[RFC 0148] Pipe operator #148

piegamesde · 2023-05-23T18:24:10Z

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nixos-discourse · 2023-05-23T18:24:30Z

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roberth

Nix is already considered to be overly complex by many. Of course we know that Nix solves hard problems and can't do so without exposing intrinsic complexity, but this needed complexity is already more than most people expect to have to deal with, coming from the broken but "easy" traditional methods.

This makes the addition of accidental complexity to Nix disproportionately harmful. It makes newcomers more likely to reject Nix, depriving themselves of a real solution to their packaging and deployment problems, while depriving us from valuable contributions.

I am opposed to the pipe function, and skeptical of a function application operator; especially one that reverses the evaluation order compared to normal function application.

rfcs/0148-pipe-operator.md

roberth · 2023-05-25T11:18:17Z

rfcs/0148-pipe-operator.md

+
+## `builtins.pipe`
+
+`lib.pipe`'s functionality is implemented as a built-in function.


pipe is not a good function because the order is not obvious. It's already available practically everywhere as lib.pipe. It's also hardly ever used, so doing this for performance strips it of the last argument for making it a builtin.

Builtins need to satisfy more stringent requirements, they take resources from the Nix team, and they can never be changed or removed.

I would recommend to always use a library, which can change things arbitrarily, thanks to being locked or pinned.

It's also hardly ever used

I think this is due to documentation issues rather than due to this function itself being bad. While it has a lot of potential of misuse, it can actually make code more readable if you put in the effort for your code to be readable from top to bottom.

I would recommend to always use a library, which can change things arbitrarily, thanks to being locked or pinned

This... makes sense, but then I'd make an argument that parts of nixpkgs.lib that manipulate generic data should exist outside of builtins and outside of nixpkgs - in a separate repository. It is not weird: we already have library flakes like flake-utils or flake-parts.

This discussion is outside of the scrope of this RFC, though; I merely disagree that lib.pipe not seeing much usage is due to technical reasons.

Shepherds currently believe this conversation is resolved, outcome: inclined to leave pipe in lib.

pipe is not a good function because the order is not obvious.

@roberth for what it’s worth, the commit where I introduced it has a rationale for the order in its message.

It's not a problem with your contribution, but with the constraints of the Nix syntax.

lib.pipe is a necessary step, and its adoption shows that it is useful to many, so thank you for creating it!

rfcs/0148-pipe-operator.md

roberth · 2023-05-25T11:35:13Z

rfcs/0148-pipe-operator.md

+## Change the `pipe` function signature
+
+There are many equivalent ways to declare this function, instead of just using the current design.
+For example, one could flip its arguments to allow a partially-applied point-free style (see above).


I think you could call this curried style.
While it is point free in the technical sense of not having a variable name to carry the data flow, point-free is generally only used in a context where higher order functions are used for more arbitrary data flows. For many, it also carries the connotation of the synonymously used pointless style.

I've never heard that term and the resources I've read so far all talked about "point-free" programming style. Google does not seem to know much either, so some links would be appreciated.

The flipped pipe function is just function composition on a list, eta expanded. Eta expansion / reduction do not change the meaning, so these two functions would be indistinguishable.

That means that we can divide the problem and drop some unnecessary terminology.

pipe = flip composeFunctions (and composeFunctions = flip pipe by virtue of a flip law)

composeFunctions is isomorphic to pipe

composeFunctions returns a function, which makes it easier to use where a function is expected: the application of higher order functions such as map.

A lot of that is just overly formal thought. Instead we can simplify this section to:

flip pipe is equivalent to function composition applied to a list. By using a function that composes a list of functions instead of pipe, we get back a function, which can be readily used in higher order functions such as map.

point-free is also point-less, in a semantic sense 😛

Shepherds currently believe this conversation can be resolved by accepting or rejecting the suggestion below.

rfcs/0148-pipe-operator.md

piegamesde · 2023-05-25T12:50:17Z

I am very well aware of the language's complexity. I am also aware of the fact that some of its users are very technical and into functional programming languages, and for many others this is a first contact with these concepts.

