Me: "Don't give me two different names with subtle variations,
  give me one strong name."
  You: "The difference between 'testify and 'conversionify isn't subtle.."

Yes I still use eq. It's useful in some cases, no question, but not so often that it merits thinking about what to call it. If I were creating a runtime from scratch, I'd give it a long name, like pointer-equal or something. If I find a better use for the name I'll override it without hesitation.

Names that take up prime real estate are like defaults. Having a bunch of similar-sounding, equally memorable words that do almost the same thing is akin to having large, noisy menus of preferences. They make the language less ergonomic, they make it harder to fit into people's heads. "What does upcase do to a list again? Does it use this coerce or that one?"

If the default doesn't work for someone of course they'll make their own up. This is lisp. They're empowered. I don't need to consider that case. My job as a language designer is to be opinionated and provide strong defaults.

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If I were creating a runtime from scratch, I'd give [is] a long name, like pointer-equal or something.

Yeah, I think it's a bit unfortunate that 'iso has a longer name than 'is. I've thought about calling them '== and '=== instead, or even '== and 'is for maximum brevity, but those are more confusing in the sense you're talking about, in that their names would be interchangeable. I can't think of a good, short name for 'iso that couldn't be mistaken for the strictest kind of equality available. "Isomorphic," "equivalent," "similar," and maybe even "congruent" could work, but "iso" is about as far as they can be abbreviated.

...Well, maybe "qv" would work. XD It has no vowels though. Thinking about Inform 7 and rkts-lang makes me strive for names that are at least pronounceable; I remember Emily Short mentioning on her blog about how a near-English language is easier for English typers to type even when it's verbose, and rkts posting here about how it's nice to be able to speak about code verbally. I think it's come up a few times in discussions about the name 'cdr too. ...And I'm digressing so much. XD;;;;

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I'm only arguing the general case for coercions.

Update: After reading over the ancestor comments more carefully, I guess this wouldn't address akkartik's concern. Nonetheless, I kind of like id.

> "Isomorphic," "equivalent," "similar," and maybe even "congruent" could work

Hmm, how about if id was a low-level beast that converted any object into a say ptr type that contained its address. Now instead of (is a b) you'd say:

  (iso id.a id.b)

In Java, everything uses equals() where it can, but then it's not easy to get == behavior when it matters. Technically you can use a wrapper:

  public final class Id
  {
      private Object x;
      public Id( Object x ) { this.x = x; }
      
      public int hashCode() { return System.identityHashCode( x ); }
      public boolean equals( Object obj )
          { return obj instanceof Id && ((Id)obj).x == x; }
  }

Back in terms of (iso id.a id.b), this would have the observable effect that (iso id.id.a id.id.a) would be true for all a.

  id.4 ; evaluates to 4 or the address where this 4 is stored?

To stretch my analogy to breaking point, that's like saying I wish we could build a hundred yards into the ocean, it would make such fine waterfront property :) Some things are 'prime' but not real estate.

Ok that is probably not clear. I think I'd use is or iso, but not both.[1] iso is perfect because it is short and precise and conjures up the right image. I'll probably never find a use for is. That's ok. Good names minimize cognitive overhead, but there's still some overhead. The best name is the one you don't need, and there's enough good names that we don't have to microoptimize.

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I'm not sure which variant is "the same regardless of who and why you ask". In common lisp there's 30 years of rationalizations why this is 'intuitive':

  * (eq "a" "a")
  nil

  arc> (is "a" "a")
  t

If you created syntax for hash-table literals you may want this to be true:

  (is {a: 0} {a: 0})

(Again I'm less certain than I seem.)

By "the same regardless of who and why you ask," I mean something at least as picky as "the same regardless of what algorithm you try to use to distinguish them." Notice that the pickiest equality operator in a language is the only one that can satisfy that description; any pickier one would be an algorithm (well, a black-box algorithm) that acted as a counterexample.

I think 'eqv? is this operator in standard Scheme and 'eq? is this operator in any given Scheme implementation (but I dunno, maybe it's more hackish than that in practice). Meanwhile, I think 'is acts as this operator in a "standard" subset of Arc in which strings are considered immutable. (A destructive algorithm can distinguish nonempty mutable strings.) I avoid string mutation in Arc for exactly this reason.

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So the semantics of is are actually quite fungible.

Yeah, especially once you get down to a certain point where nothing but 'is itself would allow you to compare things, then 'is is free to be however picky it wants to be. A minimally picky 'is would solve the halting problem when applied to closures, so something has to give. :)

Yeah perhaps I've been too conservative in designing wart, and I shouldn't be so afraid of proliferating types. Hmm, perhaps if there was a way to automatically coerce types as needed.. Say I wrote a function that can only handle lists, and pass it a string, the runtime dynamically coerces to string. If there's no coercion to string, it finds a coercion through lists of chars.. Hmm, I wonder if people have tried this sort of dynamic searching for paths through the coercions table. If I could do that I'd be more carefree about spawning new types. Just create a new 'test type, write a coercion to 'fn, and everything from before would continue to work.

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And in your approach you'd have to hardcode some coercion in every primitive.

If you're talking about putting the wrapper on the target type, like (rep:as special-coersion-wrapper x), that's still more verbose than special-coercion.x.

Whether you care about that verbosity depends on how often you need special-coercion. I just don't want some rare concept taking up space in the namespace.

