View Source SimpleAgent (simple_agent v0.0.8)

SimpleAgent is a simplification/abstraction layer around the base Elixir Agent module.

Often times, Agents are used to store a simple value, such as an atom or an integer. This is used as a flag or a count which multiple processes can access/update. In these cases, the full Agent module is used with boilerplate closure code that is repetative, adds noise to the code, and can be eliminated. For example, to create an agent, update the value, then retrieve that value, you would run:

{:ok, agent} = Agent.start_link(fn -> nil end)
Agent.update(agent, fn _ -> :final_value end)
final_value = Agent.get(agent, fn val -> val end)

SimpleAgent boils these calls down to a more readable:

agent = SimpleAgent.start!
SimpleAgent.update! agent, :final_value
completed = SimpleAgent.get! agent

For Integer manipulation, SimpleAgent takes this code:

{:ok, agent} = Agent.start_link(fn -> 0 end)
IO.puts Agent.get_and_update(fn val -> {val + 1, val + 1} end)
IO.puts Agent.get_and_update(fn val -> {val - 1, val - 1} end)
IO.puts Agent.get_and_update(fn val -> {val + 1, val + 1} end)
IO.puts Agent.get_and_update(fn val -> {val + 1, val + 1} end)

and boils it down to the more readable:

agent = SimpleAgent.start! 0
IO.puts SimpleAgent.increment! agent
IO.puts SimpleAgent.decrement! agent
IO.puts SimpleAgent.increment! agent
IO.puts SimpleAgent.increment! agent

SimpleAgent is very useful in testing. For example:

test "foo calls bar 3 times" do
  bar_call_agent = SimpleAgent.start! 0
  :meck.new(Bar)
  :meck.expect(Bar, :bar, fn -> SimpleAgent.increment!(bar_call_agent) end)
  Foo.foo()
  assert SimpleAgent.get?(bar_call_agent) == 3
end

Why only simple types?

When a complex state such as a map or a dict is in use, the correct way to manipulate the complex state is in the Agent server via a closure. This prevents the entire state from being copied from the Agent Server to the Client (see the Agent docs for more information on this). For states with these complex types, you should use the full Agent module. SimpleAgent is for those cases where the "entire state" is a single simple Integer, String, or Atom (including nil, true, and false).

Features:

  • Simple types and updates reduce chances of errors, so all calls raise exceptions instead of requiring boilerplate pattern matching, and start!/2 is available instead of start_link
  • No closures are required.
    • get!/1 uses &(&1)
    • update!/2 takes the value instead of a function and uses fn _ -> value end
  • nil support
    • start!/2 defaults the initial value to nil when not specified
    • nil?/1 checks for the nil state
    • clear/1 sets the nil state
  • increment!/2 and decrement!/2 allow for simple manipulation of integer states.

Summary

Functions

Resets the current state to nil

Decreases the value of the current state by count (default 1). Raises error if current state is not an integer

Returns true or false if the current state of the specified agent is the specified value

Returns the current state of the agent. If the agent has an invalid type, raises an exception

Increases the value of the current state by count (default 1). Raises error if current state is not an integer

Returns true or false if the current state is nil

Starts an agent with the specified initial value, or nil by default. Second optional parameter is the standard GenServer options list.

Updates the state to the new value. Returns the new value.

Types

@type agent() :: Agent.agent()
@type valid_types() :: atom() | integer() | String.t()

Functions

@spec clear(agent()) :: :ok

Resets the current state to nil

Link to this function

decrement!(agent, count \\ 1)

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@spec decrement!(agent(), integer()) :: integer()

Decreases the value of the current state by count (default 1). Raises error if current state is not an integer

@spec equals?(agent(), valid_types()) :: boolean()

Returns true or false if the current state of the specified agent is the specified value

@spec get!(agent()) :: valid_types()

Returns the current state of the agent. If the agent has an invalid type, raises an exception

Link to this function

increment!(agent, count \\ 1)

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@spec increment!(agent(), integer()) :: integer()

Increases the value of the current state by count (default 1). Raises error if current state is not an integer

@spec nil?(agent()) :: boolean()

Returns true or false if the current state is nil

Link to this function

start!(initial_state \\ nil, options \\ [])

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@spec start!(valid_types(), GenServer.options()) :: pid()

Starts an agent with the specified initial value, or nil by default. Second optional parameter is the standard GenServer options list.

Return values

Returns the pid of the server to be used in subsequent calls to other SimpleAgent functions.

@spec update!(agent(), valid_types()) :: valid_types()

Updates the state to the new value. Returns the new value.