This is why I suggest picking only one of these operators as a compromise. I don't have any overly strong preferences here, but I make a point against function composition instead of argument piping, because the former leads to the so-called "point-free" programming style which tends to be more confusing for new users. I picked |> because of its similarity to the existing lib.pipe and the |> operator in Nickel, but if you are concerned about reversing control flow, then <| is a viable alternative too.

I strongly oppose calling this proposal "accidental" complexity. I spent a lot of time thinking about the options and trying to balance expressiveness and complexity, finding a compromise that makes solving real Nix problems easier. The Nix language is full of weird quirks users eventually have to face: we even have a wiki article collecting such instances, and it is far from exhaustive. Feel free to call these accidental complexity. But nothing about this proposal is accidental.

roberth · 2023-05-25T13:13:40Z

To be clear, I don't use accidental to describe this RFC. I only used it in accidental complexity, which I've used as a synonym for extrinsic complexity. I can see that you've put a lot of thought into it, and I respect that, but that does not necessarily make the change a net positive.

expressiveness

A language with more syntax may be easier to write, but is not more expressive unless the syntax comes with semantics that were not already covered by existing features and combinations of them.

makes solving real Nix problems easier.

What is a real Nix problem? At least the lack of a syntax won't end up on the quirks page.

Are the quirks a problem? Probably. I'd be happy to see a proposal that simplifies the language or makes the syntax easier to learn, but those are hard problems with a lot of inertia, and up-front costs that aren't "repaid" for years into the future.
Pragmatically, I think the real Nix problems aren't in the syntax. We might reach a stage where we could simplify the syntax and possibly redo it to make it more accessible to people who use more commonplace languages. That's not in the coming five years though, unless we radically expand the Nix team, which is rather resource constrained today.

suhr · 2023-05-25T13:26:31Z

Nix is already considered to be overly complex by many.

And more people consider Nix awkward to read and write rather than being complex. Mostly because the standard library is poor and not discoverable.

|> would definitely make Nix less awkward to use.

nixos-discourse · 2023-05-25T23:51:39Z

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/pre-rfc-pipe-operator/28387/10

rfcs/0148-pipe-operator.md

piegamesde · 2023-05-26T17:15:08Z

A language with more syntax may be easier to write, but is not more expressive unless the syntax comes with semantics that were not already covered by existing features and combinations of them.

I disagree with your interpretation of the word "expressiveness". Because you can (almost) always already solve all existing problems with existing syntax in (almost) any programming language. That's the point of being Turing complete. Therefore the question then becomes, how well can a problem be expressed in the language. And I think this holds true even for changes that are purely syntactical, like this one.

When adding new syntax to the language, one goal may indeed be to make it easier to write. Another one, and IMO much more important, is does it make the language easier to read. Depending on the feature and the language those may coincide, or one may have the other as a side benefit, or they may contradict each other. I recently learned Haskell, so I am very well aware of the trap of having a lot of powerful operators that allow it to write programs concisely and without parentheses, but which comes at the cost of readability.

What is a real Nix problem?

Problems that one might face when writing Nix code, either for some flake/shell/system configuration or when contributing to Nixpkgs. What I mean with this, is that I don't want to solve problems that for example mostly occur when trying to solve AdventOfCode in Nix.

KFearsoff

While the lib.pipe function is pretty cool (thanks for introducing it to me!), I think going through with this RFC would take gigantic effort, and the positive results (if any) will be seen only after a few years. And I'm thinking those positive results aren't worth it, because they won't make Nixlang significantly easier to read for beginners, and they don't solve any technical issues with Nixpkgs or Nix.

rfcs/0148-pipe-operator.md

KFearsoff · 2023-05-27T14:36:54Z

rfcs/0148-pipe-operator.md

+
+## `builtins.pipe`
+
+`lib.pipe`'s functionality is implemented as a built-in function.


