You can define implementations for one or more of the following types:
Any | Atom | BitString | Float | Function |
Integer | List | Map | PID | Port |
Record | Reference | Tuple |
The type BitString is used in place of Binary.
The type Any is a catchall, allowing you to match an implementation with any type. Just as with function definitions, you’ll want to put the implementations for specific types before an implementation for Any.
You can list multiple types on a single defimpl. For example, the following protocol can be called to determine whether a type is a collection:
| | defprotocol Collection do |
| | @fallback_to_any true |
| | def is_collection?(value) |
| | end |
| | |
| | defimpl Collection, for: [List, Tuple, BitString, Map] do |
| | def is_collection?(_), do: true |
| | end |
| | |
| | defimpl Collection, for: Any do |
| | def is_collection?(_), do: false |
| | end |
| | |
| | Enum.each [ 1, 1.0, [1,2], {1,2}, %{}, "cat" ], fn value -> |
| | IO.puts "#{inspect value}: #{Collection.is_collection?(value)}" |
| | end |
We write defimpl stanzas for the collection types: List, Tuple, BitString, and Map. But what about the other types? To handle those, we use the special type Any in a second defimpl. If we use Any, though, we also have to add an annotation to the protocol definition. That’s what the @fallback_to_any line does.
This produces
| | 1: false |
| | 1.0: false |
| | [1,2]: true |
| | {1,2}: true |
| | %{}: true |
| | "cat": true |