Bikeshedding the Hack topic token

[js-choi]

This issue is for bikeshedding the spelling of the topic token in Hack pipes, branching off from #75 (comment). For more context, see the Hack pipes proposal and the wiki home page.

The table has its own editable page on the wiki. Please read this table first before contributing to this issue.

Please also keep discussion on topic: bikeshedding the topic token for Hack pipes. For other topics, search for other existing issues. Thank you!

Old obsolete questions

These currently are the most topical bikeshedding questions that I see now:

1. What is the optimal tradeoff in writability (easily typed ASCII soup, e.g., ?, or easily typed privileged valid variable, e.g., $) versus readability (less easily inputted non-ASCII syntax, e.g., ■)?

2. Related to question 2: Can non-ASCII Unicode syntax characters be considered for the pipe placeholder, or must they be categorically excluded?

   A list of all possible Pattern_Syntax Unicode characters is available.

3. If question 3’s answer is that non-ASCII Unicode syntax may be excluded, which non-ASCII syntax character would be visually understandable and/or less difficuldif­fi­cultt?

   Many non-ASCII symbols are easily inputted in certain OSes. For instance, in macOS, several dozen typographic characters are directly typable using Option or Option + Shift (image of keyboards with various active modifier keys via Macworld article. It may be worth determining if there is an intersection of these easily typable non-ASCII characters across the default keyboard layouts of many OSes.

4. For nullary operators vs. valid variable identifiers: How important is it that the placeholder be statically analyzable?

   A nullary operator can be always be statically recognized in the RHS. In contrast, a valid identifier can be statically recognized only if the rule is: “Tacit function calling may occur only when the RHS is a bare identifier, rather than allowing the RHS to be any arbitrary expression without a placeholder.”

5. Operator vs. identifier again: How often would someone want to use an identifier from an outer lexical context of the same name as the pipe placeholder?

   If the placeholder is $, how often would a programmer want to use jQuery’s $ or another externally defined $ in a pipe’s RHS, as well as the pipe placeholder? How often would they be surprised if they could not, without defining a dummy variable for the outer $? How bad of a footgun (i.e., bug by programmer-unexpected behavior) would this be?

6. How much should the pipe placeholder stay compatible with @rbuckton’s higher-order functional operators, which possibly would use {…}?

7. @rbuckton proposed a partial-application placeholder that may be explainable by multiple topic placeholders. In other words, the pipeline syntax might be completely unifiable with the PA syntax with the right enhancements.

   Assuming a future shorthand “pipe-function operator” like +> (which would be an abbreviation for x=>x|>), then @rbuckton’s proposal’s f(?, 3) would instead be +> f(?, 3), which in turn would be shorthand for x=>x|> f(?, 3).

   Creating non-unary functions could be done
   by adding numbers to placeholders,
   such as ?0, ?1, ?2, etc.
   For instance, example.sort(+> ?0 - ?1)
   would mean example.sort((x, y) => x - y).
   (?0 would be equivalent to plain ?.)

   Which placeholders would have any problems with staying forward compatible with such a future proposal?

As of 2022-07-11, I like ^^ then more distantly $_, @@, %%, and #_.

[ljharb]

There's a few choices that use ? but might avoid some of the confusion:

• <?>
• {?}

Also, I find ^^ confusing in that what it does is not related to what ^ does.

[aikeru]

FWIW, I really like ^^. I find it clever, and illustrative in that it points up, as though at the previous expression which is likely to be the LHS of the pipeline operator, reminding you where the value comes from.

[ljharb]

Pipelines will be single-line just as often as multiline; so "points up" and "points at the previous" won't consistently be the same.

[littledan]

I take it that, if we go down the Hack-style path, we're not trying for any kind of path for the placeholder to be subsumed by @rbuckton's partial application proposal, and therefore should avoid using the same token, is that right?

[gilbert]

My 2c: I personally really like $ as the placeholder variable binding. Because it's a valid identifier, it feels like an actual variable, which it certainly is (albiet smaller in scope). It also prevents the Hack-style proposal from claiming another operator that might be useful for future proposals.

I think the "jQuery problem" is probably overblown for these reasons:

• How many libraries out there actually expose a global $? I would imagine very few.
• The pipeline operator does not impede using these libraries. Rather, using these libraries impedes using the pipeline operator, which is not a big deal.
• jQuery would hardly benefit from the pipeline operator anyway since it uses heavy method chaining.
• If you're not using global scripts and using modules instead, then you can easily rename your import / require variable to something other than $.
• _ would still available, and is much more commonly used for utility functions (lodash, underscore, rambda, etc.), although becoming less so due to static ES module imports.