It's also hardly ever used

I think this is due to documentation issues rather than due to this function itself being bad. While it has a lot of potential of misuse, it can actually make code more readable if you put in the effort for your code to be readable from top to bottom.

I would recommend to always use a library, which can change things arbitrarily, thanks to being locked or pinned

This... makes sense, but then I'd make an argument that parts of nixpkgs.lib that manipulate generic data should exist outside of builtins and outside of nixpkgs - in a separate repository. It is not weird: we already have library flakes like flake-utils or flake-parts.

This discussion is outside of the scrope of this RFC, though; I merely disagree that lib.pipe not seeing much usage is due to technical reasons.

KFearsoff · 2023-05-27T14:37:52Z

rfcs/0148-pipe-operator.md

+like line numbers when some part of the pipeline fails.
+Additionally, it allows easy usage outside of Nixpkgs and increases discoverability.
+
+While Nixpkgs is bounds to minimum Nix versions and thus `|>` won't be available until


I think this is a great argument against going through with this RFC.

This would be an argument for never making any changes to improve the language at all, ever. Unless you see some shiny alternative language to which we'll realistically all migrate to within the next five years, any improvements to the Nix language are still expected to be beneficial.

I've actually thought about proposing this change for quite some time now, and the main thing that held me back was the idea is that Nix is a "lost cause" and that I should rather spending my energy towards some successor. But eventually, I realized that realistically, we'll be stuck with Nix for a long time to come, so better invest the energy to make it a bit more comfortable …

Also note that outside of Nixpkgs, the feature will be available almost immediately. There is a lot of Nix code outside of Nixpkgs, all these flakes, shells, and users' system configurations. Projects like home-manager, devenv, nur, etc. are all free to migrate to newer Nix versions at their own pace. We shouldn't ignore these.

rfcs/0148-pipe-operator.md

piegamesde · 2023-05-29T17:22:43Z

I think going through with this RFC would take gigantic effort

Honestly, the most effort in this RFC is convincing people, and maybe also the coordination across many projects. But from the technical side of things, this is actually pretty easy to implement.

and the positive results (if any) will be seen only after a few years.

This only applies to Nixpkgs, see #148 (comment)

And I'm thinking those positive results aren't worth it, because they won't make Nixlang significantly easier to read for beginners, and they don't solve any technical issues with Nixpkgs or Nix.

They also won't make it significantly harder for beginners to read Nix code IMO, while bringing quality of life improvements to everyday Nix development.

I don't understand why only "technical" problems should be worth solving.

edolstra · 2023-05-31T13:23:26Z

This RFC is now open for shepherd nominations!

piegamesde · 2023-06-05T22:37:45Z

@AndersonTorres because I don't know every programming language and just didn't come across OCaml during my research. Furthermore, the related work section is intended to roughly cover the design space, not to be an exhaustive list of every programming language.

piegamesde · 2023-06-06T15:27:09Z

Updated the text according to some of the initial feedback. The more I think of it, the more I'm leaning towards also having <| in the language. I'll try to conduct a larger Nixpkgs survey soon to get some more data on that question. (Function concatenation operators are still out IMO)

rfcs/0148-pipe-operator.md

AndersonTorres · 2023-06-06T21:10:16Z

I was thinking on it, precisely, two pipe operators! It makes the language more orthogonal: if we have arg |> function, it looks reasonable to have function <| arg as its reverse operator.

The problems are:

if we are to elevate pipe to builtins and pipe == |>, where is the builtin for reverse pipe? The apply previously suggested looks good to me;
how the pipes should interact when in a same statement (a |> b <| c, a <| b |> c)?
I suggest to APLize: same precedence, right-to-left associativity (a |> (b <| c), a <| (b |> c))
a problem for the parser team: how should the errors be reported?

rfcs/0148-pipe-operator.md

piegamesde · 2023-06-12T09:09:51Z

I nominate @maralorn, who has a lot of experience with functional programming languages and programming language design.

piegamesde · 2023-06-13T09:17:29Z

@roberth, what about you, would you like to be a shepherd? I'd rather like to have critical voices on board from the beginning …

maralorn · 2023-06-14T10:10:45Z

I nominate @maralorn, who has a lot of experience with functional programming languages and programming language design.

