Newbies might get the impression – as I did – that Emacs was all about functional programming because all I knew back then was C, SQL, and some weird proprietary stuff at work. We’re not an encyclopedia and not the One Manual, thus we can have multiple points of view, no problem. The only difficulty is presenting them well without confusing newcomers. :) – AlexSchroeder
What is important here is to characterize functional programming (briefly) and relate those characteristics to EmacsLisp. This is about Emacs Lisp, and some of it is about Lisp as a consequence, to the extent that some things true of Emacs Lisp are true of Lisp in general, and some of those things are pertinent wrt functional programming. There is not one characteristic that I listed that is not pertinent to situating Emacs Lisp wrt functional programming.
As Alex points out, there is room for multiple views. But it is absolutely incorrect to think that Lisp is not an imperative, procedural language. That is not a matter of opinion. Lisp programmers know this.
And that holds for all Lisps, including Scheme, but with the exception of something sometimes referred to in academic papers (e.g. about program transformation or proof) as “pure Lisp”, which has been neutered of all possible side-effects, which includes
‘put*’, etc. “Pure” Lisp is an abstraction that no one sees in normal programming life.
It is somewhat more a matter of opinion whether Lisp should be called a functional language, that I’ll grant you. That’s why Wikipedia finessed the question, calling it a “functional flavored language”.
It is true that there are lots of people who are not very familiar with (true) functional languages, lambda calculus, or combinatory logic, and who have the vague notion that Lisp is a functional language because of its functional appearance or the fact that 50 years ago it was the only thing even close to a functional language.
That misconception does not make it so. For 30 years now we have had real functional programming languages, and nowadays it is nothing but a misnomer to refer to Lisp the same way. Lisp is to a functional language what Mock Lisp was to Lisp: it has a similar appearance and little more.
For the record, I am a strong proponent of functional languages and declarative languages generally (have been for 30 years). I also enjoy programming in Emacs Lisp. – DrewAdams
Fair enough, then I will explain the substance problems which I’ve previously suggested be dealt with at Wikipedia.
In the same way that multiple opinions are welcome on this Wiki, multiple paradigms are available in the Lisps. You are getting hung up on the “true” and “pure” definitions of “functional programming” (probably because you are familiar, as you say, with recent languages that enforce it). Again, pure functional languages are “an abstraction that no one sees in normal programming life”. Discussing pure functional programming is of little value here as well. Wikipedia? Maybe. I’m sure you’ll agree that Lisp has imperative and procedural shortcomings as features, not as bugs. I never wrote anything here that said Emacs Lisp or even Lisp was or wasn’t a functional programming language. You have, however. The name of the page is “functional programming”, not “functional programming language” or “pure functional programming”. However, the functional programming capabilities of modern Lisps are complete. It obviously can’t be said for McCarthy?’s 1959 Lisp system nor for Emacs Lisp. ObjectOrientedProgramming? considers some languages to be pure and some not, but that would also be a rather academic exercise.
I say bring back the list of functional programming features as they apply to Emacs Lisp. Let’s incorporate Alex’s suggestions and I welcome Drew’s corrections, but have seen none of them. – AaronHawley
I specifically mentioned that Lisp “supports multiple programming styles or paradigms, including functional”. You can program in a functional style to some extent. That does not make Lisp a functional language.
Pure functional languages are not “recent” (30+ years is pretty old now as programming languages go, considering that Fortran and Lisp, the oldest, are themselves only 50 years old). Nor, unlike “pure Lisp”, are they “an abstraction that no one sees in normal programming life”. That someone might not be familiar with functional programming does not make them an academic exercise. They let you go beyond talking the talk (functional style) and start walking the walk (functional programming).
Discussing functional programming might, as you suggest, be of little value here. That is why I removed the page and changed the (very few) existing links to it to links to the Wikipedia page for functional programming. But you didn’t allow that.
In 2011 (and for a long time now), “functional programming” means what is described at that Wikipedia page. In short, it means programming using a purely functional language – or at most programming with a not-quite-pure language such as ML (which language you know, IIUC), but avoiding side effects. That’s not Lisp, I’m afraid. For someone to try to really use a Lisp as a functional language is not so trivial. You can do it, but you can also use C that way, or Prolog, or even Fortran (simulated recursion - used for things like CAD/CAM).
If we must have a FunctionalProgramming page on ThisWiki, then it should clarify the relation of EmacsLisp to functional programming (duh). And that means, first and foremost, pointing out that it is not a functional language and in what ways (how) it is not. That entails briefly listing the characteristics of a functional programming language and pointing out which of them are missing/present and to what extent.
Wrt your text:
Finally, I am fine with our removing this apparently controversial page and simply linking to Wikipedia for information about functional programming. I already tried that once as a reasonable compromise, but you simply reverted everything back to your own text, for the Nth time.
