On Sun, Apr 08, 2012 at 04:22:07PM -0400, Liam Healy wrote:

Is it necessary to duplicate functionality in alexandria?

Though it's not my place to judge that, I thought it worth making the proposal on the basis that WHEN-LET and IF-LET provide very similar functionality, and are already in alexandria. I originally modeled PROG1-LET after these, and have since found it to be useful with reasonable frequency. Notably, all of these constructs are simple and obvious in both behavior and expansion, and provide some functionality in addition to creating a binding. As far as I can tell, metabang-bind is not described by any of that.

Certainly. I've been experimenting with a compiler backed by LLVM, and using its IR builder, I often wish to create a function, define its body, then return the function. This usually takes the form of:

(prog1-let (func (llvm:add-function ...))  (setf (llvm:linkage func) :internal [...]

i'd use a custom macro here called (with-new-llvm-node ...) or somesuch, and that would also make the code a whole lot more easier on the brain when reading. and if it's a regular and often repeated construct in the codebase, then i'd even introduce an implicit -node- variable in that macro (which is colored specially in my emacs). IMHO. --  attila Notice the erosion of your (digital) freedom, and do something about it! PGP: 2FA1 A9DC 9C1E BA25 A59C  963F 5D5F 45C7 DFCD 0A39 OTR XMPP: 8647EEAC EA30FEEF E1B55146 573E52EE 21B1FF06

On Sun, Apr 08, 2012 at 12:33:55PM +0200, Tamas K Papp wrote:

I usually ... implement this pattern using the ANAPHORA library. Your macro would of course be a better solution if, for some reason, one doesn't want to use an anaphoric macro

Yeah, that's the idea. As with all the other macros in bindings.lisp, it's just a cleaner version of the same functionalith ANAPHORA provides. I took the presence of when-let and if-let to imply that this was established as a reasonable thing for alexandria to do.

but IMO it would be cleaner to...

I agree that the multiple binding use-case is a bit weak, but opted to support it for the sake of consistency with the other *-let macros already in alexandria. Returning them as multiple values is an interesting way to develop that into something more useful. I'm not sure I can see that being taken advantage of often, but it doesn't detract from the common use case, so I see no reason not to support it.

On Sun, Apr 08, 2012 at 06:24:33PM +0600, Attila Lendvai wrote:

it's not substantially shorter than the alternatives

True. However, the same can be said of every other macro in bindings.lisp, and this does not seem to have precluded their inclusion. It certainly does not preclude my glad use of them.

not a very regular pattern in my experience

Certainly this depends on the style of the code in which you are working. Many of my uses of it pertain to interacting with foreign libraries which are characteristically highly imperative, in ways that Lisp often is not. Nonetheless, interacting with such libraries is something I do often, and I do not expect I am alone in that.

makes code less readable in cases where a simple aprog1 is not desirable

I'm not entirely sure what cases you're referring to. As a matter of style, I prefer not to use macros which implicitly bind names, and therefore make use of constructs of the sort found in bindings.lisp in place of anaphora. Thus I would argue that aprog1 is in general never desirable. In cases where aprog1 is objectively unusable, such as nested forms, then this would seem to be nearly equivalent, except that the explicitly named binding allows unambiguous reference to be made, which is hardly less readable.

and it adds quite some complexity when reading the code.

This is indeed subjective. I find it to be clearer to read than the alternatives, but perhaps that's just me? Anaphora would seem to be widely used for a reason, though, and this is but a minor adjustment to that same concept...

On Tue, Apr 10, 2012 at 10:13:33AM +0300, Nikodemus Siivola wrote:

WHEN-LET and IF-LET have a fairly common use-case ... where in a typical case the /relative/ reduction in lines of code is a substantial 33% -- and they are also "classic": been around forever, re-invented independently by several people.

Though I'm no judge of classicality, judging from the response on this list the anaphoric form, aprog1, is fairly well known and used. It certainly isn't much shorter than the obvious approach with let, but I find it to be clearer to read: while let is often used for intermediate values of only local interest, prog1 clearly expresses that the value in question is to be returned by the form, without requiring that every possible codepath within the form be considered.

For the single-binding case there is also little chance of "guessing wrong" what it actually does. In case of PROG1-LET, I actually assumed...

This surprises me. WHEN-LET and IF-LET establish a pattern of "bind a value and pass it as the first argument of the corresponding form," which generalizes directly to my implementation of PROG1-LET. On this basis I had assumed that the expansion was so obvious as to barely merit explanation beyond that implied by the name itself.

in particular because I would write that as just a LET, without using PROG1 at all

Perhaps my assumptions are skewed due to my familiarity with this pattern's nearly equivalent incarnation in aprog1, which I have used to the same end in the past, but I don't seem to be unusual in that regard.

A "bind values, do stuff, return the bindings" -macro similar PROG1-LET would IMO fit fine in Alexandria, but PROG1-LET is not a good name for it.

I'm glad to hear you approve of the general idea. I'm certainly not attached to the name--the entire PROG class of names has always struck me as a gratuitously non-obvious throwback--and only named it in that manner for consistency and assumed obviousness-of-function in the presence of the existing binding macros. Given that I seem to have overestimated this effect, that rationale breaks down. I have named similar abstractions 'returning' in the past, though 'returning-let' seems a bit unwieldly. Do any superior names occur to you?

I also suspect it only gains true utility if it returns multiple values, but then we're back at looking for use-cases...

As previously mentioned, I find this macro useful primarily not to make code more concise, but more obvious of meaning, and feel that its existence is adequately justified by that alone. That said, it would hardly be impaired by returning multiple values and thereby reducing boilerplate in the (perhaps uncommon) use-case of a binding multiple-value-prog1.