I am pretty certain there are a lot of people with more experience on this in the wider community. That doesn’t prevent me from having opinions on this particular bikeshed, though. 🤣

My availability depends on the timeframe here. I will be very busy in the next few weeks, after that I’d be on board.

piegamesde · 2023-06-14T11:07:29Z

I've procrastinated writing this RFC for at least half a year, so waiting two more months won't be the end of the world

roberth · 2023-06-15T15:14:05Z

would you like to be a shepherd?

I can do that

nixos-discourse · 2025-02-05T20:00:47Z

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/lix-mismatch-in-feature-name-compared-to-nix/59879/1

nixos-discourse · 2025-02-17T16:23:40Z

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piegamesde · 2025-02-18T18:28:48Z

The pipe operators have been now implemented for quite a while, now I'd like to know about common usage patterns. Especially:

What are the real-world use cases for <|?
Are there use cases where mixing |> and <| is actually useful / would have been useful? (Lix allows it, CppNix forbids it)

keysmashes · 2025-02-19T19:51:35Z

What are the real-world use cases for <|?

While writing open-location-code.nix, I frequently found myself reaching for Haskell's $, which AIUI is essentially what <| is. For example, the following snippets:

let
  digitVal = digit: builtins.floor (builtins.stringLength (builtins.elemAt (builtins.match "(.*)${digit}.*" digits) 0) / 2);
  # ...
  lat' = builtins.floor ((lat + 90) * 8000);
  long' = builtins.floor ((long + 180) * 8000);
  encodeLatMsd = lat: valDigit (mod lat 20);
  encodeLongMsd = long: valDigit (mod long 20);

could have some parens removed:

let
  digitVal = digit: builtins.floor <| builtins.stringLength (builtins.elemAt (builtins.match "(.*)${digit}.*" digits) 0) / 2;
  # ...
  lat' = builtins.floor <| (lat + 90) * 8000;
  long' = builtins.floor <| (long + 180) * 8000;
  encodeLatMsd = lat: valDigit <| mod lat 20;
  encodeLongMsd = long: valDigit <| mod long 20;

On the other hand, I don't know if you would count open-location-code.nix as a "real-world use case"...

Examining the results for the GitHub code search query language:nix "<| ", you see some slightly more interesting ideas, including use with lib.mkForce/lib.mkIf:

{
  "containers/registries.conf".source =
    lib.mkForce
    <| toml.generate "registries.conf" {
      /* multi-line expression elided */
    };
};

lib.mkIf config.shell.direnv.enableCompletionAutoloadingWorkaround
  <| lib.mkOrder 2000 "..."

There's also some more complex usage in an Advent of Code solution here: https://github.com/V1K1NGbg/Advent-Of-Code-2024/blob/755b780a3416dc03508a209dc8c072a8e2aa938c/2.2.nix, including this fun line using both <| and |>:

concat [ (range 0 (n - 1) |> (map (elemAt xs))) ] <| groupsOf n (tail xs)

VTimofeenko · 2025-02-21T20:24:54Z

Should the feedback be provided in this issue or through some other means?

I don't want to spam here, but I do want to let the record show that I absolutely love this feature just as it is.

nixos-discourse · 2025-03-03T16:22:04Z

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nixos-discourse · 2025-03-17T16:15:40Z

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nixos-discourse · 2025-03-31T15:34:38Z

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kevincox · 2025-04-29T15:50:03Z

RFCSC: @rhendric are there any updates on this PR? What needs to be done to move it forward?

nixos-discourse · 2025-04-29T15:51:37Z

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GetPsyched · 2025-05-12T15:10:39Z

RFCSC:

Since there has been little activity for quite a while, we'll draft this in the next RFCSC meeting (2 weeks from now), unless the shepherds indicate that they'll pick this back up soon.

nixos-discourse · 2025-05-12T15:32:25Z

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nixos-discourse · 2025-05-16T16:38:34Z

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MattSturgeon · 2025-05-17T02:40:49Z

What is needed to ratify this RFC?

