lua4swift: General type checker for custom Lua Userdata types

CustomLuaBinding.swift

import Lua

protocol CustomLuaBinding: CustomTypeInstance {

    /// This is a 'type checker' function.
    /// It is used to validate expected types when binding a method to a Lua-wrapped type.
    ///
    /// e.g.
    /// ```
    /// type.createMethod([Foo.arg, Bar.arg, Baz.arg], _ fn: ....)
    /// ```
    /// ...where `Foo`, `Bar`, and `Baz` adopt `CustomLuaBinding`
    ///
    /// - Note: This probably never needs to be explicitly implemented;
    /// the generic solution provided in the protocol extension should suffice all possible custom types.
    /// - Returns: `nil` if the value matches this type; or the name of this class for informing an unexpected type.
    static func arg(_ vm: VirtualMachine, value: Value) -> String?
    
    @discardableResult
    static func addType(to vm: VirtualMachine) -> CustomType<Self>
}

extension CustomLuaBinding {
    static func arg(_ vm: VirtualMachine, value: Value) -> String? {
        guard value.kind() == .userdata, let userdata = value as? Userdata else { return luaTypeName() }
        let instance: Self = userdata.toCustomType()
        return type(of: instance) == self ? nil : luaTypeName()
    }
}