For me, when I was writing my spider solitaire program, I had a couple of global variables, the-deck and the-piles, which represented the state of the game. the-deck was a list of randomly permuted cards, which were integers from 0 to 51 (or 0-25 with two suits, or 0-12 with one suit). the-piles was a list of piles, which were lists of cards (the zeroth element was the top of the pile), and the cards were of the form (list card revealed?), where "card" = 0-51 and "revealed" = t/nil. (Yes, a card is an integer in the deck, but a list of an integer and t/nil on the board. This does not need to be changed.)

To flip over the top card of the nth pile, I went:

  (= (cadar the-piles.n) t)

  (forlen i the-piles
    (push (list pop.the-deck t) the-piles.i))

  (let (a b) (split the-piles.i n)
    (= the-piles.i b
       the-piles.j (join a the-piles.j)))

  (push (deepcopy:list the-deck the-piles) prev-states)

  (let (a b) pop.prev-states
    (= the-deck a the-piles b))

  (= the-piles
     (join (take (- n 1) the-piles)
           (list:cons (list (car the-piles.n) t) (cdr the-piles.n))
           (drop n the-piles)))

  (= the-piles
     (map [cons pop.the-deck _] the-piles))

  (= the-piles
     ; ... oh god I have to know whether i>j
     ;(join (take (dec:min i j) the-piles)
     ;      ... egad I may as well just say (if (< i j) ...
     ;(if (< i j)
     ;    (join (take dec.i the-piles)
     ;          (list:drop n the-piles.i)
     ;          (cut the-piles i dec.j)
     ;          (list:join (take n the-piles.i) the-piles.j)
     ;          (drop j the-piles))
     ;    (join ... egad the repetition is intolerable
     ; hmm, this'll work
     (with (new-i (list:drop n the-piles.i)
            new-j (list:join (take n the-piles.i) the-piles.j))
       (if (< i j)
           (join (take dec.i the-piles)
                 new-i
                 (cut the-piles i dec.j)
                 new-j
                 (drop j the-piles))
           (join (take dec.j the-piles)
                 new-j
                 (cut the-piles j dec.i)
                 new-i
                 (drop i the-piles)))))

  (push (list the-deck the-piles) prev-states)

Functional programming is a useful tactic in some cases. For example, I use a "chain-length" function, which might, say, take the list (7♥ 8♥ 9♣) and return 2, the number of revealed cards of consecutive rank of the same suit. I first thought about making it take "i" as an argument, referring to the ith pile in the-piles, but I made it take the list "xs" instead, and that allowed me to use it for other purposes: e.g. determining whether a move of n cards from pile i to j will create a longer single-suited chain. (Like, moving that 7♥ 8♥ chain onto a 9♥. I have a couple of AI-assistance procedures that look for moves satisfying conditions like that.) The expression is:

  (is (+ n chain-length:the-piles.j)
      (chain-length:join (take n the-piles.i) the-piles.j))

  ; Move a chain of n cards from i to j.
  (zap [let (taken leftovers) (split _.i n)
         (copy _ i leftovers j (join taken _.j))]
       the-piles)
  
  
  ; Reveal the top card of the nth pile.
  
  (def copdate (orig . kvs)
    (apply copy orig (mappend [list _.0 (_.1:orig _.0)] pair.kvs)))
  
  (zap [copdate _ n [cons (list _.0 t) cdr._]] the-piles)

  ; Reveal the top card of the nth pile.
  
  (def copdate (orig . kvs)
    (apply copy orig (mappend [list _.0 (_.1:orig _.0)] pair.kvs)))
  
  (zap [copdate _ n [cons (list _.0 t) cdr._]] the-piles)

  (set:cadar the-piles.n)

  (def zipdate (func subject . indices)
    (iflet (first . rest) indices
      (copdate subject first [apply zipdate func _ rest])
      func.subject))
  
  (def zet (subject . args)
    (apply zipdate [do t] subject args))
  
  (zap zet the-piles n 0 1)

   (= the-piles
       (join (take (- n 1) the-piles)
             (list:cons (list (car the-piles.n) t) (cdr the-piles.n))
             (drop n the-piles)))

To clarify, I'm not mostly interested in eliminating side effects because of some obsession with purity, but rather for performance sake. I've been pursuing so many lang design choices like fexprs/first-class macros that are supposed to really cripple performance, that I'm starting to worry my hobby implementations aren't even going to run. I'd heard that Haskell and Clojure get a big performance boost from enforced immutability/no side effects because it guarantees that most operations can safely be run concurrently, so I've just considered it an option.

