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lib/once.ex
defmodule Once do
@moduledoc """
This module provides a mechanism to perform actions only once, using a unique
key provided by the caller to determine whether the action has already been
performed this session.
"""
use Agent
@doc """
Starts the agent that keeps track of seen keys.
"""
def start_link(_opts) do
Agent.start_link(fn -> Map.new() end, name: __MODULE__)
end
@doc """
Checks if a key has been seen before. If the key has not been seen, it
returns `{:error, :not_seen}`. If the key has been seen, it returns `{:ok,
value}` where `value` is the value associated with the key, or `true` if no
value was specified.
"""
def get(key) do
Agent.get(__MODULE__, fn seen ->
with {:ok, value} <- Map.fetch(seen, key) do
{:ok, value}
else
_ -> {:error, :not_seen}
end
end)
end
@doc """
Marks a key as seen. If the key has not been seen before, it returns `true`
and updates the internal state. If the key has already been seen, it returns
`false` without updating the state.
"""
def set(key, value \\ true) do
Agent.get_and_update(__MODULE__, fn seen ->
if Map.has_key?(seen, key) do
{false, seen}
else
{true, Map.put(seen, key, value)}
end
end)
end
@doc """
Emits a warning (using `UI.warn/1`) if the message has not yet been emitted
during this session.
"""
def warn(msg) do
if set(msg) do
UI.warn(msg)
end
:ok
end
end