Hi Mark! Mark H Weaver <m...@netris.org> skribis:
> I have a few comments on your v2.0.2-70-gcf14f30 commit: > >> + (define (pure-expression? x) >> + ;; Return true if X is pure---i.e., if it is known to have no >> + ;; effects and does not allocate new storage. Note: <module-ref> is >> + ;; not "pure" because it loads a module as a side-effect. >> + (let loop ((x x)) >> + (match x > [...] >> + (($ <application> _ ($ <primitive-ref> _ name) args) >> + (and (effect-free-primitive? name) >> + (not (constructor-primitive? name)) >> + (every loop args))) > [...] >> (define *effect-free-primitives* >> - '(values >> + `(values >> eq? eqv? equal? >> = < > <= >= zero? >> + * - / 1- 1+ quotient remainder modulo > > As currently implemented, `pure-expression?' does not behave as its > header comment claims, because in most cases the arithmetic operators > _do_ allocate storage. Sorry, the headers are misleading: it’s not about whether a primitive allocates new storage, but whether it allocates storage for a *mutable* object. For instance, that ‘+’ allocates storage for a bignum doesn’t matter as they are immutable. > Unfortunately, there's another complication as well. As you know, there > are plans to eventually add support for arbitrary-precision floats, at > which point the result of arithmetic operators will depend not only on > their arguments, but also on the value of a fluid which specifies the > desired precision. Yes, good point. Well, wait and see? :-) Surely the current assumption will have to be revisited then. But I can’t tell exactly how until we have an actual API, which will probably be in 2.2. [...] > Even today, the arithmetic operators are extensible with GOOPS, and for > some representations it is advantageous to control the behavior of the > associated methods using fluids. Note that arithmetic operators were already inlined before peval. > For now, one reasonable compromise would be for peval to perform > arithmetic operations only when the results are exact. Allowing peval > to safely perform inexact arithmetic is a more difficult problem. You must be right (GCC uses MPFR for constant folding). I’ll change primitives.scm to that effect, then. Thanks for your feedback! Ludo’.