Blade is written in Groovy right now, and to make partial evaluation possible, everything is done in continuation-passing style. Partial calculations' continuations are held in stasis until the definitions they're waiting for become available. I guess it's related to cooperative multithreading and single-assignment variables, but I'm kinda playing it by ear.

Back when my work on Blade was petering off, the main reason was that Groovy's syntax wasn't very friendly for continuation-passing style. I really missed macros, and I wasn't in the mood for writing a complicated Groovy AST transformation. I wanted to just port it to Arc or something, but I had trouble writing Arc code itself without getting hung up in all the abstraction leaks (particularly unhygienic macros) and absent features (particularly OO-style type-based dispatch and weak references).

Meanwhile, thanks to ranting about Blade here, I realized it wouldn't be a good REPL language. Single-assignment has gotta paint people into corners at a REPL, and the alternative would be to compile the whole application over and over, using some meta-runtime to orchestrate the compiles. But what runtime?

Also, I began to suspect Bllade's design itself would result in something ugly to use and possibly even unusable. A declaration's behavior would depend on definitions, and a definition's value would be what the declararions said it was, and naturally there'd be deadlocks. The deadlocks were easy to avoid in toy examples, but I didn't know what limitations and gotchas Blade would develop once it matured. (I still wonder about that.)

So a plan fell into place: I could pursue a language that had similar high hopes for extensibility as Blade, but in a REPL-friendly form. That language's evaluation model would be much more familiar to me, and it would be closer to any of the languages I might choose to implement it in, so I'd probably be able to implement it faster and more confidently. Then I could use the resulting language--and the lessons I learned from it--to experiment with different variations on CPS and finish Blade.

So I started working on that language, and I called it Penknife. ^^

> Meanwhile, thanks to ranting about Blade here, I realized it wouldn't be a good REPL language. Single-assignment has gotta paint people into corners at a REPL,

> So a plan fell into place: I could pursue a language that had similar high hopes for extensibility as Blade, but in a REPL-friendly form.

So the REPL was a large part of it. I've heard too that single-assignment is problematic for REPLs, but I don't quite understand why. Can't you just use let everywhere you'd use = and def ordinarily?

  ; Scribbling at the REPL

  arc> (= x 5)
  5
  arc> (def add1 (n) (+ n 1))
  #<procedure: add1>
  arc> (add1 x)
  6

  ; Single-assignment version

  arc> (let x 5)
  nil
  arc> (let add1 [+ _ 1])
  nil
  arc> (let x 5
         (let add1 [+ _ 1]
           (add1 x)))
  6

Could this style of REPL'ing address the problems posed to REPLs by single-assignment, or are there larger problems I'm ignoring?

Well, single-assignment can have variables that are introduced without values and then assigned to later. It's a bit of a relaxation of pure FP that (I think) doesn't sacrifice referential transparency. Hmm... actually, a lambda that assigns to a binding outside itself is not referentially transparent, so oh well. :-p

In any case, Blade isn't directly supposed to be a single-assignment or pure sort of language. I just want to view the space of Blade declarations as a completely orderless set, letting the compilation of each one run in parallel semantically, and I still want it to have an intuitive kind of determinism. Inasmuch as I can orderlessly orchestrate side effects in ways I find bearably intuitive, I'll probably add side effects to Blade.

Blade side effects would also be limited by the fact that I want to build up to an IDE that rewinds the compile when you edit. In fact, I'm almost certain my current plan won't work well with that. I expect custom declaration syntaxes to be supported by core-syntax declarations that happen to branch based on the declarations they find by reading the project tree, but then editing any part of a file would invalidate the branching itself and cause recompilation of all the custom-syntax declarations in that file. Seems there'll need to be a more edit-resistant kind of branching. I already expect a bit of IDE cooperation with this--it doesn't have to be as hackish as diffing plain text--but that's where my ideas leave off.

...Actually, I've got a bit of an idea already. XD A declaration that observes a file's contents in order to branch can limit its observation to some method that doesn't rely on the contents of the individual declarations, and then each of the branches can observe just its own part of the file. The core syntax already works like this, essentially splitting the file based on appearances of "\n\n[".

Anyway, you've gotten me thinking and ranting. ^_^

---

Would this style of REPL'ing address the problems posed to it by single-assignment, or are there other problems I've ignored?

The kind of problem I expect to happen is that I'll define something once, I'll define something based on that, and then I'll realize the first thing had a bug, but I won't be able to redefine it as simply as I can in Arc. Once I do enter the correct version, I'll have to paste in all the things that depend on it too.