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How do I... Set default values?

In order to make your actions more flexible and reusable, you probably will want to provide default values in some cases. This isn't difficult, but you have a few options on how to do it:

• function parameter defaults (optionally combined with pattern matching)
• using Map.get(map, :key, default_value) (or Keyword.get)
• using value_that_could_be_nil || fallback_value

Here is a fairly advanced example using of these options to make an action that can be used in many different ways:

@doc """
Reply to a feed item.  Can be called in different ways:

1) `reply(context, feed_item_id, "reply text")` where `feed_item_id` is a string to
   reply to a specific feed item

2) `reply(context, feed_type, "reply text") where `feed_type` is one of `:post`,
   `:asset`, or `:reply` to reply to a randomly chosen feed item of the specified
   type

3) `reply(context, "reply text")` to reply to a randomly chosen feed item of a
   randomly chosen feed type

You can also leave off the reply text in any version to use randomly generated text.

In addition, you can supply a 4th paramater as a string to use as the reply name,
otherwise `context.username` will be used.
"""

def reply(
      _context,
      type \\ Enum.random([:post, :asset, :reply]),
      text \\ random_text(),
      reply_name_override \\ nil
    )

def reply(context, feed_item_id, text, reply_name_override)
    when is_binary(feed_item_id) do
  reply_name = reply_name_override || Map.get(context, :username, "no name")
  # ...
end

def reply(context, :asset, text, reply_name_override) do
  # special handling for replying to assets
  # ...
end

def reply(context, other_type, text, reply_name_override)
    when other_type in [:post, :reply] do
  # ...
end

There's a lot going on here, so let's unpack it.

First, if you define a function that has default values and multiple clauses, you need to make a "function header" which specifies the defaults but has no body. You will get a descriptive error if you forget. If you don't have multiple clauses, you can put your default values in your normal function definition. In this case, the first def line is the header, specifying default values.

This reply function always has an arity of 4, even if you call it with less arguments. context is the only required parameter. The default values will be applied if you don't specify those parameters yourself.

A common trick is to make a parameter optional by providing a default parameter of nil. We do this for reply_name_override, then use || to provide a fallback. In this case the fallback has a fallback (with Map.get), since we don't know if username exists in the context.

For type and text we define default values in the function definition. Note that the code after the \\ only runs if an argument for that parameter wasn't provided, so the various random functions will run with new random values each time this action is called. Using a call-time function like this is a powerful trick.

Finally, the various pattern matching variations of the function define how to respond in each case.

Putting it all together makes an action that is very flexible and reusable.