Close… when a: var, or t

Does anyone do this or similar.
Here I am using a spec’d identity function to “cast” a binary to an opaque type which will instruct dialyzer to warn when I try to use it in normal strip operation.

defmodule MyID do
  @opaque t(x) :: x

  @spec id(binary) :: t(binary)
  def id(x) do
    x
  end

  def run do
    id = id("alice")
    String.upcase(id)
  end
end

Defining a specification

Type variables with no restriction can also be defined.

That reads as if it is simply unconstrained rather than unified.

In my experience dialyzer wants specifics, e.g.

  @type my_t(t) :: list(t)
  @type my_int_t :: my_t(integer())
  @spec id(t) :: t when t: my_int_t
  @spec id(t) :: t when t: integer()
  @spec id(atom()) :: atom()
  def id(x) do
    x
  end

  @spec run() :: r when r: atom()
  def run() do
    id(:foo)
  end

  @spec run2() :: r when r: integer()
  def run2() do
    id(10)
  end

  @spec run3() :: r when r: my_int_t
  def run3() do
    id([10])
  end