Example for non-specializable interfaces

demo.zig

// some comments:
// this is not actual zig code (as it uses stuff not available in zig)
// it's an example used in a discussion about if "polymorphous interfaces" are required
// or if everything can be solved by specialization

const std = @import("std");

// interfaces are inherent "fat pointers"
// that store both vtable as well as a pointer to the object.
// also functions here omit the self field as the object type is not known at interface
// declaration time

/// this interface allows converting an object to or from a string value
const TextConvertible = interface {
    fn toString(a:*Allocator) error{OutOfMemory}![]u8;
    fn fromString(text : []u8) error{FormatError}!void;
};

/// Interface used in ui object
const Widget = interface { … };

/// A text box in the user interface that 
/// allows having a value binding
const TextBox = struct {
    value : ?TextConvertible, /// binding is refreshed when the user types in something
    textBuffer : [256]u8,
    …
};

const Point = struct {
    x : i32,
    y : i32,

    implements TextConvertible;
    
    // here the self argument is required
    fn toString(pt : Point, a:*Allocator) error{OutOfMemory}![]u8 {
      return std.format.string("x={}, y={}", pt.x, pt.y);
    }
    
    fn fromString(pt : Point, text : []u8) error{FormatError}!void {
      std.format.parse("x={}, y={}", text, &pt.x, &pt.y) catch rerturn error.FormatError;
    }
};

pub fn main() !void {
    var ui = …;

    var coords = Point { .x = 0, .y = 0 };

    var coordinateBox = TextBox.create(TextConvertible(&upperCoords)); // casting a pointer to the interface creates a fat pointer
    var nameBox = TextBox.create(TextConvertible(&lowerCoords));

    ui.appendWidget(Widget(&coordinateBox));
    ui.appendWidget(Widget(&nameBox));
}