Are nulls the best representation for unused refinements?


The Refinements are their own arguments change is in. And it's a good thing, whose goodness I cannot overstate. It's paying off now and will pay off in the future.

But unifying the refinement and argument has one hitch: what if you want a refinement to be literally BLANK!? I'd hoped it wouldn't be common. But @giuliolunati is using BLANK! to represent a NaN (not-a-number) state...which I have advocated for.

There's a way to do it, though it's kind of ugly. When you say a refinement is [<opt> blank! ...] you are asking for the refinement to be null if unused. This way, blank can be passed as having meaning.

But... that's a bit sketchy. And there's something sketchy about unused refinements being BLANK! in the first place. MAKE FRAME! creates an empty frame with all nulls...for a reason. And those clearly should mean "not specified". For refinements that means not provided. We wind up having to mutate nulls into blanks when the function runs...and it's an "unclean"-feeling thing.

What if /REFINEMENT was used to access refinements?

What's not very likeable is having to use GET-WORD!s all the time, to avoid errors through access of an unset variable:

foo: function [a /ref [integer!]] [
    if :ref [
        print "This is annoying"
        print ["But you only need it for checking" ref]

But something that I've thought about often is the idea that accessing a refinement might look good if it were done with the refinement itself:

foo: function [a /ref [integer!]] [
    if /ref [
        print "This makes sense and actually adds value"
        print ["Still not needed on *every* access" ref]

This might fit into a form of GET that is explicitly for the purpose of NULL checking. We then might make GET-WORD! error on NULLs, so it would be clearer what the purpose of the GET was for (e.g. specifically to suppress function execution). This is something I've felt is pretty necessary for a while because you can't tell at first glance exactly what the point of a GET-WORD! usage is...and here we'd know when it was trying to tell us that the variable may be null.

So to be clear--it wouldn't really have anything to do with refinements. You would be able to use /foo on any word, and have it gloss over the NULL-ness. It would basically be like a GET-WORD! of today. It looks a bit cleaner, if you ask me, and you can type it without shift. :slight_smile:

It would mean losing refinements as an inert class. But as they are PATH!s now, it's not obvious that they should be inert. And with @word, @at/path, @(at group), @[at block] on the horizon...there's several other more clearly inert value options coming.

One downside of making PATH! with blank at the head evaluate would be incompatibility with historical Rebol's inertness of REFINEMENT!. There's no way for Redbol to subvert the evaluator for that. Code would have to be changed to QUOTE the refinement. Changing the way GET-WORD! worked to not allow NULL would similarly be something emulation couldn't override.

Another thing that would be a bit annoying would be using this with a new APPLY syntax, where you wanted to use refinements pass-thru in the expression itself, you'd have to put the refinement in a GROUP!

foo: function [... /dup [integer!]] [
    apply append [... /dup (/dup) ...]
    apply append [... /dup :/dup ...]  ; or maybe this?

That's if you didn't know if it was used or not, since you could use plain dup if by that point you knew it was used. It's not a deal-breaker by any means...pretty easy workaround as such things go. But it's something to bear in mind.

Intentionally Specializing With NULL
NULL, first-class values, and safety
Quoting Decisions In Practice: NULL Refinements

I just noticed I was doing something that wouldn't work in a refinements-are-null world. It seemed like a problem for a minute, until I realized... it wasn't new, and we now have a pretty good solution.

The issue was that in COMPOSE, nulls vaporize. So if you have something doing a code emitter, like:

emit compose [something/refinement plain-arg (/refinement-arg)]

You'll get an error because there will be a missing arg to /refinement. There's no way to put a null into a block. can get around it with quoting:

emit compose [something/refinement plain-arg '(/refinement-arg)]

Which you probably want to do anyway with generic content being run through evaluation. (What if it had been a GROUP!? What if it was an ACTION!, or WORD!)

Pretty neat.


I think this proposal is a good idea. Distinguishing blanks from nulls is important. If we don't make unused arguments null, we aren't using the nullness of null to its full advantage.

