[racket-dev] Proposal for a "no-argument"

From: Vincent St-Amour (stamourv at ccs.neu.edu)
Date: Mon Jul 2 11:49:51 EDT 2012

I really like this.


At Sun, 1 Jul 2012 09:27:00 -0400,
Eli Barzilay wrote:
> There rare cases where it is useful to have a value that means that no
> argument was passed to a function.  In many of these cases there is a
> plain value that is used as that mark, with the most idiomatic one
> being #f, but sometimes others are used.  IMO, while such uses of #f
> are idiomatic, they're a hack where an argument's domain is extended
> only to mark "no argument".
> A more robust way to do that, which has become idiomatic in Racket is
> to use (gensym).  (And as a sidenote, in other implementations there
> are various similar eq-based hacks.)  IMO, this is an attempt to
> improve on the #f case by guaranteeing a unique value, but at its core
> it's still a similar hack.
> Recently, I have extended the `add-between' function in a way that ran
> against this problem at the interface level, where two keyword
> arguments default to such a gensymed value to detect when no argument
> is passed.  Natually, this "leaked" into the documentation in the form
> of using `....' to avoid specifying the default value and instead
> talking about what happens when no argument is given for the keywords
> in question.
> After a short discussion that I had with Matthew, the new version uses
> a new keyword that holds the unique no-value value, to simplify
> things:
>     (define (foo x #:nothing [nothing (gensym)] [y nothing])
>       (printf "y is ~s\n" (if (eq? y nothing) 'omitted y)))
> The idea is that this does not depend on some specific unique value,
> since one can be given.  For "end-users" of the function, there is no
> need to know about this.  It's useful only for wrapper functions which
> want to mirror the same behavior, and call `foo' in a way that makes
> their own input passed to it, including not passing it when its own
> input is missing.  In this case, you'd do something like this:
>     (define (bar #:nothing [nothing (gensym)] [x nothing])
>       (foo 10 x #:nothing nothing))
> This works, but I dislike this solution for several reasons:
> 1. Instead of finding a solution for the `gensym' problem, this
>    approach embraces it as the proper way to do such things.
> 2. But more than that, it also exposes it in the interface of such
>    functions, which means that "simple end users" need to read about
>    it too.  There is no easy way to somehow say "you souldn't worry
>    about this unless you're writing a function that ...", and if you
>    look at the current docs for `add-between' you'd probably wonder
>    when that #:nothing is useful.
> 3. There is also a half-story in this documentation -- even though the
>    docs look like the above function definition, you obviously would
>    want to define a single global gensymmed value and use it, to avoid
>    redundant allocation.  By the way the docs read, the above does
>    look like the way to do these things, and I can see how a quick
>    reading would make people believe that it's fine to write:
>      (define (foo)
>        (define (bar [x (gensym)])
>          ...)
>        ... call bar many times ...)
> I considered a bunch of alternatives to this, and the one closest to
> looking reasonable is to use the #<undefined> value: it makes some
> sense because it is a value that is already used in some "no value"
> cases.  However, it is probably a bad idea to use it for something
> different.  In fact, that's how many languages end up with false,
> null, undefined, etc etc.
> (As a side note, a different approach would be to use a per-argument
> boolean flag that specifies if the corresponding argument.  Since this
> started with a documentation point of view, I'm assuming that it won't
> be a good solution since it doesn't solve that problem -- a function
> that uses it similarly to `add-between' would still need to avoid
> specifying the default.)
> Instead, I suggest using a new "special" value, one that is used only
> for this purpose.  The big difference from all of these special values
> is that I'm proposing a value that is used only for this.  To
> "discourage" using it for other reasons, there would be no binding for
> it.  Instead, there would be a fake one, say `no-argument', which is
> used only as a syntax in a default expression and only there the real
> no-argument gets used -- so the value is actually hidden and
> `no-argument' is a syntactic marker that is otherwise an error to use,
> like `else' and `=>'.  (I'm no even suggesting making it a syntax
> parameter that has a value in default expressions, because you
> shouldn't be able to write (λ ([x (list no-argument)]) ...).)  The
> only real binding that gets added is something that identifies that
> value, or even more convenient, something like `given?' that checks
> whether its input is *not* that value.
> To demonstrate how this looks like, assume that the kernel has only a
> three-argument `kernel-hash-ref', and you want to implement `hash-ref'
> on top of it without using a thunk (which avoid the problem in a
> different way).  The so-far-idiomatic code could be as follows:
>     (define none (gensym)) ; private binding
>     (define (hash-ref t k [d none])
>       (cond [(not (eq? d none)) (kernel-hash-ref t k d)]
>             [(not (has-key? t k)) (error "no such key")]
>             [else (kernel-hash-ref t k 'whatever)]))
> Using the new idiom, it would be:
>     (define default-nothing (gensym)) ; private binding
>     (define (hash-ref t k #:nothing [nothing default-nothing] [d nothing])
>       (cond [(not (eq? d nothing)) (kernel-hash-ref t k d)]
>             [(not (has-key? t k)) (error "no such key")]
>             [else (kernel-hash-ref t k 'whatever)]))
> And using my suggestion:
>     (define (hash-ref t k [d no-argument])
>       (cond [(given? d) (kernel-hash-ref t k d)]
>             [(not (has-key? t k)) (error "no such key")]
>             [else (kernel-hash-ref t k 'whatever)]))
> Note that the code is essentially the same, only now there's no need
> for the gensym hack or any of the similar things, and the interface to
> the function is back to its uncluttered form.  The documentation would
> use `no-argument', which would be linked to a description of how/when
> to use it when people need to do so, while for most people the
> description is clear from the name.
> Two notes:
> 1. You might notice that there's no real need for the fake binding
>    since it's just syntax.  For example, the same could be done with
>    just dropping the default expression, as in
>      (define (hash-ref t k [d]) ...)
>    Keeping the binding there is useful since the syntax is still the
>    same (eg, macros don't need to change since it looks like just
>    another expression), and in case it is later desirable, it's easy
>    to replace it by an actual binding (see below).
> 2. Unlike the #:nothing keyword, this is not 100% robust, since you
>    *can* grab the actual value and pass it.  For example:
>      (hash-ref t k ((λ ([x no-argument]) x)))
>    I think that this is not a problem in practice, at least no a new
>    one, since it's essentially the same situation as with
>    #<undefined>, where it is possible to write an expression that
>    evaluates to it.
>    In addition, it is not 100% robust in that you can write broken
>    code like:
>      (define (hash-ref t k [d no-argument])
>        (kernel-hash-ref t k d))
>    and end up with the no-argument value as a result.  But this is
>    also not a new problem, since the same mistake can apply to the
>    other cases too.
> -----
> [Footnote:
> Like I said, further on, there is the possibility of having a proper
> `no-value' binding.  There are some obvious uses for it (eg, call a
> function with two optional values and specify only the last); these
> uses are arguably ones that should use keywords instead, but maybe
> there's still a point for older functions.  Regardless of whether it's
> a good idea or not, I think that if the need comes, then it could be
> implemented similarly to a `Maybe' thing.  To abuse the names from a
> different language, add a `some' constructor that is optional for any
> value other than no-value, which makes it a kind of a quotation -- so
> inputs like `5' are the same, `no-value' is the same as above, and
> (some x) is the same as x for any value, including `no-value'.  (And
> functions would need to check if it's a `some?'.)  But that's just a
> random though, not related to the above in any way other than showing
> how that syntax leaves this possibility open.
> ]
> -- 
>           ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
>                     http://barzilay.org/                   Maze is Life!
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