The article well written. It didn't throw me or make me confused. I simply was bothered by the lack of nuance in regard to mutation. Trillions of pixels have been burned describing pure functions and giving small examples of immutability and recursion. Trees have even been killed.

What makes Chapter 3 of On Lisp or SICP useful is they're Third Acts. They go deeper than pure functions. They're pivotal because that's where we start talking about the messy world where mutation is necessary. It's where we admit that there's a rationale for the way Java works even if it's implementation could stand improvement.

What makes Clojure interesting is the way in which it implements that depth. Lisp went on a journey to Java and was transformed by the experience. Clojure is not just another Lisp - that side of the family has always had shared structure and a propensity for functions. There's `is` and 'testing` and agents and vars and `.` and `..`. JavaScript the good parts is a book. Java the good parts are in Clojure. Clojure is an Act V not an Act I.

The book chapter aside, I think you might actually have it backwards. Hopefully I'm not misunderstanding you terribly! The reason why PG has to spend as much time as he does warning programmers about hidden ways in which Common Lisp mangles your data structures (nconc, for example) is because Common Lisp does not come with immutable data structures out of the box. My impression is that he's telling you how to work as well as you can (strive for purely functional code) with the tools at hand. Clojure, on the other hand, was built so that you could code without having to worry about that stuff by default.

Also, you say that lisps have "always had shared structure." I'm not sure what you mean by that? Clojure implements _structural sharing_, which is what allows it to have persistent, immutable data structures. Common Lisp certainly doesn't do the same.

I'm not sure what you mean by "Lisp went on a journey to Java and was transformed by the experience." I think you're saying that Rich Hickey designed Clojure to be a lisp that's more Java-like? Could you explain how? My impression is that Rich Hickey thinks OO is broken (see the talk "Are we there yet?"), as is the notion of mutability as implemented by Java. Clojure's interop with Java is a great convenience, but the emphasis is still on functional programming. I don't think I understand what you're saying, or maybe it's just that we disagree?

Finally, from clojure.org: "Clojure is predominantly a functional programming language, and features a rich set of immutable, persistent data structures. When mutable state is needed, Clojure offers a software transactional memory system and reactive Agent system that ensure clean, correct, multithreaded designs." Maybe it's these state management features you're concerned about? If so, then I would suggest that vars, atoms, refs, and agents are not as inspired by Java as you're saying. Or maybe we just disagree that, the majority of the time, you should be writing pure functions and using immutable data structures in Clojure?

PG isn't warning any experienced Common Lisp programmer about `nconc`. The [n] at the front means it's destructive...there's also `nsubst` and `nreverse` and `nstring-capitalize`. They all exist in Common Lisp to allow a programmer to avoid consing and the subsequent garbage collection. In other words, they exist to allow a programmer to optimize their code. They exist as exceptions to idiomatic Common Lisp practice of returning values in lieu of mutating locations. Sure it is not as idiomatic in Common Lisp as in Clojure in part because Common Lisp is largely a product of its time and that time was largely single threaded.

Avoiding mutation in Common Lisp is even less idiomatic than it is in Scheme - where there are not destructive versions of various functions by default and idiomatic practice is mutation is signaled with a bang [!]. But again, the preferred approach is persistence and writing side-effect free functions. Indeed at the front edge of Scheme, Racket implements immutable data structures by default.

Idiomatic functional style and defaulting to immutable data structures are not what makes Clojure unique as a Lisp - it's knob twiddling, not a new paradigm. It has some great built in syntax that allows for the best part of Lisp-2 programming - i.e. symbols representing a collection acting as functions over the collection and the association of a meta-data map with a symbol like a plist. But it's evolutionary as a Lisp.

On the other hand, though not unique, idiomatic functional style and defaulting to immutable data structures are unusual in relation to the Java platform. Its relationship to the Java platform is what makes Clojure unique. STM is not a product of it's Lisp heritage. Neither is it's implementation of objects with via interop [largely eschewing objects despite availability is also idiomatic Lisp].

Understand, I'm writing to think. What you've written is just a starting point I can use as an excuse for saying what I was primed to say anyway. Like I said in my original comment, I was already drinking from the Clojure firehose.

So what's the upshot? Well what did your article say that hasn't already been said before about functional programming? Why not point people to Chapter 3 of Graham? It's more convincing for those who need convincing and more in depth for those who don't. Why not point people to The Value of Values, it's again more convincing for tire-kickers; a comfortable sermon for the newly converted, and a reference point for those already in the cult.

What really makes Clojure unique?

I don't think I read On Lisp the same way. From 3.1: "If a function is advertised as destructive, that doesn't mean that it's meant to be called for side-effects. The danger is, some destructive functions give the impression that they are. For example,(nconc x y) almost always has the same effect as (setq x (nconc x y))". Calling it dangerous sounds like a warning to me. I definitely agree, though, that he makes the case that the preferred approach is writing side-effect free functions. I'm not sure what you mean by "persistence" though, because Common Lisp doesn't have persistent data structures.

I don't know if its emphasis on functional programming and values makes Clojure unique, but that's definitely one of its core design concerns. The "article" is a chapter from a book on Clojure, and learning to write in a purely functional style is essential to learning Clojure. The point of the chapter isn't to get to the heart of what makes the language unique, it's to offer instruction on how to use it.

