The Extempore philosophy

Note

This was once a blog post—corrections/improvements welcome.

Extempore is a programming language and runtime environment designed with live programming in mind. It supports interactive programming in a REPL style, compiling and binding code just-in-time. Although Extempore has its roots in ‘live coding’ of audiovisual media art, it is suitable for any task domain where dynamic run-time modifiability and good numerical performance are required. Extempore also has strong timing and concurrency semantics, which are helpful when working in problem spaces where timing is important (such as audio and video).

These two goals—dynamic flexibility and close-to-the-metal control—seem at odds. Extempore tries to offer both by supporting both a high-level dynamic language (Scheme) and a low-level ‘C like’ language (xtlang) simultaneously, with tight integration and transparency between the two. A running Extempore process will compile both valid scheme and xtlang forms. Scheme objects (lists, closures, continuations, etc.) coexist with the xtlang’s types and pointers to allocated memory, and with a few ‘helper functions’ data can even flow through both languages fluidly.

What’s scheme, and what’s xtlang?

This is all a bit abstract, so let’s look at a couple of examples:

(define scheme-closure
  (lambda (a b)
    (let ((result (* a b)))
      (print "result = " result)
      result)))

(scheme-closure 4 5) ;; prints "result = 20", returns 20
(scheme-closure 2.4 2) ;; prints "result = 4.8", returns 4.8

(bind-func xtlang_closure
  (lambda (c:double d:i64)
    (let ((result (* c (i64tod d))))
      (printf "result = %f\n" result)
      result)))

(xtlang_closure 4.5 2) ;; prints "result = 9.000000", returns 9.0

Here, scheme-closure is a Scheme closure (a closure is a function along with its enclosing scope). It’s just a regular Scheme closure, it takes two arguments (a and b), which can be any number; anything for which number? returns #t. Closures are first-class objects in Scheme, and scheme-closure is no exception. It can be passed to map, apply, and friends.

xtlang_closure, on the other hand, is an xtlang closure. xtlang (unlike Scheme) is a new language, and the Extempore executable provides the xtlang compiler. Like Scheme, xtlang is has an s-expression based syntax.

xtlang_closure is also a closure which takes two arguments, and xtlang uses the lambda form to build closures, just like Scheme. In fact, xtlang_closure does the exact same thing as scheme-closure does—it takes two arguments, multiplies them together, then both prints and returns the result. One thing that’s different in the xtlang version, though, is the presence of type annotations for the arguments: they’re the (blue) parts of the symbol name following the colon. The types should be familiar: double for a double-precision floating point number, and i64 for a 64-bit (signed) integer. Unlike Scheme—which is dynamically typed, and will silently coerce floats into ints and other things like that—xtlang is statically typed. Not every type needs to be specified, the compiler will infer types when it is unambiguous, but the compiler will never silently coerce types. This is by design—the whole point of using xtlang in Extempore is to make things explicit. If you want more dynamic typing, then there’s always Scheme.

The xtlang compiler uses an LLVM backend to generate high-performance machine code. Basically, Extempore’s xtlang compiler generates the LLVM IR, and this is then passed to LLVM for compiling and linking.

So why two languages?

Why introduce this confusion? Why not just pick one language or the other (or design a new language which has aspects of both)? By way of explanation, let’s do a bit of numerical processing. Say we want to calculate the highest common factor of two integers a and b using a brute-force approach:

(define hcf-scheme
  (lambda (a b)
    (letrec ((hcf (lambda (i)
                    (if (and (= (modulo a i) 0)
                             (= (modulo b i) 0))
                        i
                        (hcf (- i 1))))))
      (hcf (if (< a b) b a)))))

(hcf-scheme 10 15) ;; returns 5

(bind-func hcf_xtlang
  (lambda (a:i64 b)
    (let ((hcf (lambda (i)
                 (if (and (= (modulo a i) 0)
                          (= (modulo b i) 0))
                     i
                     (hcf (- i 1))))))
      (hcf (if (< a b) b a)))))

(hcf_xtlang 10 15)  ;; returns 5

The code for Scheme (hcf-scheme) and xtlang (hcf_xtlang) is identical except for an i64 type annotation on the first argument a in hcf_xtlang and a letrec instead of a let in hcf-scheme. Both functions use tail call recursion, and are written in a fairly ‘scheme-like’ way. Although there is only the one type annotation, hcf_xtlang is strongly (and fully) typed. The types of all the other variables and the return type of the closure are all inferred by the compiler from the type of a: the function hcf_xtlang takes two i64 arguments and returns another i64. In more complex functions there may be a greater need to specify the types of the variables, but often just a few type annonations can unambiguously determine everything in scope. The xtlang type reference has more info on how type inferencing works in the xtlang compiler.

