[r6rs-discuss] Interpreters need not apply?

From: AndrevanTonder <andre>
Date: Wed Mar 7 09:51:49 2007

On Wed, 7 Mar 2007, Pascal Costanza wrote:

> The terms "compiler" and "interpreter" are not well-defined. But AFAICT, a
> compiler typically works in two phases: A translation from one representation
> to another one, where the latter is typically a representation that can be
> executed by some interpreter (for example, a CPU).

No r5rs-conformant Scheme interpreter will start evaluating a form
without macro-expanding it first. R6RS requires nothing more than this.

> An interpreter typically works in one phase: It evaluates expressions one by
> one without doing any sort of whole-program analysis.

But any serious existing Scheme interpreters will expand and syntax-check
each form before evaluating it. In particular, an r5rs lambda body has to
be expanded as a whole before it can be evaluated - no one would dream of
expanding and evaluating r5rs internal definitions one by one. R6RS mandates
nothing more than this. An R6RS library is a single form containing a
sequence of internal definitions (in a glorified letrec* subexpression) that
can be read, expanded/analysed and evaluated once, just as one would do every
toplevel form in an r5rs interpreter. The same applies to an R6RS "toplevel
program", which is also a glorified letrec* expression. In my implementation,
it can be entered at the prompt as a single form, e.g.,

> (program
      (import (r6rs))
      (display "Hello world"))

  Hello world

R6RS does not preclude a traditional toplevel in which one can evaluate naked
expressions. My implementation in fact provides a traditional toplevel for
interactive REPL development, at which one can type

> (import (r6rs))

  unspecified

> (display "Hello world")

  Hello world
  unspecified

> Interpreters are interesting because they sometimes have better performance
> characteristics than compilers. Especially when code is loaded or generated
> on demand at runtime, a compilation step may incur a much larger overhead
> than "pure" interpretation. For example, this is the case when the loaded or
> generated code is only executed once or just a few times.
>
> This is, BTW, one of the reasons why "modern" virtual machines, like those
> for Java, Smalltalk or Self, are so efficient: They simply defer compilation
> until there is enough evidence that certain hotspots actually benefit from an
> extra compilation step.

Do you consider these language specifications more interpreter-friendly than
r6rs? If I remember correctly, all Java code has to be encapsulated in
classes. These have to be assembled into a well-formed program, which is
sytax-checked and typed-checked, before anything will run. Does r6rs mandate,
in your view, a heavier burden than this?

Regards
Andre
Received on Wed Mar 07 2007 - 09:51:04 UTC