Redbol emulation will be able to give the old behavior by converting unused refinement arguments from nulls to blanks. People who expect refinements to be evaluatively inert in Redbol will have to use QUOTE...or if they don't care about their code actually still running in Rebol2/Red, they can use apostrophe as '/refinement.

Helping drive my belief is the feeling that "paths with inert things at their head are inert" is the wrong direction. I'd wondered if <tag>/2 should be #"a" or if it should be easier to handle something like that as inert. But now that we have generalized quoting, you can say '<tag>/2 if you need to...or if you want it to be permanently inert you can say @<tag>/2. Making a plain path inert doesn't feel right...why would (<tag>)/2 act any differently from <tag>/2?

I think the net result will be intuitive. Switching GET-WORD! to not be null tolerant would line up with the historical behavior of GET not being null tolerant without /ANY, which we might consider re-establishing (though I've pretty much firmly proven that SET-WORD! needs to be null tolerant).

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I think this proposal is a good idea.

Consider the following ways to deal with a loop with NEXT, that ultimately returns NULL when it runs out of input:

while [x] [... x: try next x]  ; (1)

while [:x] [... x: next x]  ; (2) how you had to do it w/o the TRY before

while [/x] [... x: next x]  ; (3) how you'd do it w/o the TRY in new proposal

Since accessing a null variable (unset) through a plain word will give an error, to use such a plain word you have to use TRY. If you didn't want to do that, you previously would say (2). But this proposal says that (2) would error now...only disarming functions. So you'd do it as (3).

Likeable Points:

  • I've been uneasy historically with the lack of communicativeness of why a GET-WORD! was being used. Is it to disarm a function, or is it because the variable may be NULL? Here you can tell at the callsite...the PATH! starting with a BLANK! means you're specifically asking for null tolerance.

  • The optional-ness of a /refinement visually ties together with the access pattern for optionally-null things.

  • On top of that, I think it looks better (slash is a "cleaner" character to lead with) well as being a bit more noticeable.

  • You don't have to hit shift on most keyboards to get the slash

Problem Observed In Practice:

It has traditionally been easy to have a PATH! or SET-PATH! in your hand and transform it via as get-path!. But because leading-slash paths are not a distinct datatype, you wind up with some complexities when you're trying to produce /.. programmatically.

Among the complexities that arise is the danger of producing a double-slashed path unnecessarily, One imagines a set of transformations running where each one is sticking on an extra leading slash:

compose '/((some-get-path-that-already-has-a-leading-slash))

Basically, since the GET- and SET- and plain PATH! distinctions are mutually exclusive and cannot compound, they interchange particularly smoothly. That gets compromised in meta-programming when a GET-PATH! can't give back NULL and you have to futz around with more slashes in the path somehow.

I don't think this is a deal-breaker - it's just something I observed. Historical Rebol made you step outside the bounds of using a SET-WORD! or SET-PATH! and use SET/ANY if you wanted to assign an UNSET!. So who's to say GET-PATH! has to be all things to all people and allow NULL, when SET-PATH! isn't willing to assign a VOID!?

Plus, REFINIFY is easy enough to write (following the pattern of -ify meaning "leave it as it is if it was already"), if there's concern about accumulating leading slashes.


To further my point here, in Rebol2:

>> unset 'foo
>> get 'foo
** Script Error: foo has no value

If get 'foo for an unset variable is an error, why shouldn't do compose [(to get-word! 'foo)] also be an error, with you having to do something "more extreme", like do [get/any 'foo]?


The more I thought about the epicycles of this problem, the more it bothered me.

When you add it on top of changing how "refinements" (blank-headed PATH!) act in a way that will break Redbol compatibility in a way that can't be worked around, the proposition sinks.

(...and even if it were a good thing, I also don't like shaking things up and requiring function bodies to say if /refine instead of if refine this close to the conference.)

