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!

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.

1. pipe = flip composeFunctions (and composeFunctions = flip pipe by virtue of a flip law)
2. composeFunctions is isomorphic to pipe
3. 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.

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.

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.

Plan for moving forward/stabilizing feature in Nix

Now that the experimental feature has been released, it may be time to start talking about a plan for evaluating it.

I see a number of possibilities:

• We do nothing and hope it happens organically that enough enthusiasts try the feature out to give it a good trial. In all likelihood, people will only comment if they dislike something about the feature, which means we could receive little or no feedback and be left wondering if enough people have looked at it. This requires least effort but is likely to take the longest.
• We promote the feature in official Nix channels and set up some sort of polling for both positive and negative feedback, with an expectation of how long the poll will be running before moving this RFC to FCP. This requires a bit of up-front work and may be biased towards people who pay attention to official Nix channels, but has the advantage of resolving faster than the first possibility.
• We devise an actual experiment to measure people's ability to learn the new operators. This might take the form of a survey asking people to rate the readability of various Nix expressions, some with the new operators and some without, and testing if there is a statistically significant difference in the ratings. We could send the survey to a representative sample of community members instead of (or in addition to) the public polling from the previous possibility, to try to control for bias. I'm not aware of the Nix community undertaking this level of rigor in the past but it would help us bring a stronger case for assuaging people's concerns about the mental cost of adding new operators. It would be a substantial investment of time from a few people (I'm happy to be one of those people but it probably shouldn't be me alone).

Anyone else have thoughts?

We do nothing and hope it happens organically that enough enthusiasts try the feature out to give it a good trial. In all likelihood, people will only comment if they dislike something about the feature, which means we could receive little or no feedback and be left wondering if enough people have looked at it. This requires least effort but is likely to take the longest.

I think it would be beneficial to give the new feature more visibility and promotion, rather than leaving it as an experimental feature indefinitely.
Collecting feedback through some form of polling or voting mechanism could provide valuable insights.
Additionally, introducing new features in a programming language is a significant event, especially for a community-driven project like Nix. It's likely that experienced PL experts would be needed to assess its impact and stability.

(Attempting to corral feedback into a thread.)

@illustris [#148 (comment)]

nix-repl> double = x: x*2

# current syntax
nix-repl> 3 |> (x: 1 + x) |> double |> toString
"8"

# proposed alternative
nix-repl> 3 |> x: 1 + x |> double |> toString

# current syntax
nix-repl> 3 |> (x: 1 + (x |> double)) |> toString
"7"

# proposed alternative
nix-repl> 3 |> x: 1 + (x |> double) |> toString

Both of those proposals seem, to me, to be ‘more likely’ to mean something else:

3 |> (x: (1 + x |> double |> toString))
3 |> (x: (1 + (x |> double) |> toString))

In other words, if |> binds more loosely than the lambda-forming :, it would be the first thing in the language to do so and thus would be very surprising to me. So far in the language, a lambda always extends as far rightward as it can.

Partially applied pipes

To me this smells like function composition with extra steps. Function composition is discussed in the RFC as well, and while I am not completely opposed to it, my fear is that it reduces code readability given a lack of type annotations. In x = |> inc |> toString it is a lot less intuitively clear that x is a function.

This is one of these questions where I'd like to see more usage to find out whether this is a thing that comes up sufficiently often in practice that it is worth dealing with.

(Side note: I think we would get this feature for free if Nix had operator sections)

Lambdas inside pipe

While I agree that not requiring parentheses here would be nice, especially given that one goal of the pipe operator is to reduce parentheses, I don't see any way this would be realistically implementable without throwing the entire language under the bus. Making the binding strength of abstractions context dependent just doesn't sound great

Making the binding strength of abstractions context dependent just doesn't sound great

agreed, it’s a recipe for disaster

Lambdas inside pipe

Making the binding strength of abstractions context dependent

Right. I didn't think that through. This is a bad idea.

Partially applied pipes

In x = |> inc |> toString it is a lot less intuitively clear that x is a function

In my opinion it is slightly more clear than, for example,

concatLines = concatStringsSep "\n"
getBin = getOutput "bin"

or any other partially applied function. The open |> (or |) at the start suggests to me that this is a function.

Is it allowed to backport the pipe operator to the Nixpkgs minimum nix version?

Do major feature additions like this usually get a backport?

Even if it was backported, wouldn't it defeat the purpose of having a minimal version that's likely to be installed by most users?

A backport would still be a new release, even if it was based on an old major version number.

Sometimes. Support for zstd cache compression was backported to 2.3, so it's within the realms of possibility.

The reason we have a minimum version of 2.3 not so much to support people who haven't updated Nix in several years, it's because later versions of Nix have unresolved regressions in things that some people depend on.