You are entitled to your own opinion, but not to your own facts. If you nevertheless find that your opinion trumps the characteristics of functional programming as I’ve laid them out here, then I propose (again) that we remove this page and simply link to Wikipedia. You can take it up with them. What we should not do is mislead people by having this page indicate that Emacs Lisp is about functional programming. It is not. At all. In fact, unlike Scheme and Common Lisp, Emacs Lisp is not even very supportive of progamming in a functional “style”. – DrewAdams
Ok. So we can have a list that tries to “clarify the relation of EmacsLisp to functional programming”. You suggest that some of the features aren’t “a necessary characteristic of functional programming”. I never did nor will I say that they do, but we can agree they are qualities which bear some significance and are worth mentioning for EmacsLisp’s functional programming-ness. To use the OOP analogy again: Message passing, polymorphism, inheritance these are qualities of OOP but their existence or absence do not necessarily make or break a language’s OOP-ness. – AaronHawley
Those OOP features/characteristics are not so much implementation concerns/techniques as are memoization, lexical scoping, and tail-recursion optimization.
And OOP is far fuzzier a notion that is functional programming. At some point or another nearly everything has been called “object-oriented”. Not so, for functional programming features.
Anyway, I repeat my proposal that we remove this page. Any page of ThisWiki that needs to link to an explanation of FunctionalProgramming can link to Wikipedia. That is a reasonable compromise wrt our differing points of view about this. There were only a couple pages that linked here anyway. – DrewAdams.
Indeed, the OOP analogy is less than perfect.
I’m not looking for a compromise, just a simple page explaining the orientation of Emacs Lisp to functional programming. I think it’s deserving such a page on the Wiki. A shorter version of the page also lends itself to emphasizing the links to Wikipedia whose material I think we both agree is more useful. – AaronHawley
(first draft removed) – AlexSchroeder
This is an improvement, especially the intro.
That “Lisp is an imperative, procedural language, like C and Fortran but unlike C and Fortran it is also an applicative language” may be true, but it unfairly equates Lisp with these languages on a page about functional programming. I’m sorry, but Lisps’ functional programming abilities are not hobbled as C and Fortran’s in this category. The confusing comparison doesn’t even seem worth mentioning.
Further, “What makes Lisp stand out from other procedural languages is the degree to which it uses expressions that return values and the ease of higher-order programming (in particular,
‘eval’).” This is duplicative of the previous bullet, and the last part isn’t even entirely true of Lisp and especially Scheme. Keep the discussion to Emacs Lisp, no? You could change the sentence to apply to Emacs Lisp rather than just Lisp, but Elisp isn’t deserving a superlative on “higher-order programming”.
I’d still like to see mentions of optimized tail recursion and lazy evaluation since they’re relevant to Emacs Lisp.
Also, I’d like to try using Alex’s suggestion about the commonly seen style used in Emacs Lisp – with its emphasis on functional programming but reliance on imperative and procedural features. – AaronHawley
Yes, it looks fine. – AaronHawley
Sorry, but Emacs Lisp cannot usefully be written in a declarative style (whatever “style” might mean here, which is probably nothing). Emacs Lisp is simply not declarative. Yes, you can write a purely declarative, recursive definition of fibonacci or some such in Emacs Lisp, but such toys really have little to do with programming using Emacs Lisp.
If you want to say that you can write Emacs Lisp code so as to minimize side effects, then just say so. That is clear and correct.
Things are much stronger than “nothing prevents you from using imperative, procedural style”. Emacs Lisp is inherently imperative and procedural. You’ll be hard pressed to use Emacs Lisp much without side effects. It’s simply not how it is used (usable) as a language. Telling users, especially newbies, this kind of thing is extremely misleading.
The most important thing we can tell users about the relation between Emacs Lisp and functional programming is that there isn’t any. Or there is very little such relation. Word it as you like, but anything that gives a different impression, such as it’s a realistic choice to program with Emacs Lisp as if in a functional language, just leads people down the garden path and sets them up for disappointment and confusion.
Our advice to Emacs Lisp newbies should be, first of all, to learn the nature of Lisp – what’s particular/special about it. We should advise newbies to avoid destructive functions (esp. list-modification functions), but also avoid pretending that Emacs Lisp is functional (cascading maps, reduces, etc., creating mountains of conses)…
You simply do not program in Lisp the same way you program in Haskell (or in C, for that matter). You cannot, at the end of the day. And you should not try, at the beginning of the day. Lisp is peculiar. Users need to know about list structure and symbols etc. In Emacs they need to know about keymaps and such.
I grant you that if someone is new to Lisp and completely unfamiliar with functional programming, learning about declarative, recursive definitions, higher-order functions, and such can be an eye-opener. But that is hardly learning Lisp; it is only a first step in learning, and it is hardly typical of Lisp programming.
I’m sorry, but it is completely misguided to give the impression that Lisp is a flavor of functional language, in the sense that you program it the way you program, say, Haskell. To do that is to do users a real disservice. They deserve better.