I don't follow. In Arc I never say (coerce x 'sym); I always say sym.x instead. Neither of those is more hardcoded than the other, IMO; sym.x hardcodes the 'sym global binding, while (coerce x 'sym) hardcodes the 'coerce global binding and the 'sym type symbol, but they're both ways of looking up the same separately specified behavior.

...Oh, by "primitive" do you mean a function like 'sym? I think it's just a matter of perspective whether 'sym or 'coerce is the more basic concept. IMO, 'coerce acts primarily as a way to offload some functions into a less convenient namespace (which I could do myself with a verbose naming scheme), and secondarily as a way to add an extra dimension of meaning to type tags when they're not attached to types (which strikes me as suspicious wrt extensibility...).

Hmm, I guess the 'type function itself breaks polymorphism. I wouldn't arbitrarily limit the programmer from using 'type, and I know your 'defgeneric relies on it almost intrinsically, but now I've persuaded myself a bit more that I'd rather avoid dealing in specific type tags whenever possible.

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Ok, enough generalities in this sideshow. Focus on my other responses :)

I believe the choice between 'testify and 'conversionify is very clear. There's no way 'all and friends would use 'conversionify, and there's no way my hypothetical update to 'copy would use 'testify.

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You're trading off verbosity in setting things up for a new type (which I hope will be relatively painless) with just not being able to override certain decisions.

Yeah, I might have called it 'coercify if not for the confusion it would cause. ;) And so far I only have one (niche) use in mind for it, so it doesn't have to be nice and brief.

Actually, it would make more sense to call it "transformify." Both "coerce" and "convert" have the connotation of at least sometimes transforming from one kind of value to another. For my purposes, the function [+ 20 _] is a valid behavior--it certainly makes sense to 'copy a data structure but add 20 to one of its parts--and yet the inputs and outputs of that transformation are of the same kind.

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may be a case where the primitives themselves need multiple coercions.

Nah, you can resort to (as test x) and (as conversion x), with 'test and 'conversion--or 'transform--being callable types.

This is the same thing I meant when I said "you can make a 'test type to turn 'testify into a coercion."

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I want to try really hard to have just one coercion per type combination, and let users create new types when they want a different conversion. You point out that's verbose, but it may not be overly so, and at least it's overrideable.

Feels like I'm talking past you a bit.

When users want different conversions, they can already define new "anything -> type-foo" global functions, and those are already extensible/overrideable. There's no need to add a framework for that, even if it does manage to be a tolerably simple framework with a tolerably brief API.

Code that chooses between the super-similar testify.x and checkify.x is just as doomed to being arbitrary and hardcoded as if it were to choose between (as test x) and (as check x). The 'coerce framework doesn't help with this hardcoding issue.

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And I'd definitely try to designate one of them the 'default' coercion like arc already does.

I don't generally agree with designating one operation as default simply based on the types it manages, like "anything -> type-foo." For a more obvious example, it would be especially silly to identify a default "number x number -> number" operation.

Still, I think we agree on what's important. If the point of operation X is to reshape things to expose their fooish quality, then we don't want an operation Y whose point is also to reshape things to expose their fooish quality.

You draw the line farther toward "reshape things to expose their _____ish quality" and embrace 'coerce as a way to abstract the _____ away. As for me, I suppose I've come to believe _____ is an intrinsically subjective, second-class concept.

At one point I wanted Penknife to have a type type, with operations like list'ish.x, list'ify.x, [inherits list seq], and [new.list a b c], which could then be abstracted into other utilities like [def list my-list a b c]. It's been a while, and now I've kinda given up looking for a great advantage; I think the one plus would be having only a single global variable to attach the list documentation to, and I'm not especially excited about that. ^_^

These days I have the vague notion that a user-defined type which extends all the right things oughta be an equally pure representation of _____ish quality; the correspondence between _____s and types isn't one-to-one.

coerce, ify, transform, these seem really similar to me. Why bother splitting such hairs?

Ah, right. ^_^ Yeah, most of the times I "use" these things, I'm just making some other abstract nonsense utility. It could be a while until I have another down-to-earth application to try this stuff on.

coerce, ify, transform, these seem really similar to me.

Well, I don't distinguish "ify" from "coerce" except when it helps tell apart the subtle variants we're talking about in this conversation. :-p The subtle variants wouldn't all go into the same language, I hope.

"Transform," on the other hand, seems more general. Really, 'transform as a type wouldn't mean anything other than "unary function, preferably pure." I could very well be splitting hairs by drawing a distinction between that context and the everyday "function" context. However, I think there are some things I want to treat differently between those cases, like ints (multiplication in function position, constant functions in transform position).

If you're going to eschew coerce why not get rid of all trace of the types?

Actually, that's how I think about it. Coercion is a technique to focus output ranges or widen input ranges so utilities are easier to use. If I consistently want a certain kind of focusing--and only then--I'll put it into a utility.

I name those utilities to correspond not with specific types but with informal target sets, usually based on the extensible utilities they support. However, any given tagged type means little except the extensions associated with it, so I think my approach has a natural tendency toward corresponding names anyway.

Don't call it [this], call it [that].

That's a nice way to think about it.

"I don't expect to come up with witty names as fast as I change my mind about the designs of things. :) For now I'm happy resorting to cookie-cutter names like 'fooify, 'fooish, and 't-foo"