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Muack as a development runtime static typing system

Muack as a development runtime static typing system

Ever consider a static type system in Ruby? You could actually see a lot of
asserts inserted in the beginning of some methods in some libraries. For
example, there are assert_valid_key_size, assert_kind_of, etc, in
dm-core, and assert_valid_keys, assert_valid_transaction_action,
and various random asserts in activerecord.

You could find them by searching against raise ArgumentError because
rails is much less consistent and sometimes it's hard to find a pattern in
rails. But you get the idea, those ArgumentError would much help us debug
our code from misusing the API, and that's exactly the point of type system,
or more specifically, static type system.

We could also use some static analysis tools to do something like this, for
example, there's ruby-lint. However, as you might already know, since
Ruby is so dynamic, static analysis tools cannot really do a great job if
our code is quite dynamic. Of course we could write it more statically,
and treat our static analysis tools better, but that might not be the spirit
of Ruby somehow.

Alternatively, it would be great to do this static type checking
dynamically... I mean, in the runtime rather than compile time. This
means it would be much more accurate, just like those asserts in the
above examples.

However, if we're doing those checks in a hot path, for example, right
inside a loop looping over a million times, this would definitely slow
things down if we're checking them in the runtime. Even if we put $DEBUG
guards around those check, we're still suffering from checking the flag.

It would be great if we could actually just remove those checks in
production, while turn it on when we're developing or debugging.
Muack could actually fulfill this desire, as it could inject codes
externally and seamlessly, and we could remove them anytime when we
call Muack.reset, or, simply don't do any stubs in production config.

Consider we have two classes:

Food = Class.new
User = Class.new{ attr_accessor :food }

And we could make sure User#food is always a kind of Food by putting this
into a development config or so:

Muack::API.module_eval do
  any_instance_of(User) do |user|
    stub(user).food = is_a(Food)

And then if we're trying to set a food other than a Food...

u, f = User.new, Food.new
u.food = f # ok
u.food = 1 # raise Muack::Unexpected

This could go wild and we could customize our own domain specific argument
verifiers. For example, we could do this to check if the food is frozen:

Food = Class.new
User = Class.new{ attr_accessor :food }

FoodFrozen = Class.new(Muack::Satisfy) do
  def initialize
    super lambda{ |actual_arg| actual_arg.frozen? }

Muack::API.module_eval do
  any_instance_of(User) do |user|
    stub(user).food = FoodFrozen.new

u = User.new
p u.food = Food.new.freeze # ok
p u.food = Food.new        # raise Muack::Unexpected

Please check Arguments Verifiers (Satisfy) section for more argument
verifiers details.

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