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).
Laurent On Sun, Jun 3, 2012 at 8:40 PM, Neil Van Dyke <[email protected]> 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. > > ____________________ > Racket Users list: > http://lists.racket-lang.org/**users <http://lists.racket-lang.org/users> >
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