Describes an argument.

When declaring a command, the :arguments entry is a keyword list that accepts these settings, all optional:

• :required - A boolean. Arguments are required by default.
• :type - :string, :integer or :float. Defaults to :string.
• :doc - A string to document what the argument is for. This is not used at the moment but future releases will exploit it.
• :cast - A fun accepting the value, or a {module, function, arguments}, returning a result tuple. See the "Casting" section below for more information.
• :repeat - A boolean, defines a variadic argument that accepts several values. This can only be enabled on the last argument.

Casting

When the :cast option is a fun, it will be called with the deserialized value. When it is an "MFA" tuple, the deserialized value will be prepended to the given arguments.

Cast functions input

What is a deserialized value though? By default it will be the raw string passed in the shell as command line arguments. But if the :type option is also defined to :integer or :float, your cast function will be called with the corresponding type. If that type deserialization fails, the cast function will not be called.

Returning errors

Cast functions must return {:ok, value} or {:error, reason}. The reason can be anything, but at the moment CliMate doesn't do any special formatting on it to display errors (besides calling to_string/1 with a safe fallback to inspect/1). It is advised to return a meaningful error message as the reason.

Compilation commands

When commands are declared as a module attribute, you cannot use local functions definitions or the compilation will fail with "undefined function ...". You need to use a remote form by prefixing by __MODULE__, or use an MFA tuple.

defmodule MyCommand do
  use Mix.Task

  @command name: "my command",
           arguments: [
             some_arg: [
               cast: &__MODULE__.cast_some/1
             ]
           ]
end