Programming Elixir ≥ 1.6

Specs for Functions and Callbacks

The @spec specifies a function’s parameter count, types, and return-value type. It can appear anywhere in a module that defines the function, but by convention it sits immediately before the function definition, following any function documentation.

We’ve already seen the syntax:

@spec function_name( param1_type, …) :: return_type

Let’s see some examples. These come from the built-in Dict module.

@type key   :: any
@type value :: any
@type keys  :: [ key ]
@type t     :: tuple | list    # `t` is the type of the collection

@spec values(t) :: [value]              
@spec size(t) :: non_neg_integer        
@spec has_key?(t, key) :: boolean       
@spec update(t, key, value, (value -> value)) :: t       

Line 6

values takes ...