The feature is already available an an experimental feature; does it need to be made not experimental before the RFC is done, or can the RFC be considered done before then?

Maybe it's also worth considering what criteria needs to be met for it to come out of being experimental?

I'd propose that 1) being confident no changes are needed and 2) having waited a long enough period for feedback should be enough to make the feature stable.

If the RFC will finish before the feature is stable, the RFC should make clear what criteria is needed for it to come out of stable.

Otherwise we risk the feature remaining experimental indefinitely, because no one agrees what is needed for it to be stable 😁

piegamesde · 2025-05-17T09:25:10Z

See also #148 (comment)

I think the pipe operator is pretty uncontroversial, however I am still unclear as to whether or not we want the backpipe operator. Symmetry and the occasional use case would speak in favor of that, ambiguities around a canonical formatting and readability as well as the precedence question against it. Additionally, the backpipe operator kind of intersects with function concatenation, so that would require more investigation and design work to figure out what is best here

roberth · 2025-05-17T15:41:25Z

To me, the arguments against the back pipe weigh more heavily.

rhendric · 2025-05-17T16:17:22Z

As a personal anecdote, in my personal configs, <| currently appears four times and |> zero.

Some (well, one) of these cases could be rejiggered to use |> without any material loss of readability, but the pattern for which I find <| most useful is:

someAttrSetFn
  <| {
    ...
  }
  // lib.optionalAttrs predicate {
    ...
  }

Such expressions are often already within a few sets of multiple-line-spanning parentheses, and adding yet another parenthesis pair to the stack is substantially less readable to me than using <| to indicate ‘everything below here, up to a closing delimiter, is part of the same argument’.

KFearsoff · 2025-05-19T00:40:18Z

It seems that everyone agrees with |> addition, and it has been added to Nix implementations for a while. Does it make sense to stabilize |> without a feature flag, and morph this RFC into discussing if <| will make it into no-feature-flag state?

piegamesde · 2025-05-19T09:50:21Z

The text is a bit outdated, but this RFC is and always has been only about the forwards pipe operator. The backpipe is only discussed in prior art and the alternatives section. Implementing both pipe operators at once always has been a matter of experimentation to find out if it is actually worth it (turns out, a bit but not really).

My suggestion would be to clean up the RFC and FCP it as-is, and leave the backpipe operator (or function concatenation) for someone else to champion.

rhendric · 2025-05-19T19:33:48Z

Are there any other programming languages that include or allow users to define an operator equivalent to |> but not <|? (| from shell-like languages doesn't qualify. That's process composition, not application: it joins the output of one process to the input of another, resulting in a process-like thing that still has an input and an output.)

If not, what makes Nix special so that we would want to do so? Are all those other languages wrong and we think we're doing language design better here?

Is there any other purpose for which we might want to reserve the sequence of characters <| in the language that would not be confusing in the presence of |> as specified here? Anyone who would be confused by <| meaning apply-to-the-left after learning about |> meaning apply-to-the-right? How many more people would be confused by the absence of <|?

RFC author has the right to write the RFC however they want, but this is a troubling place to cleave the proposal in my opinion. The language with just |> and no indication of getting <| is arguably in a worse place than without either. And if <| ends up being a shoo-in for that reason, what would we get out of the extra procedure of a second RFC?

arnarg · 2025-05-19T19:54:30Z

Are there any other programming languages that include or allow users to define an operator equivalent to |> but not <|? (| from shell-like languages doesn't qualify. That's process composition, not application: it joins the output of one process to the input of another, resulting in a process-like thing that still has an input and an output.)

