Evaluate left argument before right

[littledan]

Looking at the source of this module, it seems to transform x |> f directly into f(x). However, any standardized proposal will likely evaluate x first, before f.

[SuperPaintman]

So, I don't fully understand what you mean.

Example in a proposal

[littledan]

For example, if I have

let log = [];
function f() { log.push("f") }
function g() { log.push("g") }
f() |> g()

After that code runs, log should be ["f", "g"].

I don't see examples in the proposal, probably because it doesn't really make sense to write code this way.

[SuperPaintman]

Yeah, that's right. These ways are equivalent:

Calling in calling:

let log = [];
function f() { log.push("f") }
function g() { log.push("g") }

f() |> g

console.log(log) // => ["f", "g"]

Pipeline:

let log = [];
function f() { log.push("f") }
function g() { log.push("g") }

g(f())

console.log(log) // => ["f", "g"]

And, as I understand it, when we use a pipeline, we say:

f()  // Get result of this function
|>   // and put it as a first parameter in to
g    // that function, and call it.

Or did I misunderstand you?

[littledan]

Sorry, there was a typo in my example, and it seems like you fixed it in a direction that's different from what I was going for. It should be:

function g() { log.push("g"); return () => {} }

Then, the code f() |> g() is related to the code g()(f()).

But! They differ in the evaluation order of f() vs g(). In the pipeline example, f() runs first. But in the normal function call code, g() goes first.

This can be fixed by desugaring into something like this:

let __arg = f();
let __fn = g();
__fn(__arg)

[SuperPaintman]

Oh, I understood what you mean, but I didn't find an opinion on this case in proposal.
I think, you'd better ask this question in proposal repo: https://github.com/tc39/proposal-pipeline-operator
Please let me know later what they think about this.

Or my implementation is wrong?

[littledan]

Well, that proposal doesn't have specific enough spec text to say things one way or another.

I'd be really surprised if it went any other way than left-to-right (and I think it would be shot down in committee if so), but it would be fine with me if you feel like waiting for more solid confirmation first.

[littledan]

I made a spec text proposal at tc39/proposal-pipeline-operator#51 which evaluates the left argument before the right one. I'd be interested in any feedback you have.