[racket] multiple-value sugar in "let"* forms

From: Laurent (laurent.orseau at gmail.com)
Date: Sun Jun 3 16:10:15 EDT 2012

Such an improvement would be very welcome (I don't like to have to split my
lets because of one expression returning multiple values), and I vote for
#2 (using square brackets makes it sufficiently readable IMO), unless
someone spoils it all with an overseen big drawback... But I would be quite
happy with "((values ID ...+) VAL-EXPR)" or even "(values (ID ...+)
VAL-EXPR)" (although this one looks weird to me).


On Sun, Jun 3, 2012 at 8:40 PM, Neil Van Dyke <neil at neilvandyke.org> wrote:

> What do people think about this syntax addition for multiple values in
> "let" and "let*" (and perhaps "letrec")?
> I like using multiple-value returns, but two problems with using them with
> the "let"* family:
> 1. "let-values" and "let*-values" are long identifiers, and increase
> rightward drift, sometimes making the difference between a line break
> needed before right-hand-side (RHS) or not (which line break is ugly in a
> "let"* form).
> 2. If I have an existing "let"* that's not a *"-values" that already has a
> few clauses in it, and I want to add a multiple-value LHS to it, I have to
> go to the trouble of converting it to a "-values", which requires editing
> each clause to add parentheses.
> So, I've been wondering what would be a good way to support multiple
> values on a per-clause basis.  Options I see:
> 1. Change "(ID VAL-EXPR)" to "(ID ...+ VAL-EXPR)".  I am shying away from
> this one because it seems error-prone, and the standard indenter won't know
> a good way to break such a clause across multiple lines, but it does have
> some appeal.
> 2. In addition to "(ID VAL-EXPR)", have "((ID ...+) VAL-EXPR)".  This
> could work.  One drawback I see is if in the future someone wants to have
> some funky generalized-"set!" selectors that have a parenthesized form, and
> have those work with the "let"* forms.  I'm not currently interested in
> such "set!" selectors.  Another small drawback is that we'll sometimes see
> "(let (((ID" three parentheses sometimes, like we do with "let-values",
> which is not the most visually pleasant thing to see.
> 3. In addition to "(ID VAL-EXPR)", have "((values ID ...+) VAL-EXPR)".
>  This avoids the hypothetical problem with some future "set!" selectors,
> since I doubt is going to use "values" for the name of such a selector with
> any other purpose.  This still has the three-parentheses problem like
> option #2.
> 4. In addition to "(ID VAL-EXPR)", have "(values ID ...+ VAL-EXPR)" or
> "(values (ID ...+) VAL-EXPR)".  This avoids the three-parens of #2 and #3,
> it's not ambiguous with "values" used as a variable name, but it doesn't
> seem very Scheme-y, it has similar indent problem to option #1, and we
> still have rightward drift because of the "values" keyword.
> I lean a little towards option #2, but I think #1 or #3 would still be an
> improvement.  #4 seems less-Schemely syntax, so I am reluctant to go there.
> For each option, the transform to Racket's canonical "let-values" is
>  trivial.
> I don't know whether there are implications for TR syntax.
> A separate question -- which I don't care about, but someone else might --
> is whether there should be ``rest'' ID in the LHS that gets a list of any
> remaining values from the RHS.  I think this came up in a SRFI discussion a
> long time ago.  I almost never have occasion to use such a feature, and it
> could complicate the syntax and the Racket compiler, so I'm not interested
> in a ``rest'' in the LHS.
> Neil V.
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