undefined | Better HN

0 pointsANTSANTS11y ago0 comments

> This makes no sense. You want feature X (the ability to return multiple values) without having to either (a) pick apart a simple structure containing those values or (b) assign names to those values, letting the runtime system pick them apart (or never stick them together, its choice) for you.

I was actually aiming for (b) here, just incorporated directly into the normal function call and return syntax. Why shouldn't you be able to return multiple values in registers or on a results stack without "tupling them up"? It's clearly supported by the hardware. By "call-values" (sp) hack, I meant that needing to use "call-with-values" and "values" is the hack. Lua, for instance, lets you (with very clean syntax) return multiple values from a function, and assign the results to separate variables, during which at no point is an intermediate table constructed:

  thing, err = getthing()
  if thing == nil then print(err) return nil end
  do_stuff(thing)

I don't think I got it quite right with my angular brackets, but I was trying to think of a nice, clear syntax for multiple return values that wouldn't look out of place in a Lisp.

Maybe I was being misleading by using an example that resembled option types. I wasn't trying to bring static typing into this, though I am interested in static type systems and curious about static Lisps, it's just that emulating simple option types for error handling purposes is a very common usage for multiple return values in dynamically typed languages (and, cough, static ones like Go that have lame type systems).

> data Maybe a = Nothing | Just a

> No matter what kind of cleverness you do in memory or whatever, in the end this type is a tuple of two values: (constructor, data).

Is that true? "Maybe" seems like the perfect candidate for a nullable pointer representation, or at least some kind of small tag. I'd be surprised if GHC represents Maybe as a full blown tuple.

0 comments

belovedeagle11y ago

I still don't see how you're advocating a solution of anything other than call-with-values under a different name.

I don't know Racket itself, so let me switch to common lisp which has the same "problem"... except it comes with a super-simple macro to do exactly what you want, apparently:

(multiple-value-bind (quotient remainder) (floor x y) ;; some code referring to quotient and remainder )

which (excepting perhaps edge-cases I'm not considering, and definitely excepting some error-checking) is just

(defmacro mvb (vars form . body) `(multiple-value-call (lambda ,vars ,body) ,form)

The same thing should work in Racket, modulo syntax and names.

Asking a lisp programmer to do everything with special forms/primitive language elements is like asking a Haskell programmer to do everything in the IO monad. Sure, it's possible, it will work just fine; but... it's just wrong.

By the way, the existence of the #'values function by no means implies there's some data structure being created; indeed, that's the whole point of the #'values construct. Anything you could do with #'values you could do in a list which you then destructured; #'values is to be used when the common-case is to throw away the "structure" and just use each value individually or not at all, like with the return values of #'floor. So in fact #'values is exactly what you want; I'm not sure why it's a "hack" when lisp programmers write it under one name and "the solution" when you write it under a different name.

Ah, it occurs to me that maybe racket doesn't work like this, but in common lisp at least, the transiency of values can be seen in that when you say, e.g., (+ (floor a b) c), the remainder of a/b is thrown away; just the first value is used. That's another huge benefit over structures-to-be-destructured, but obviously doesn't work if the extra values are always important.

The point of my discussion with Maybe is that /even if/ it's just a nullable pointer, conceptually it's essentially a [restricted] tuple. (pointer-is-null,data). There's still two pieces of data there, even if the representation of one of them is cleverly folded into generic object representation or what-have-you. By definition it can't be done with a single piece of data: (Maybe a) is not the same type as (a), for all a. Even if (Maybe Word16), e.g., were represented in a single value at a single place in memory, that value would have to hold more than 16 bits. Then it's viewable as a... tuple, again; one element in 16 bits and the other in the remaining bits.

j / k navigate · click thread line to collapse