Helpful compile time errors

compile_time_errors.cr

class Response
end

abstract class BaseAction
  abstract def call : Response
  
  def redirect(location : String, permanent : Bool = false)
    # do stuff
  end
  
  compile_error redirect(_location, status : Int32)
    <<-ERROR
    The status defaults to 301, but you can pass `permanent: true` for a 302.
    ERROR
  end

  # No catch all is defined so it just uses the default compile error if no `redirect` method matches,
  # and the status is not an Int32
  
  # If one of the return types is Nil
  compile_error call : Nil
    <<-ERROR
    You must always return a Response from call, but it currently returns a `nil` value. This often happens because you 
    have a branch that doesn not have an `else`, or you forgot to call `render` or `redirect`
    ERROR
  end

  # Define a custom error if `call` returns something other than 
  # Response or Nil (because Nil would be caught by the more specific compile_error)
  compile_error call
    <<-ERROR
    You must always return a Response from call. Remember to call `render` or `redirect` at the end of your action.
    ERROR
  end
end

class Users::Index < BaseAction
  def call
    redirect("/somewhere", 302)
    if some_condition?
      Response.new
    end
  end
end