[mAAdhaTTah]

Is it at all possible to use $ in the partial application proposal? If we're trying to leave open the door for a partial application proposal in the future, we could end up with two different syntaxes for partial application / placeholders, one inside pipelines and one outside.

[littledan]

@mAAdhaTTah I don't think it's possible for partial application to use $ unless there's a token which is used to start off the partial application expression. The current partial application proposal does not have such a token.

[js-choi]

@littledan: I take it that, if we go down the Hack-style path, we're not trying for any kind of path for the placeholder to be subsumed by @rbuckton's partial application proposal, and therefore should avoid using the same token, is that right?

From what I could tell, syntactic partial application could use the same placeholder as Hack style iff syntactic partial application were forbidden in every placeholder-using pipe’s RHS. This could be a footgun—it creates a hidden context dependency between two very different results, which may cause the programmer to accidentally commit a mode error—but if we logically proceed anyway:

Proposal 2: Hack Style Only and Proposal 4: Smart Mix would forbid syntactic partial application in the RHS of all pipes—or, rather, the parser would always interpret the presence of a RHS placeholder as a pipe placeholder, not a partial-application placeholder. For instance, if |> is always a Hack pipe and ? is the placeholder for both |> and partial application, then f(?, 3) would be partial application, and x |> f(?, 3) would be f(x, 3).

Proposal 3: Split Mix would forbid syntactic partial application in the RHS of Hack pipes. It could also forbid it in the RHS of F-sharp pipes but it could also allow them. If |> is an F-sharp pipe, |: is a Hack pipe, and ? is the placeholder for both |: and partial application, then f(?, 3) would be partial application, x |> f(?, 3) would be f(?, 3)(x) (or it could be a syntax error), and x |: f(?, 3) would be f(x, 3).

If that analysis is correct, I’ll add it to the original post’s ? option. The problems of hidden contextual dependency—and of visual confusion with nullish coalescing and optional method chaining (assuming that syntactic partial application would stick with ?)—would remain.

[ljharb]

I would be very opposed to using any valid identifier - including $. It is a very common pattern to do var $ = document.querySelectorAll as well, for example - it’s not just about globals.

Shadowing bindings is fine when users explicitly choose the identifier - it would not be ok with me that a magic implicit binding would appear that shadows my code just for using an operator.

[barneycarroll]

I prefer personally ?, but I think it's worth unpacking 'visually confusing'. I think the statement that it is visually confusing in relation to the two other stage 1 proposals is misleading — those statements are more visually confusing with each other and with the ternary operator than anything else. Even if both of those get through, the ? placeholder proposal should not be held back on their account.

The ambiguity only creeps in when placeholders are nested in further expressions — @gilbert's original example being a placeholder in a ternary expression: ? ? : foo. This is the kind of scenario where the other two stage 1 operator proposals become problematic, when ?? can be read as either an optional chain on the placeholder.

It's a truism that nested ASCII operations are difficult to parse, but I think the placeholder is a special case inasmuch as it already bears the application-space cognitive weight of a deferred value within a special lexical scope of a higher order operation (the pipeline). So the situation only becomes complicated when we allow that the placeholder be used as an operand within a further expression nested inside a pipeline. Might it be worth outlawing that particular condition? — that is to say, a placeholder must be standalone, invocable only with ( or , either side of it?

In passing, I am firmly against the idea that an otherwise valid generic reference (like _ or $) be special cased. There is a special kind of semantic ambiguity introduced in scenarios like that of this in class arrow methods, when lexical parsing rules change based on higher order context. To do that with something which is otherwise up to user semantics is by all accounts a really imposing proposal (I foresee eslint rules to try and retro-ban them to compensate for this, invalidating previously legitimate and self-explanatory code).

[stephaneraynaud]

What about parameters with a name ?
for example:

anArray
  |array> pickEveryN(array, 2)
  |array> array.filter(...)
  |filteredArray> makeQuery(filteredArray)
  |query> await readDB(query, config)
...

Then one could write something like this:

anArray
  |$> pickEveryN($, 2)
  |$> $.filter(...)
  |$> makeQuery($)
  |$> await readDB($, config)
...

I think this would improve code readability, as there may be a lot of transformations applied. If I want to read one particular line, I won't know what's passed to the function and what's for.

Other proposal, reuse arrow functions : [edit: actually, I noticed this is proposal 1]

anArray
  |> array => pickEveryN(array, 2)
  |> array => array.filter(...)
  |> filtered => makeQuery(filtered)
...

I like this proposal because it can be used with destructuring and stuff like this, while being readable.

