Aww, you know I can't resist talking about my hobby horses. XD
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. ^^
Altogether, I don't think this was based on any bad experience with pure functional programming. It was more about my expectation of those bad experiences, together with the frustration of writing CPS code in longhand Groovy.
> 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
REPLs should provide a convenient way to grab commands from the history anyway, so you can just keep building on your previous let. It is more cumbersome, but you also avoid some of the nasty surprises that come after you've accumulated a complex state in your REPL.
Could this style of REPL'ing address the problems posed to REPLs by single-assignment, or are there larger problems I'm ignoring?
Update: In the particular REPL example I've shown, there's no need for the style transformation since nothing gets redefined. Hopefully you can still gather what I mean from it! ^_^
Can't you just use let everywhere you'd use = and def ordinarily?
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. ^_^
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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.
To draw a connection, it's similar to how when you want to redefine a macro in Arc, you have to re-enter all the code that used the macro in order for it to use the new one, whereas fexprs wouldn't have this problem.
