[BUG] Confusing behavior with structs and closures about mojo

[abduld]

The basic idea is that your set_x and set_y need to return a new instance of the class, since you are chaining the method calls. Try

struct Line:
    var x: fn (F32) -> F32
    var y: fn (F32) -> F32

    fn __init__(inout self):
        self.x = constant
        self.y = constant

    fn __init__(inout self, x: fn (F32) -> F32, y: fn (F32) -> F32):
        self.x = x
        self.y = y

    fn set_x(self, x: fn (F32) -> F32) -> Self:
        return Self(x, self.y)

    fn set_y(self, y: fn (F32) -> F32) -> Self:
        return Self(self.x, y)

    fn evaluate(self, points: DynamicVector[Vec2]) -> DynamicVector[Vec2]:
        var out = DynamicVector[Vec2]()
        for i in range(points.__len__()):
            let p = points[i]
            let x = p[0]
            let y = p[1]
            out.push_back(Vec2(self.x(x), self.y(y)))
        return out

which should allow you to write

    let e = Line().set_x(my_x).set_y(constant).evaluate(points)

[abduld]

This is an interesting and useful pattern. We will make sure we have an example of how to do this in Mojo ASAP

[lsh]

This also brings up the question: how are closures stored on a struct? In Rust, one would likely Box the closure. Would this allocate in Mojo to store the closure?

[lsh]

Thanks for looking into it! That makes sense. I wonder if there could be a self^ which moves self into a method

[nmsmith]

Why not allow r-values to be mutated in this way? Is it not possible to come up with a semantics for this?

Oh wait, I see: the original definition of set_x isn't returning anything.

I don't understand why set_x can't be written as follows, though:

fn set_x(inout self, x: fn (F32) -> F32) -> Self:
    self.x = x
    return self

[sa-]

The basic idea is that your set_x and set_y need to return a new instance

This is a lot like scala's copy function on a case class which creates a shallow copy.

See "Copying" for more details here: https://docs.scala-lang.org/tour/case-classes.html

[lsh]

I was trying to trigger moves rather than copies in this case, but I'm not sure that's possible right now

[Mogball]

@lsh there's a typo in your code

    fn set_y(self&, y: fn(F32) -> F32):
        self.x = y

[Mogball]

Closing this for now.