.com/
I have found a concatenative language (Factor, specifically) to be extremely useful for use in a workflow system. In particular, since it's not applicative, there's no API between "words", so it's damn near trivial to combine workflow sequences together -- no API is necessary. Each sequence of words (think of it as a workflow) can consume and or produce as much data as needed -- again, without any API coordination whatsoever.
In an applicative language this is explicit and it is probably the main reason for the fact that it is relatively easy to understand code in any procedural language once you've learned one.
(On second thought, this post's title echoes Backus's. Maybe they would include FP.)