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

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Sun Jun 3 21:47:14 EDT 2012

Years and years ago, we experimented with a let+ form that did things
like this, but nowadays define works in so many contexts that it seems
like the natural thing to use in situations like this. Of course, I'm
sure there are situations where define is suboptimal (eg, see Eli's
thread with the up and down arrows) but I'm finding that I quite like
using it everywhere.


On Sun, Jun 3, 2012 at 1:40 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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