Here's the recycled cons at the car of ANSI Common Lisp section 12.4:

Common Lisp includes several functions that are allowed to modify list structure. These functions are destructive for reasons of efficiency. Though they may receive conses passed to them as arguments, they are not meant to called for their side-effects.

For example, delete is a destructive version of remove. While it is allowed to trash the list passed to it as an argument, it doesn't promise to do anything. This is what happens in most implementations:

   > (setf lst '(a r a b i a))
   (A R A B I A)

   > (delete 'a lst)
   (R B I)

   > lst
   (A R B I)

As with remove, if you want side-effects, you should use setf with the return value:

   (setf lst (delete 'a lst))

Because garbage collection and homoiconicity can sometimes feel like magic, it can be hard to think of Common Lisp running closer to the metal than a language like C. But it does. Cons cells are locations in memory, and symbols are pointers and there's nothing in between. A programmer doesn't even get `free(array)`. Memory locations can be shared willy-nilly because just as in Clojure, two distinct lists/sequences can share a tail. From ANSI Common Lisp section 12.8, "Constant Structure":

   > (defun arith-op (x)
       (member x '(+ - * /)))
   <function:arith-op>

   > (setf lst '(as it were)
   (AS IT WERE)

   > (nconc (arith-op '*) lst)
   (* / AS IT WERE)

   > (arith-op '-)
   (- AS IT WERE)   ;; bad

   > (arith-op 'as) 
   (AS IT WERE)     ;; even worse

"Oh Shit!" moments like this are why Lispers like Graham developed a functional programming style. It's a lot of what motivated the design of Scheme. It's not really what motivated Clojure because Java already solves this problem. Clojure is designed to solve the problems Java doesn't more easily.

That fundamental goal is why what makes Clojure unique matters when explaining Clojure. It's also what makes Clojure a less than ideal vehicle for teaching functional programming style - it's designed for programmers who are solving problems on the JVM [or CLR or V8], and not really so much as a general purpose language. It's designed around interop. As weavejester says in his linked talk, Clojure is a Java library.

There are better languages for teaching functional programming - Scheme/Racket. What makes them objectively better is that helping people learn to program in a functional style is one of the problems they are trying to solve, and they have almost four decades of development toward that goal. Education is entirely orthogonal to Clojure, even more so than Java with its two decades of introductory text development and promotion via vocational arguments in CS departments. And, Racket/Scheme isn't trying to solve JVM problems.

Teaching functional programming style is hard, but a largely solved problem because the internet allows pointers to excellent materials.

http://learncodethehardway.org/blog/AUG_19_2012.html

You're right about being able to point to the same tail from two different lists in common lisp, of course. I'm not very experienced with scheme so I can't comment much on that. Clojure is different in the way it implements its data structures, so "structural sharing" in that context refers to its use of tree structures to implement vectors, maps, etc.

Still, though, I think you have it wrong about Clojure's design. You mentioned the talk "the value of values." Immutability and functional programming are core to Clojure's philosophy. This is pointed out on the clojure.org home page. I'm not sure why you think it's not?

Also, Clojure is meant to be a general-purpose language. Also from the home page: "It is designed to be a general-purpose language". So I'm not sure why you think otherwise? I started learning Clojure with 0 experience with the JVM, and many others are doing the same.

I'm also not sure what you mean by "education is entirely orthogonal to Clojure." I think you're commenting on the value of producing an introductory Clojure book? In any case - you mention that "there are better languages for teaching functional programming." The point of Clojure for the Brave and True is not to teach functional programming, it's to teach Clojure.

If the purpose of Clojure for the Brave is not to teach functional programming, why bother teaching it? Why go through the motions when not fully committed? Is it better than pointing the student to excellent resources despite those resources using another form of Lisp?

To put it another way, is anyone really going to know Clojure without knowing the something about SICP? Is there anything out there that more fully captures Clojure's definition of functional programming than On Lisp or Learn You a Haskell for Great Good?

OK so here's where we differ. It's in our definitions [what could be more Clojuresque] of "learning Clojure". Sure, Clojure's syntax takes an hour versus the thirty minutes Ableson takes to cover Lisp syntax including the substitution model. Then what?

Well I'm saying the two things a person needs to learn Clojure are the practices already described in four decades of Lisp's literature and the Clojure interop. Without either piece, I won't know Clojure.

http://norvig.com/21-days.html

The book covers functional programming because Clojure places a heavy emphasis on functional programming, and learning it is essential to learning Clojure. The book is not "Functional Programming for the Brave and True, with Clojure as the Medium." That doesn't mean that I'm merely "going through the motions" and am "not fully committed" to helping people understand functional programming, and it's bizarre, presumptuous, and rude that you would suggest that. At this point, it seems to me like you're merely being argumentative.

>Well I'm saying the two things a person needs to learn Clojure are the practices already described in four decades of Lisp's literature and the Clojure interop.

I agree. However, the Lisps all have subtle differences. For example, Clojure's interop is one difference, but it's not the only difference. These subtle differences still need to be explained in that Lisp's context to receive a full understanding of that Lisp. This is what I feel nonrecursive has achieved with his writing about FP in Clojure.