sentinel

Module nu.core.sentinel.

Sentinel atoms: the queries that observe EMPTY.

The one core family that is not a Python builtin: IsEmpty / NotEmpty ask whether a value IS EMPTY, and Fallback picks the first value that is not. Every other query propagates an EMPTY operand; these observe it, so they are the only core atoms that do not guard - the compile thunk reads the raw child value with no EMPTY short-circuit.

They live in core because they are reused everywhere (the Form base exposes them as is_empty() / not_empty() / fallback(), flows branch on them, callers guard on them). Sort: all ScalarQuery (Q).

Fallback

The first of its children that is not EMPTY.

Fallback(value, *alternatives)

Path nu.core.Fallback. Kind ScalarQuery, sort scalar_query, cardinality scalar. Arity None (1 required).

Arguments

Yields

The first present child. EMPTY only when every child is EMPTY.

Notes

• Checks presence, not truthiness: 0, "", False and None are present values and are kept.
• Short-circuits: children after the first present one are never evaluated.
• Reached as x.fallback(a, b, ...) on every Form, which also keeps the result in x's Form.

Example

from nu.lang.sentinels import EMPTY
nu.run(nu.Fallback(EMPTY, 0, 5))[0]
nu.run(nu.Fallback(EMPTY, EMPTY))[0]

0
<EMPTY>

IsEmpty

True if its one child yields the EMPTY sentinel.

IsEmpty(value)

Path nu.core.IsEmpty. Kind ScalarQuery, sort scalar_query, cardinality scalar. Arity 1 (1 required).

Arguments

Yields

A plain bool, never EMPTY.

Notes

• Accepts EMPTY rather than propagating it: this is one of the few core atoms that does not guard, since observing EMPTY is the whole point.

Example

from nu.lang.sentinels import EMPTY
nu.run(nu.IsEmpty(EMPTY))[0]
nu.run(nu.IsEmpty(5))[0]

True
False

NotEmpty

True if its one child does not yield EMPTY.

NotEmpty(value)

Path nu.core.NotEmpty. Kind ScalarQuery, sort scalar_query, cardinality scalar. Arity 1 (1 required).

Arguments

Yields

A plain bool, never EMPTY.

Notes

• Accepts EMPTY rather than propagating it, same as IsEmpty.

Example

from nu.lang.sentinels import EMPTY
nu.run(nu.NotEmpty(EMPTY))[0]
nu.run(nu.NotEmpty(5))[0]

False
True