RosettaCodeData/Task/Add-a-variable-to-a-class-instance-at-runtime/Morfa/add-a-variable-to-a-class-instance-at-runtime.morfa

import morfa.base;

template <T>
public struct Dynamic
{
    var data: Dict<text, T>;
}

// convenience to create new Dynamic instances
template <T>
public property dynamic(): Dynamic<T>
{
    return Dynamic<T>(new Dict<text,T>());
}

// introduce replacement operator for . - a quoting ` operator
public operator ` { kind = infix, precedence = max, associativity = left, quoting = right }
template <T>
public func `(d: Dynamic<T>, name: text): DynamicElementAccess<T>
{
    return DynamicElementAccess<T>(d, name);
}

// to allow implicit cast from the wrapped instance of T (on access)
template <T>
public func convert(dea: DynamicElementAccess<T>): T
{
    return dea.holder.data[dea.name];
}

// cannot overload assignment - introduce special assignment operator
public operator <- { kind = infix, precedence = assign }
template <T>
public func <-(access: DynamicElementAccess<T>, newEl: T): void
{
    access.holder.data[access.name] = newEl;
}

func main(): void
{
    var test = dynamic<int>;

    test`a <- 10;
    test`b <- 20;
    test`a <- 30;

    println(test`a, test`b);
}

// private helper structure
template <T>
struct DynamicElementAccess
{
    var holder: Dynamic<T>;
    var name: text;

    import morfa.io.format.Formatter;
    public func format(formatt: text, formatter: Formatter): text
    {
        return getFormatFunction(holder.data[name])(formatt, formatter);
    }
}