As for how you want to word this page, I leave it up to you. – DrewAdams
I added a stronger objection and replaced the original text. – AlexSchroeder
Yes, that’s true. In order to get things done in Emacs Lisp, you need to make side effects to buffers, keymaps, window configurations, association lists, text properties – in addition to assigning values to global symbols (variables).
Off topic but, I browsed the Introduction to Emacs Lisp manual and was surprised how little functional programming and “the nature of Lisp” was revealed. It presents lists, variables, functions, commands but then launches into frobbing buffers. Clearly, the emphasis was on “Emacs” and not the “Lisp”. This trade-off is unfortunate but likely more practical for teaching readers how to get things done in Emacs Lisp. – AaronHawley
I just finished reading Doug Hoyte’s Lisp is not functional. I am convinced. – Alex
Never heard of him. The book looks like it’s just self-published on Lulu. I have nothing against Lulu, but a questionable source. I’ve added quotes from David Turner and Anton van Straaten. – AaronHawley
– AaronHawley 2012-11-15 23:23 UTC
Seems to me you all basically agree what Elisp is like as a language and are arguing mainly about semantics, but the semantics of something that doesn’t have a universally accepted and precise definition.
In my experience, some people use “functional” loosely, to mean that a language has functions as first-class objects (which usually also implies it has useful functions like apply, map, reduce, etc.). Other people use “functional” more strictly, to mean that the language is side-effect free. The first camp often call the latter “pure functional”.
I’ve always thought of Lisp as a functional language, and also a procedural language, but not a pure functional language. So I guess I fall into the first camp. But I have no problem with people who prefer to reserve “functional” for languages like ML and Haskell, as long as it’s clear what’s meant.
Doug Hoyte’s argument is premised entirely on the assumption that “functional” means functions are side-effect free. That’s fine in a book, where you can choose to define it that way explicitly. But a wiki like this should try to reflect common usage, not get bogged down in religious wars over definitions. The fact that there’s no broad consensus about what “functional” means makes this harder. But a significant proportion of people use “functional” in a looser sense, to mean a language with first-class functions. (Otherwise Doug Hoyte wouldn’t have needed to write his rant in the first place!) And those people might come looking for a discussion of functional-flavoured Elisp features on this wiki.
The current FunctionalProgramming page is terrible. Most of it is pointless evangelizing about what “functional” should mean, based largely on appeals to authority. If the page is going to be a useful addition to this wiki, it needs to get the semantics out of the way in a sentence or two (acknowledging that different people mean different things by “functional”), and then go on to explain something useful. Like some of the functional-flavoured features of Elisp which people more used to C or Fortran might not be so familiar with. Bonus points if there are examples showing how to do the same thing in both a procedural and a more functional style.
Personally, I’d delete everything except the first sentence (if necessary replacing it with links to Wikipedia and other more appropriate discussion forums for debating the definition of “functional”). Then we could put some useful content here.
Just my 2c.
– TobyCubitt 2012-11-21 12:45 UTC
Sounds good to me! :)
– AlexSchroeder 2012-11-21 12:52 UTC
Deleting all but the first sentence sounds good to me too. Alternatively, keep the first two sentences. And I agree that the appeals to authority here are silly.
But if we keep just those one or two sentences, there is perhaps even less reason to keep the page at all. I say we just delete the page and let folks google to find out about “functional programming” (and even “Emacs Lisp”). Just one opinion. Let them explore and learn in their own directions and to the extent that they are interested.
FWIW, both of the camps you mention, Tobi, refer to the purely functional languages as “purely functional.” In fact, that term is used more by the purely functional camp (you have it backwards) – they are the ones who care most about the difference. The term itself was first used by those who developed such languages, to emphasize what was different (and new at the time) about them compared with what it had already been the practice to call functional languages (e.g. Lisp).
– DrewAdams 2012-11-21 14:13 UTC
Thanks Drew, I stand corrected.
I agree we could delete the whole page and not lose much at the moment. It seems like there’s scope for useful content, though, if someone was prepared to take the time to write it… (This is not me volunteering ;-)
– TobyCubitt 2012-11-21 18:22 UTC
Useful content, sure. But should it be on Emacs Wiki? Perhaps the general discussion of what functional languages are and aren’t should happen elsewhere. Emacs Wiki might be more appropriate when talking about “useful functions like apply, map, reduce, etc.” or closures or the lack of tail recursion etc.
– AlexSchroeder 2012-11-21 19:08 UTC
I completely agree. I was proposing getting rid of the discussion about what are functional languages, and replacing it with content demonstrating the functional aspects of Elisp: e.g. using mapcar instead of a loop, what closures are useful for (now that we finally have them!), etc. Unlike a debate over the definition of “functional”, I think this type of thing could be useful to people, and would be appropriate for the Emacs Wiki. But since I’m not volunteering to write it, my opinion counts accordingly :-)
– TobyCubitt 2012-11-27 10:29 UTC
FWIW, I agree with both of you.
– DrewAdams 2012-11-27 15:06 UTC