Re: Inconsistency with expressions between definitions

["Dr. Arne Babenhauserheide" <arne_bab@web.de>]

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Hi,

Linus Björnstam <linus.bjornstam@fastmail.se> writes:

> When you are not referencing x before defining y everything works as
> you want. There is no, so to say, temporal dependency on how the
> things are bound. When you introduce (display x) before actually
> defining y you force letrec* to bind x to the unspecified value,
> because display has side-effects and you don't move around
> side-effecting code.

This is a technical explanation. It answers "how does this happen?"
(thank you for that!), but not "why is this the correct behavior?".

The core problem I see: if you inject some logging code between the
defines, the behavior changes.

I would expect that referencing a variable that can’t yet be used in an
intermediate expression (between defines) would not cause a (potentially
subtle) behavior change, but would throw an error: variable used in
expression that depends on later define.

Racket does not support defines using later defines at all:

$ racket
> (define (using-later-variable)
>      (define x y)
>      (define y #t)
>      x)
> (using-later-variable)
y: undefined;
 cannot use before initialization
 [,bt for context]

Best wishes,
Arne
-- 
Unpolitisch sein
heißt politisch sein,
ohne es zu merken.
draketo.de

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