There are probably a lot of issues with those proposals, but my main idea is that variables like "$" or "^^" lack of readability and are not really elegant (from my point of view).

[mAAdhaTTah]

Whichever token we choose is going have an effect on the placeholder syntax, either by producing two different placeholder syntaxes (which I'd prefer to avoid), by superseding the placeholder syntax (effectively killing it), or by forcing its hand (requiring it to use the same token). We should probably decide which approach we're taking and keep that in mind as we bikeshed the token for use in the pipeline.

---

@bisouduperou I think we ran into problems with a similar proposal for await, e.g. this:

x |$> pickEveryN($, 2)

could be parsed as:

x | ($ > pickEveryN($, 2))

[charmander]

Reusing arrow functions’ arrow works very well for most purposes, but not for await.

[aikeru]

Honestly I just really hope any form that has a placeholder variable of any shape or spelling gets adopted, because it adds tremendous power and flexibility to an operator that would otherwise be much more limited and constrained... Thanks for letting me add my 2c.

[js-choi]

In #84 (comment) I used a ## nullary operator as a pipe placeholder. It looks nice and it doesn’t mess up GitHub’s current JavaScript syntax highlighting, but it probably would be quite visually confusing with private fields, so I don’t actually think it’s a good idea, heh.

[gilbert]

Ok, if $ is too commonly used in user-land, then how about $$, like the actual Hack pipe operator?

[mAAdhaTTah]

@gilbert I was also thinking about $_ as another possibility.

[stephaneraynaud]

Let's try to not end up with smileys please $_$

[js-choi]

Ah, $_ brings back dormant memories of Perl…

For what it’s worth, Perl 6 actually turns Perl 5’s $_ into an impressively unified concept of a lexical “topic variable”, which may be lexically bound using a given block or a for block and which is implicitly used by many other types of statements, operators, and functions, making for rather pithy tacit code. Think of it like linguistic topics. And in general, Perl 6’s design (fun fact: they actually released 6.0 some time back) is worth checking out you want to dive into the heart of some ingeniously mad and madly ingenious PL design.

I am in no way suggesting that an implicit topic variable be a thing in JavaScript beyond Pipe Proposal 2/3/4, though it’s pretty fun to think about.

[ljharb]

@gilbert tbqh, i very strongly would object to any valid identifier there - it’s not just about “commonly used” for me, it’s about “ever used”.

[gilbert]

@ljharb I think such a strong stance to take would make sense if $$ were made globally available, but here it's only on the right-hand side of a pipeline – a very small scope, of which a programmer consciously introduces by writing |> in the first place.

[ljharb]

Magically introducing an implicit binding that shadows something defined in an outer scope is with-like behavior, and after we all worked so hard to remove with from strict mode, I’d think we’d be loath to reintroduce it.

[gilbert]

I agree it's implicit, but I would not equate to with. Yes we'd be loath to reintroduce with, but this is not remotely the same thing.

[rbuckton]

PowerShell also uses $_ as a topic variable in a number of places:

// pipes
Get-Files | ForEach-Object { Out-Host $_ };

// catch blocks
try { ... }
catch [System.Exception] {
  Out-Host $_.Exception.GetType().FullName; 
}

// filters/functions
function Foo {
  begin { ... }
  process { Out-Host $_; }
  end { ... }
}

Of course, that's because every variable in PowerShell is prefixed with $...

[aikeru]

Ah, $_ brings back dormant memories of Perl…

PowerShell also uses $_ as a topic variable in a number of places:

FWIW, Ruby uses this too. If JS were to adopt $_ as a topic variable or something like it, there's lots of precedent.

[jridgewell]

there's lots of precedent.

Node's also set precedent with _ being the last evaluated expression, but if it's defined in-scope it the variable you set it to.

[js-choi]

There is also Safari Web Inspector’s use of $1, $2, … to denote previously evaluated expressions in its console REPL, I suppose.

I used to be very against using a valid identifier for a pipe placeholder, but being reminded of Perl and Ruby made me realize that it might not be that weird. Then again, that there is an implicit binding at all in a Proposal-2,3,4 pipe is relatively unprecedented in ES. with was very bad especially because of its mucking up of static optimization, which Proposal-2,3,4 pipes do not have at all. But could using a valid identifier make static analysis of a pipe’s RHS—e.g., to determine whether there is no placeholder in the RHS—more difficult?

I plan to have my Babel plugin for Proposal 4 support configuration of what its placeholder is, to encourage experimentation (if its tokenization permits it). What its default placeholder should be, I don’t yet know. Maybe a non-ASCII placeholder by default might be interesting after all, haha.

