Re: [Kawa-commonlisp-dev] Switching to Guile's #nil

[John Cowan]

Per Bothner scripsit:

> Apropos here a a related note from John Rose:
> http://mail.openjdk.java.net/pipermail/lambda-dev/2013-June/010171.html

[snip]

> This seems hard to avoid with the "Guile approach": Unless you know
> a list is a "Scheme list" then you have to test for both LList.Empty
> and #!null.
> 
> Code that does:
>   (eq? lst '())
> will be faster than
>   (null? lst)
> but the latter also handles Lisp lists.  Is that a good tradeoff?

Based on the above-referenced note, perhaps it is not actually faster,
given that a null check can often be folded in.  (Smalltalk has two
separate subclasses of Boolean, namely True and False -- both singleton
classes, but that's neither here nor there -- and it's perhaps a pity
that Java didn't do the same.)

> One could make "better Scheme/Lisp compatibility" a configuration
> option.  Not clear what the default should be.  Of course one
> might argue that interpreting #!null as false and allowing it as the
> end-of-list are "friendlier" and less surprising, even to pure Scheme
> programmers.

Indeed, and I think it should be the default for that reason.  If
it turns out that (eq? x '()) is actually faster, then it can be
added to the performance hints.

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