;; first, figure out two large numbers with a common factor (133)
(println (map (lambda (x) (* x 133)) '(125219 123711))) ;; prints (16654127 16453563)

;; profile the scheme version
(let ((t (clock:clock)))
  (println 'HCF '= (hcf-scheme 16654127 16453563))
  (println 'elapsed 'time '= (- (clock:clock) t) 'seconds))

;; --result--
;; HCF = 133
;; elapsed time = 82.085036 seconds

;; profile the xtlang version
(let ((t (clock:clock)))
  (println 'HCF '= (hcf_xtlang 16654127 16453563))
  (println 'elapsed 'time '= (- (clock:clock) t) 'seconds))

;; --result--
;; HCF = 133
;; elapsed time = 0.257790 seconds

In a direct comparison, here I’ve calculate the HCF of the integers 16654127 and 16453563, which are (by design) known to have at least one non-trivial factor (133). Both functions return 133, but the xtlang one finishes over 300 times faster. I tried to use even bigger integers as input, but the Scheme version blew past the maximum runtime timeout, while the xtlang one finished in about 2 seconds :)

Now, this comparison is one datapoint: it isn’t meant to start a flame war about dynamic vs statically typed languages or anything like that. It’s a brute-force algorithm for a problem with many more elegant algorithms. What it does show, though, is that Extempore’s Scheme interpreter is slow. There are some crazy fast and efficient Scheme compilers, but Extempore’s isn’t one of them—it’s dog slow.

You may now be thinking that this pretty much rules Scheme out for anything computationally intensive in Extempore, such as audio and graphics. Well, late one night in about 2010 Andrew (Extempore’s creator) had pretty much the same realisation. At the time he was working on Impromptu, Extempore’s predecessor, which had the same Scheme interpreter. And he realised that the Scheme interpreter would need some serious work to bring it up to speed if it was going to be used for any number-crunching. At that point, he figured that he might as well write a new language, leveraging the LLVM compiler. And lo, xtlang was born (although it wasn’t called that straight away).

After working on xtlang inside of Impromptu for a while, it became clear that introducing a whole new language to a programming environment is kindof a big change. So he decided to fork the project, give it a new name, and also make a couple of other fundamental changes (open source and cross-platform) as well. Impromptu is still supported, and some of the improvements to Extempore are back-ported, but Extempore is the future. And that’s the history of Extempore and the genesis of xtlang in two paragraphs.

xtlang’s types include tuples (like C structs), arrays, SIMD vectors and pointers in addition to the float and int primitives shown in these examples. The upside of having to worry about these types is the increased performance and low-level expressiveness, which is particularly important in real-time and computationally intensive settings such as digital audio, graphics and interfacing directly with hardware. The other benefit of having a low-level type system (like C) is that it’s easy to bind to shared libraries (.dll, .so or .dylib depending on your platform) and then call into them in xtlang. You can even bind and rebind these shared libraries dynamically, switching the bindings around as you please. There’s more details about binding to C shared libraries in the examples/external directory, and in C-xtlang interop.

There’s heaps more to say about the Scheme/xtlang interop in Extempore (as well as the details of xtlang itself!), but the key point is that it’s nice to have the choice. Scheme is a great control/scripting language for triggering events, and xtlang is a nice ‘systems’ language for building infrastructure and for doing computational heavy lifting. Extempore allows the programmer to live in both worlds, as long as they have some understanding of what’s going on under the covers. And as I work with Extempore (and as xtlang matures) I find myself using Scheme less and less and xtlang more and more. The code I’m writing is almost the same (since they’re syntactically so similar), but with the performance benefits and bit-level control of working much closer to the metal. It’s even nice (most of the time, at least!) to get the compile errors, it’s better to catch type mismatches earlier rather than later.

Live programming: Interacting with the Extempore compiler/runtime

Remember the claim in the opening paragraph that Extempore is a language designed with ‘live programming’ in mind? Now, ‘live programming’ is a pretty loaded term (is the insinuation that all other programming is dead?) and as such needs some unpacking. Extempore is designed to support (and indeed make it easy for) the programmer to interact with, modify, and extend their program as it runs.

This is obviously possible in any REPL-based development environment, but often this interaction is limited to the building and debugging phase of software development, with the program being frozen (possibly compiled) upon completion and then left to run unmolested. In Extempore, on the other hand, this interactive development style is supported (and encouraged) through the whole software lifecycle—up to and including the deployment and running of the ‘final’ code. An Extempore codebase is not necessarily a static artefact: the behaviour of the system is determined by the development of the code over the whole time the system is running, and this behaviour may be differ substantially between the commencement and completion of this process.

This human-in-the-loop programming approach is exemplified by the practice of live coding or laptop performance, a “new direction in electronic music and video: live coders expose and rewire the innards of software while it generates improvised music and/or visuals. All code manipulation is projected for your pleasure.” In an artistic context this idea of improvisational live programming makes sense, but there are also many other contexts where having a human in the loop even at program execution time (to catch unforseen bugs or add hitherto unplanned functionality) is advantageous. This is a tough job for the programmer—there’s no safety net when you’re modifying the program as it’s being run—but that’s exactly why Extempore is being designed as it is: to provide as much support as possible to the programmer as they deal with this difficult (and exciting) challenge.

This ‘everything should be hot-swappable at runtime’ philosophy has a couple of implications for the architecture of the Extempore compiler and programming environment:

  1. Compilation/binding should happen as late as possible. Extempore has a couple of static dependencies baked in at compile time, but the rest of the functionality is loaded on-the-fly.
  2. Compiler-as-a-service (CaaS): the Extempore compiler is a running process, and compilation happens by interactively sending Scheme or xtlang code to the appropriate address/port. The compiler need not be running on the same machine as the programmer, and the code can also be executed in any number of running Extempore processes. And because it’s written in Scheme, even the compiler itself is reconfigurable at runtime.