The result of evaluating a CEL expression whose outcome depends on inputs that are not known, in the spirit of cel-go's types.Unknown.

An unknown carries the set of entries it depends on. Each entry pairs an attribute with the opaque reason the caller supplied:

• {:var, name} — a top-level variable bound to {:unknown, reason}.
• {:call, function, expr_id} — a custom function call that returned {:unknown, reason}.

Bind a variable as unknown by passing {:unknown, reason} (or an existing CEL.Unknown, to forward one result into another evaluation):

{:unknown, unknown} =
  CEL.eval("x < 30 && y > 1", %{x: {:unknown, :timeout}, y: {:unknown, :missing}}, env)

CEL.Unknown.entries(unknown)
#=> [{{:var, "x"}, :timeout}, {{:var, "y"}, :missing}]

Operators that can decide their result without an input do so: false && x is false and true || x is true. Every other operation that reads an unknown returns an unknown, merging the entries of all unknown operands. When an operand of a strict operation is an error, the error wins, because the outcome no longer depends on the unknown inputs. In &&, || and comprehensions an unknown wins over an error, because a known input could still decide the result.