Gleam only has the |> variant. https://tour.gleam.run/functions/pipelines/

rhendric · 2025-05-19T20:26:31Z

Okay, I'm not familiar with Gleam, so what makes Gleam special so that its creator made this design choice, and is Nix special in the same way?

VTimofeenko · 2025-05-19T21:31:36Z

Are there any other programming languages that include or allow users to define an operator equivalent to |> but not <|?

There's also

Nickel, which is actually extremely close to nix. I could not find anything about a backward pipe outside of this issue, where enum syntax was being considered.
Google BigQuery SQL extension that adds pipe
Elixir (?)
~~OCaml (?)~~(see next post in the RFC)

(?) denotes languages which don't seem to have backward pipe, but I have no experience with them beyond quick googling to prove/disprove this post.

My guess is that |> allows writing the code more naturally as a pipeline. Most languages are read left to right top to bottom and breaking the code using the pipe or pipe forward symbol allows reader to quickly glance through the code. <| breaks this use-case by having to mentally jump back. Even if <| is not truly illegal in the language axiom set (e.g. by clashing with some other syntax), it may be just not a priority and we may not find strict reasons for language authors not implementing it.

I personally don't have any <| in committed code, but I have used it to drop a debug statement here and there

rhendric · 2025-05-19T21:57:56Z

In OCaml, the equivalent of <| is spelled @@ (OCaml operators have associativity rules based on the first character of the operator, so a <| operator would have to be left-associative, which is obviously not what one wants for apply-to-the-left).

Elixir I don't know about either. Maybe there's something in the water in the Erlang ecosystem?

Most languages are read left to right top to bottom and breaking the code using the pipe or pipe forward symbol allows reader to quickly glance through the code. <| breaks this use-case by having to mentally jump back.

<| allows the reader to quickly glance through the code by reducing the number of close-parentheses tacked on to the end of a big expression. It doesn't make anyone mentally jump back to consider the verb any more than |> forces the reader to mentally jump back to consider the noun. Neither operator is necessary in expressions small enough to fit in one eyeblink; just use regular function application there. In larger expressions, one says, ‘Remember this verb; it's about to take a big input,’ and the other says, ‘Remember this noun; it's about to go on a big journey.’ If verb-first expressions were always inferior to noun-first expressions, function application wouldn't be verb-first by default in a predominating number of languages (despite the several technical advantages of postfix notation!).

VTimofeenko · 2025-05-19T22:24:52Z

Thanks for pointing out my error about OCaml, I will update my previous post.

<| allows the reader to quickly glance through the code by reducing the number of close-parentheses tacked on to the end of a big expression

Makes perfect sense, thank you! i actually have replaced a lot of nested parentheses trading them for more lines with pipes aiming for a single line of code to do exactly one thing.

Most languages are read left to right top to bottom
The "languages" in my previous statement should actually be qualified as natural languages, not necessarily programming ones.

Also, for the record, I am not against the backward pipe. Forward pipe syntax has a special place in my heart and it's easy to see how the backward pipe could be similar to someone else. Nor am I making any statements about one syntax being superior to the other one :)

arnarg · 2025-05-20T06:57:10Z

Okay, I'm not familiar with Gleam, so what makes Gleam special so that its creator made this design choice, and is Nix special in the same way?

I'm not sure why to be honest but I suspect it's similar to what was said prior.

My guess is that |> allows writing the code more naturally as a pipeline.

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Adding an operator like Haskell's '$' NixOS/nix#1845

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		## `builtins.pipe`

		`lib.pipe`'s functionality is implemented as a built-in function.

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[RFC 0148] Pipe operator #148

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[RFC 0148] Pipe operator #148

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piegamesde commented May 25, 2023

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piegamesde commented May 29, 2023

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piegamesde commented Jun 12, 2023

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AndersonTorres commented Jun 6, 2023 •

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