2021-06-04 - Codex: Dubito, ergo Cogito, ergo Sum

Last couple of weeks I learned about:

My goal is to devise and build an optimizing compiler which does not precludes AOT or JIT. In other words: I want both AOT and JIT. As a subgoal, I want to build an optimizing or optimized pattern matching program in the spirit of SRFI-200, and SRFI-204, that I call pattern combinators. One particular optimization, I think about is what Software Design for Flexibility [SDF] calls unification.

For instance given the following match expression:

(match exp
  ((a) out1)
  ((a d) out2))

Can be rewritten as follow:

(match exp
  ((a . rest) (match rest
                (() out1)
                ((d) out2))))

In that case, doing some kind of prefix compression.

match can be seen as a pattern combinator, that is a parser combinator that can traverse not only a sequence, but also a sequence made of nested sequence in a recursive way. For the purpose of the pattern combinator, a sequence is a finite acyclic ordered set. A sequence might be a list, vector or an ordered mapping. A parser combinator such as the one described in [GLL] is a good basis for a pattern combinator, except if does not support the optimizing / optimized part.

To be able to implement optimizing pattern combiner, it would require to reflect upon the match predicates, in particular have access to a predicate subsume? that allows implement a partial order over predicates. And that should be done at in the object language.

The object language is the language of the user. It is defined in opposition to meta language that is used to implement the object language. The meta language might be itself implemented in terms of another meta language until... until all the way down! That is where 3-LISP call into atoms, the physical atoms (not the atoms of the simulation).

We will just glimpse over the problem(s), and add a reflection applicative called sum, so that we can introspect how applicative and operative are made, so that in turn we may be able to optimize code at runtime from the object language.

Instead of trying to implement subsume? and the optimizing pattern compiler, I will implement only arithmetic folding (replace an arithmetic computation with its value) with an applicative based on the inferences / knowledge / theories built by an applicative called dubito.

So, given the following pseudo-code:

(define (make-frob-adder value)
  (lambda (other)
    (+ other value 36)))

(define my-frob-adder-2 (make-frob-adder 2))

(my-frob-adder-2 4) ;; => 42

A simple compiler will just compile my-frob-adder-2 into a lambda that is passed an extra environment argument called static so that the resulting procedure does not have free-variables, it will look like:

(define (my-frob-adder-2-compiled static other)
  (define value (environment-ref static 'value))
  (+ other value 36)

The trick is that if we forgo the last line of the previous program, the closure of my-frob-adder-2 is known at compile time. If we also consider the last line, the whole program can be compiled to 42: That is what is called (AFAIK) constant folding.

Let's consider the following program:

(define (make-frob-adder value)
  (lambda (other)
    (+ other value 36)))

(define my-frob-adder-2 (make-frob-adder 2))

(my-frob-adder-2 (string->number (cadr (command-line))))

It is similar to the original program, except it my-frob-adder-2 takes its argument from command-line, hence it can not be folded into an integer object, because... (command-line) is not known at compile time. So, indeed after a simple compilation, we end up with the following code:

(define (my-frob-adder-2 static other)
  (define value (environment-ref static 'value))
  (+ other value 36))

(my-frob-adder-2 (alist->environment `((value . ,2))) (string->number (cadr (command-line))))

That is naive example: an optimizing compiler can create the following optimized definition for my-frob-adder-2:

(define (my-frob-adder-2 other)
  (+ other 40)) ;; 36 + 4 = 40

So what is this all about?!1!

It is also seems a little naive given that Chez Scheme can compile to native code an s-expr at runtime, given a perfomance timer (cost center), and counters, that gather new knowledge only known at runtime, it is completly possible to implement a manual JIT [MANUAL-JIT].

So what do sum do that is new? Not much: it puts all that together.

Kernel [R-1RK] will factorize macro and procedure into operatives, it will also reify all environments: static / lexical, and dynamic. The application sum will reify in the object language the implementation of any object... including eval in other words, eval can be reifyed as a meta-evaluator or better as the "source code" in the previous meta language. sum does not only mean that the source is embedded in the program, but also prescribe that it is possible to represent in a possibly infinitly resursive way all the meta-languages. The limit being our knowledge, time, and energy to encode that as source. Does it mean that this language is self-bootstrappable, in other words that it does not require a seed: no. Even it it self-hosted and self-describing, it requires a seed [BOOTSTRAPPABLE].

In the above paragraph I glossed over (sum eval): i) does it return a meta-evaluator, a program represented in the object language that can evaluate the object language (itself), ii) does it return an evaluator in the previous meta-language. Both are possible, the latter is more interesting, because it expose the compiler tower [NANOPASS] to the object language.

eval is not the only interesting primitive procedure. In the context of Scheme, it is interesting to reflect upon a couple of procedures. One of the most interesting is call/cc, another interesting procedure is the non-standard expand and yet another everything related to the garbage collector. To stay goal-driven, a target application of that reflection, that I think may be interesting, can be to disable or customize the garbage collector for a subset of the program. Another goal might to change for a subset of the program the evaluation strategy without relying on meta-evaluation [EVALS].

In the case where (sum eval) returns the source of eval in the previous meta-language, expand, call/cc, and the garbage collector should be represented... somehow!