Object deconstruct

[machaval]

Object decomposition

It is very common to return objects and then trying to extract its parts.

For example the function splitAt of dw::core::Arrays returns a Pair that is basically an object with l and r properties So currently what we need to do is

var splits = splitAt([1,2,3], 2)
var left = splits.l
var right = splits.r
---
left[0] ++ right[0]

I think this verbose with no reason and that many modern language already solve this.

Proposal

Binding top level fields

We just allow to bind any part of the result.

var {l: left, r: right} = splitAt([1,2,3], 2)
---
left[0] ++ right[0]

Also not the entire object needs to be bind. For example if we want just to bind the r we can just

var {r: right} = splitAt([1,2,3], 2)
---
right[0]

Binding inner fields

var {error: {kind: kind}} = try(splitAt([1,2,3], 2))

This will bind the error.kind field

[TomoDC]

I'm not sure about this because:

• In the script without decomposition, the bind is explicit, it is easy to understand what is happening. That is not the case of the script that uses decomposition. Maybe it adds complexity in understanding the code.
• Maybe in the case that there is a lot of bindings, the decomposition allows you to do that with less amount of code.

I'm not sure if its usefulness is greater than the complexity that adds to read the script.

[jerneyio]

I'm also concerned about the usefulness relative to the readability. Something I see common in other languages is the ability to assign multiple variables with one statement. It'd be convenient to have something like this supported:

var splits = splitAt([1,2,3], 2)
var left, right = splits.l, splits.r
---
left[0] ++ right[0]

It makes the decomposition of objects less annoying, but has a syntax that more programmers would already be familiar with.

For what it's worth, clojure has a similar syntax, they call it "destructuring" - https://clojure.org/guides/destructuring

[machaval]

• C# has the same thing

https://docs.microsoft.com/en-us/dotnet/csharp/deconstruct#deconstructing-a-tuple

• Rust also has it

https://users.rust-lang.org/t/tuples-destructuring-assignment/11028

• Scala has it too

https://docs.scala-lang.org/tour/tuples.html

They all have it for tuples but as we don't have tuples (because I still think we don't need them) I thought we can make it for objects

[menduz]

I'm going to show some examples mixed with arrays/tuples but those are the same for DW.

C++ also have it in this shape

• https://en.cppreference.com/w/cpp/language/structured_binding

  auto [x,y,z] = vectorTouple;

Which is the same as JS/TS

• https://www.typescriptlang.org/docs/handbook/variable-declarations.html#destructuring
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment

let [x,y,z] = [1,2,3]

And JS/TS has the object destructuring in its simpler form

let {x,y,z} = vector

And with renames

let {x: horizontal,y: vertical,z: depth} = vector

// you can use horizontal now

Do not forget the rest operation:

let {x, ...resto} = vector3
// resto = {y,z}

Also for arrays:

let [first, ...resto] = [1,2,3,4]
// resto = [2,3,4]

Which in dataweave is the ~

Go, python and the WASM and the LLVM have multiple return values

func x() {
        return 1,2,3,4
}

a,b,c,d = x()