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[srfi-11] LET-VALUES wrapup (was: Re: Another vote for more parens)

This page is part of the web mail archives of SRFI 11 from before July 7th, 2015. The new archives for SRFI 11 contain all messages, not just those from before July 7th, 2015.

(Sorry for dropping this on the floor for 6 weeks, I guess we should
thank the editors for not vigilantly moving SRFIs to "final" status
when the draft period is up! ;)

Open issues:

  (a) Leave it alone, or add a level of parens to allow multiple sets of
      bindings.  In favor: me.
		 Against: Hilsdale, Sperber(?), and the MzScheme/DrScheme/... 
                 suite of programs.  Probably most everyone else, too.

  (b) If we're adding parens, do we need to introduce LET-VALUES*,

As to (a), Erik Hilsdale writes:

>Whenever I use let-values, I always write it as
>  (let-values ((Formals Exp) ...) Exp ... Exp)
>rather than this SRFI's
>  (let-values (Formals Exp) Exp ... Exp)
>I do understand that the extra parens are annoying when there's only
>one binding, which is why I frequently use one of two other forms when
>I only care to use one binding.  When I'm thinking about monads I tend
>to use 
>  (bind (Formals Exp) Exp ... Exp)
>which works just as this SRFI's version.  In most of my usual coding I
>  (with-values Exp Consumer) ; Consumer should evaluate to a procedure
>because (a) I'm pretty used to cps, (b) it's an extremely short macro,
>and (c) it only uses a trifle (two spaces) more horizontal space than

In response I can only say that I rarely think about monads and that if
I wanted to use CPS I would be fairly happy with CALL-WITH-VALUES... :-/

Looking at both my code and code written by others, the use of
LET-VALUES and CALL-WITH-VALUES varies, and clearly there are cases when
multiple bindings of multiple values are created.  This is a point in
favor of using an extra nesting level.

As to (b), adding LET-VALUES* or LET*-VALUES make sense.  LETREC-VALUES
strikes me as fairly absurd; I can see no use for it.  I see MzScheme
supports it, but it is not used even once in over 42,000 lines of Scheme
source in the MzScheme distribution.

So, proposal:  I change the spec for LET-VALUES to require the extra
level of nesting, thus facilitating

	(let-values (((a b) e1)
		     ((c d) e1))

but also allowing syntax of the form

	(let-values ((l e1)           ; grab all results into l
		     ((a b . l) e2))  ; grab two results and rest into l

and introducing a compatible LET*-VALUES form (MzScheme's name for it).


[ Mike, please hold off on finalization until the dust has settled. ]