The :undefined sentinel: predicator's representation of an unbound or
missing value.
The atom stays the runtime representation - it is pervasive in tests and
the Ruby/JavaScript siblings, JSON-adjacent, and cheap. A struct sentinel
would break instruction interchange and every existing embedding for
aesthetic gain (the instruction list is the cross-language interchange
format, ADR-0001). This module owns it publicly instead: one place that
names the atom, checks for it, and normalizes it against nil at a
JSON-shaped boundary.
Summary
Functions
@spec from_nil(Predicator.Types.value() | nil) :: Predicator.Types.value()
Converts nil to the :undefined sentinel; any other value passes
through unchanged. The inverse of to_nil/1.
For normalizing an incoming nil (JSON null, a host application's own
"missing") to predicator's sentinel at the edge.
Examples
iex> Predicator.Undefined.from_nil(nil)
:undefined
iex> Predicator.Undefined.from_nil(42)
42@spec to_nil(Predicator.Types.value()) :: Predicator.Types.value() | nil
Converts the :undefined sentinel to nil; any other value passes
through unchanged.
For embedding at a boundary that speaks nil rather than :undefined
(JSON output, a host application's own representation of "missing").
Examples
iex> Predicator.Undefined.to_nil(:undefined)
nil
iex> Predicator.Undefined.to_nil(42)
42@spec undefined?(Predicator.Types.value() | nil) :: boolean()
Checks whether value is the :undefined sentinel.
Examples
iex> Predicator.Undefined.undefined?(:undefined)
true
iex> Predicator.Undefined.undefined?(nil)
false
iex> Predicator.Undefined.undefined?(42)
false@spec value() :: :undefined
The :undefined sentinel value.
Examples
iex> Predicator.Undefined.value()
:undefined