[stephaneraynaud]

I don't think it's about weirdness but about origin. That you don't know about what an operator means is ok. You may don't know what "yield" or "await" means, because those are operators. But a valid identifier is, by default, a variable previously defined somewhere. By adding valid identifiers like "$_", you will need to ask yourself "am I in pipe ?" "does it come from pipe" "is it a global / simple variable?". I deeply encourage to avoid such (little) confusion.

While I personnally prefer meaningful placeholders ("$_" is as readable as "let thing = stuff()"), why not using @ as placeholder?

anArray
  |> pickEveryN(@, 2)
  |> @.filter(...)
  |> makeQuery(@)
  |> await readDB(@, config)
...

[tabatkins]

And more directly for this topic: this is a thread about bikeshedding the topic token. Off-topic discussions will be hidden and/or deleted.

[severinraez]

Regarding ^^: Please note that ^ is a prefix key on Swiss (and I think German and French) keyboard layouts. We type ^, a to get â. To write ^^ reliably we need to type ^ four times (where only keystrokes 1 and 3 have a visual effect). Not the ergonomics I'd wish for :) 🚲 🏠

[jods4]

@severinraez that's incorrect, at least for Swiss-French keyboard (fr-CH), which I'm using (aka QWERTZ).

When the second keypress after ^ can't be accented, then the ^ is simply printed by itself.
So although pressing ^ then e results in ê; pressing ^ then q results in ^q.

Likewise, pressing ^ then ^ again results in ^^.

More precisely:

• First press ^, nothing happens on screen. Input is waiting for 2nd character that would be accented with ^, e.g. a or i.
• Second press ^, this can't combine with previous keypress so ^^ appears on screen.

[severinraez]

@jods4 interesting! The behavior seems to be dependent on the operating system. For de-CH, on Windows it's behaving exactly as you describe. On Ubuntu 23.04, it behaves as I described.

[jods4]

@severinraez right, I should have taken OS in consideration. I'm using Windows indeed.

[Akryum]

Fedora also needs four keypresses to type ^^

[nmn]

Although ^^ is not going to be the choice due to Linux behaving the way it does, I checked macOS. When typing ^, it shows up immediately with a little blue line under it. If you type an a, e, or i, The ^ is replaced by a â. But if you type anything else, including a space or a ,, then the ^ character is left as-is and the next character is inserted after.

Looks like Linux is the only problematic OS for this.

[bhallstein]

I'm curious if the following has been considered:

Current proposals/token discussions are of the form:
foobar |> `$ ${<TOKEN>}` |> console.log

A possible alternative:
foobar <KEYWORD> foo |> `$ ${foo}` |> console.log where foo is a coder-supplied identifier that is then used as the ‘token’.

For example (not evaluated these possibilities for robustness, but just to illustrate the idea & show this might read nicely):

• foobar as foo |> `$ ${foo}`
• foobar :: foo |> `$ ${foo}`

In my view giving the programmer control over the identifier would be a desirable goal. This discussion's long duration suggests that no single choice of token such as ^^ can please everyone. Also, the programmer then has an opportunity to choose an identifier that is reads nicely in context. (However, note this would be more tricky than choosing a regular variable identifier, as the meaning of the operand will change as it is passed through multiple |>s.)

[tomByrer]

giving the programmer control over the identifier would be a desirable goal. This discussion's long duration suggests that no single choice of token such as ^^ can please everyone. Also, the programmer then has an opportunity to choose an identifier that is reads nicely in context.

I'm leaning towards of having a static identifier & not putting changing as part of the spec. Of course, someone could make their own library to have whatever tokens however they want.

IMHO 6 years is far too long to be bikeshedding anything; right now no one is getting any pipe spec into JS, instead of upsetting half of the folks & getting something out the door.

[kamoshi]

Is choosing the topic token the only obstacle blocking this proposal from moving to stage 3?

[nmn]

The "topic token" wasn't a blocker for the proposal at all from what I understand? I thought this is just us wasting time as the committee moves the proposal forward.

[zoodogood]

What about a %` (or %') token - might that be a good option?

numeric |> %` % 5

I think this option is more visually appealing and, perhaps obvious, but this more difficult to write compare to:

numeric |> %% % 5

Edited:
Although it has no logic, the construct

5%`some string literal` // old code: can be wrong

is allowed in Javascript. A compatible option would be @`

---

When it comes to three-character tokens, my favorites are |%| or |^|

|^|[5] 
^^[5]

[mybearworld]

If ` is at the end of the symbol for the topic, calling the topic as a tagged template function would look like this:

(...a) => a
 |> @```

In my view giving the programmer control over the identifier would be a desirable goal.

