Can I create private enum constructors?

Question

In Haskell, I could do something like this (example adapted from Learn You A Haskell )

module Shapes (
    Shape,
    newCircle,
    newRectangle,
    ... -- other functions for manipulating the shapes
)

data Shape = Circle Int Int Float       -- x, y, radius
           | Rectangle Int Int Int Int  -- x1, y1, x2, y2

newCircle :: Float -> Shape
newCircle r = Circle 0 0 r

newRectangle :: Int -> Int -> Shape
newRectangle w h = Rectangle 0 0 w h

... -- other functions for manipulating the shapes

This would allow me to expose the types Shapeand newCircleand newRectangle.

Does Rust have an equivalent for this?

Answer 1

In a general sense, no; Rust has no private enum constructors. Enumerations are purely public things.

Structures, however, are not, and so you can combine them to make options purely implementation details:

// This type isn’t made public anywhere, so it’s hidden.
enum ShapeInner {
    // Oh, and let’s use struct variants ’cos they’re cool.
    Circle {
        x: i32,
        y: i32,
        radius: f64,
    },
    Rectangle {
        x1: i32,
        y1: i32,
        x2: i32,
        y2: i32,
    },
}

// Struct fields are private by default, so this is hidden.
pub struct Shape(ShapeInner);

impl Shape {
    pub fn new_circle(radius: f64) -> Shape {
        Shape(Circle { x: 0, y: 0, radius: radius })
    }

    pub fn new_rectangle(width: i32, height: i32) -> Shape {
        Shape(Rectangle { x1: 0, y1: 0, x2: width, y2: height })
    }

    // "match self.0 { Circle { .. } => …, … }", &c.
}

I would advise against this as a general practice.