Re: [Scheme-reports] Draft 3 Comments: Chapter 4

[John Cowan <cowan@x>]

Denis Washington scripsit:

> Thinking about it, leaving this unspecified makes sense as one couldn't 
> use "when" and "unless" to return a value from a procedure anyway 
> without risking an unspecified return value (if the test evaluates to #f 
> or #t, respectively).
> 
> On the other hand, "begin" in an expression context also a sole 
> side-effect construct (otherwise, all expressions except the last in a 
> "begin" form would be useless) and still returns the last expression's 
> result. Given that "when" and "unless" are very similar, I find it to be 
> pretty intuitive if they have the same behavior. I'm undecided.

The difference is that BEGIN can have a value if its last argument does, but
you can't ever rely on WHEN or UNTIL having a value, so you can only
use it for effect portably.  That being so, it might as well return
an unspecified value in all cases.

> Maybe optional arguments should should be done in WG2 instead (together 
> with keyword arguments, maybe). However, this has been voted upon 
> already by the working group, so I haven't commented on the decision.

Optional arguments are on the WG2 docket.

Keyword arguments can be done in R5RS without extra machinery, provided you
are willing to export them as identifiers:  see KeywordsArcfide.

-- 
BALIN FUNDINUL          UZBAD KHAZADDUMU        cowan@x
BALIN SON OF FUNDIN     LORD OF KHAZAD-DUM      http://www.ccil.org/~cowan

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