Not having to come up with new names for every substep is a big plus of having pipe expressions - so ideally, the programmer wouldn't have to choose a name.

[tabatkins]

Quotes always come in pairs; breaking that association would only be done with an extremely good reason. That's definitely off the table as an option, I think.

[mybearworld]

* would look nice, as it's often used as a kind of placeholder value meaning "anything can go here". Of course, that's already used by the multiplication and exponent operators, so this probably shouldn't be used, but it's worth thinking about (maybe [*] or <*>?)

2
  |> addOne(*)
  |> add(*, 5)
  |> * * 2 // not great
  |> **2 // very bad

[sant123]

What about $# like shell count parameters?

value |> foo($#)

[Daniel15]

What about $# like shell count parameters?

value |> foo($#)

This looks a bit weird because JavaScript doesn't prefix variables with $ like shells do.

[sant123]

What about $# like shell count parameters?
value |> foo($#)

This looks a bit weird because JavaScript doesn't prefix variables with $ like shells do.

Yes but you can still do it, the advantage with # is that you will not break existing JS code. $# can’t be defined by anyone.

[mybearworld]

The shebang syntax #! does exist, but it'll only be ever used as the first two characters in a file (and it's only legal in that position).

[bogdanbiv]

* would look nice, as it's often used as a kind of placeholder value meaning "anything can go here". Of course, that's already used by the multiplication and exponent operators, so this probably shouldn't be used, but it's worth thinking about (maybe [*] or <*>?)

Why [*] and not [_] or |_|?

// worst case would be:
[[2, 1, 3], ["AAA", "BBB", [""]]]
  |> pickOne([_][0], "concat")
"2concat"

My impression is people hate three character tokens, at least they hate having to use ===, otherwise ___ would have been a nice pick. My impression is we need to use the worst examples to countervalidate what would not work

[mybearworld]

_ is a valid identifier right now. [_] is an array with one element which is the variable _, and |_| can appear in the context of 1|_|2. ___ is also a valid identifier.

[JAForbes]

I'd really like it if the particular binding token was reused elsewhere in the language. I think it would also be great if we could reuse or extend existing parts of the language instead of adding a new special symbol.

Maybe we can let users bind their own name by extending the existing syntax for label statements in the language.

x
|> $: f(x,$)
|> it: f(it)

And if the function is unary you can skip the name

x
|> f |> g |> it: await h(it, 2) |> i |> j |> it: await k(it, 4)

They aren't arrow functions so any concerns about performance / closure creation is allayed, and being able to name intermediate variables would probably be useful for long pipes.

I also find it pretty easy to follow what is going on here: it: await h(it, 2). We are so used to a colon after a name as a key/attribute/parameter definition the syntax largely explains itself.

If you want to get super terse, its only 2 extra characters.

|> await(#, 4)
 
// vs

|> x: await(x, 4)

But for that 2 character cost we get the ability to bind more human readable names, and reuse an existing part of the language.

And the same idea could be applied to those other language features even legacy ones like if / for and switch e.g.

switch (something(...)) it: {
   case 5: return it * 2
   default: return it * 3
} 

We saw these sort of bindings in other languages. Rust and swift have let bindings / pattern matching in loops. Zig also has a similar feature.

Currently the above syntax is invalid in JS so its not a breaking change, all that is needed is to differentiate between a label binding in an expression and a label binding directly after a continue / break command.

Brief aside: I was very much in favour of F# pipelines but I now just want this language feature to be the best that it can be no matter what the specific syntax is.

[mybearworld]

"Hack pipes favor more common expressions" and "Hack pipes might be simpler to use" seem to apply just as well to that variant of F# pipes as well.

[nmn]

@mybearworld We are no longer discussing F# pipes. I prefer them too, but Hack Pipes are more likely to actually ship and if you keep arguing we will never get any pipe operator.

Further, this issue is only about the token to be used in the case that we move forward with the Hack pipes approach. Your comment doesn't belong here.

[mybearworld]

I was responding to the comment above about extending the label syntax.

[sant123]

What about:

value |> foo(!!)?

Yeah would be cases like

value |> foo(!!bar)

But can be handled as a valid token if !! is only found. Also is not disqualified.

[mybearworld]

That would be ambiguous. foo |> !!(bar) could be foo |> (!!)(bar) or foo |> !(!bar).

[zlumer]

@nmn

if you keep arguing we will never get any pipe operator.

I'd argue that no pipe operator at all is much better than this abomination.

[nmn]

@zlumer That argument doesn't belong in this issue either. Read the title. Make that argument in a different issue.