But refinements being null when not used is good and right and true. So new plan needed...

...I now believe that the problem is that "null variables" should be fetched through ordinary WORD! and PATH!... see explanation here.

This just means adding a twist onto the responsibility matrix of NULL, BLANK!, and VOID!. A little less erroring-responsibility on the non-valued NULL state, with more error-causing into the court of the valued-VOID!. It moves closer to how Rebol2 accomplished the specific end of error messages on variables you mistype, while still being fundamentally reworked about what "unset" variables are.

I think any illusions of Ren-C being able to bring evaluator "safety" to a fundamentally unsafe language are being stripped away with time. Instead, what's actually happening is that the features are getting laser-calibrated to where an informed programmer can write short, correct, code easily... the right thing is the short thing, not something riddled with edge-case handling.

Clarity is a catalyst for safety in a language. But beyond that, the bias seems tipping ever toward being clean and powerful enough to "make your own you-made-a-mistake-catching-safety mechanisms"...

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Clarity and brevity are two fundamental qualities of good writing. I think that applies to programming as well. Also 4 concepts so close (NULL, BLANK!, NONE! and VOID!), make me look for a fundamental problem in the language, or they do not have adequate names.

I do not quite understand the usefulness of these 4 variants but should not there be one, or several but named to make understand to which functionalities of the language they relate (block, value, refinement ...)? Maybe I get it wrong.


NONE! was a Rebol2-ism (along with UNSET!), and does not come up in Ren-C except in "Redbol" emulation.

So what I describe is how these two ideas have become three:

  • NULL has properties unlike anything in Rebol2. It is not a "value" at all (and type of a null is also a null). It cannot be put in a BLOCK!. It is a transitional state representing nothing, like how a C null pointer doesn't point to any memory. It is conditionally false if it appears in something like an IF statement, and it is the result of any control construct that does not run a branch...or any looping construct that BREAKs. ELSE and THEN react to their left-hand-side being null or non-null, respectively.

  • BLANK! is a unit type...e.g. a value of type blank holds no further information (than that it is of type blank). Unlike NULL it's something that can appear in a block, and hence can be used as a placeholder--for instance if a block represents a record structure broken into N-element units, it could represent a missing field. Values of this type are conditionally false.

  • VOID! is another unit type. But it is neither true nor false...trying to test it conditionally or assign it via an ordinary SET-WORD! produces an error. It is the result of routines that deliberately want to say they aren't returning useful it is the return value of things like PRINT (e.g. x: print "Hi" is an error). Since it is a value type, it is possible that it may appear in blocks.

It has turned out that VOID! acts very much like what Rebol2 called UNSET!... and with the suggestion I'm making here it's getting even closer. I still don't think UNSET! is a good name for it... because to my mind a variable can be unset, but not a value. Hence it's actually nulled variables that are unset. But it is converging on almost entirely a terminology point.

Rebol2's NONE! and Ren-C's BLANK! were always supposed to be quite similar, just a different name. But NULL has taken over many of the duties of NONE! as an indicator of "soft failure" from a when a FIND can't find anything, or a conditional statement doesn't run any branches. BLANK! sticks around as the solution to the problem of neutral placeholders in blocks, as well as to be easily convertible from NULL via TRY in order to signal a conscious desire to opt-out of an operation.

I think the names work pretty well. Particularly pleasing is that NULL is exposed in the API as C and JavaScript's NULL. That has turned out to be very important.

Rebol2 always had trouble with #[none]-the-unit-value and none-the-word, frequently rendering it as looking like the word:

rebol2>> find [a b c] 'd
== none

As mentioned, Ren-C does that with NULL now. But if you wanted to use TRY to convert it to a blank (e.g. to quietly opt-out of a chain of operations on failure) you get a blank:

ren-c>> try find [a b c] 'd
== _

Reserving plain old _ so it's the literal form of this value...and having it be named BLANK!...has felt pretty good. But we're still in that stretch of time for evaluating decisions for their merit...