Adding support for custom error messages on guard functions

[joaomdmoura]

Relates to issue #34

This commit will add the ability to declare a custom error
message for guard functions and properly propagate it.

Guard conditions will allow the transition if it returns anything other than a tuple with {:error, "cause"}:

• {:error, "cause"}: Transition won't be allowed.
• _ (anything else): Guard clause will allow the transition.

Example:

defmodule YourProject.UserStateMachine do
  use Machinery,
    states: ["created", "complete"],
    transitions: %{"created" => "complete"}

  # Guard the transition to the "complete" state.
  def guard_transition(struct, "complete") do
    if Map.get(struct, :missing_fields) == true do
      {:error, "There are missing fields"}
    end
  end
end

When trying to transition an struct that is blocked by its guard clause you will
have the following return:

blocked_struct = %TestStruct{state: "created", missing_fields: true}
Machinery.transition_to(blocked_struct, TestStateMachineWithGuard, "completed")

# {:error, "There are missing fields"}

[wfgilman]

This looks great! Can the custom error be any data type or must it be a string?

[joaomdmoura]

@wfgilman 🤔 yup, but keep in mind this will also be used by the dashboard, so in order to the dashboard work properly it should probably be a string or a number tops, but the library itself will work just fine with any type.

[wfgilman]

Great. I’m using it outside of the dashboard. Thanks for the quick add!

…

On Tue, May 1, 2018 at 8:38 AM João Moura ***@***.***> wrote: @wfgilman <https://github.com/wfgilman> 🤔 yup, but keep in mind this will also be used by the dashboard, so in order to the dashboard work properly it should probably be a string or a number tops, but the library itself will work just fine with any type. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#35 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ASZ7OMPtphpmYD7u32UeWzJmRqavc2xPks5tuIFmgaJpZM4Tt_uF> .