Nothing¶

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description¶

Nothing

is a symbol that is automatically removed from any list in which it appears as an element: {a, Nothing, b} evaluates to {a, b}. It is the identity element for list construction, so Table[If[test, val, Nothing], ...] builds a list of just the values for which test held. Any Nothing[...] form is removed likewise. Non-list heads treat Nothing as an ordinary symbol.

Examples (5)¶

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (2)¶

In[1]:= {1, Nothing, 2}
Out[1]= {1, 2}

In[2]:= Table[If[EvenQ[i], i, Nothing], {i, 6}]
Out[2]= {2, 4, 6}

Applications (3)¶

Every Nothing simply disappears

In[3]:= {a, Nothing, b, Nothing, c}
Out[3]= {a, b, c}

A Nothing[...] form is removed too

In[4]:= {1, 2, Nothing[x, y], 3}
Out[4]= {1, 2, 3}

Keep only the elements that pass a test

In[5]:= Table[If[PrimeQ[i], i, Nothing], {i, 10}]
Out[5]= {2, 3, 5, 7}

Implementation notes¶

Algorithm. Nothing is not a builtin function but a special symbol handled by the evaluator: it is the identity element for list construction and vanishes from any List it appears in. strip_nothing (src/eval.c), called during evaluate_step, scans a List's arguments and removes every element that is either the symbol Nothing or any Nothing[...] form (an EXPR_FUNCTION whose head is Nothing). It makes one counting pass to find how many survive, and only if some element is a Nothing does it rebuild the argument array — freeing the removed nodes, keeping the rest, and invalidating the expression's cached hash.

List-specific. The rewrite fires only for the head List; for any other head Nothing is an ordinary symbol. This is what makes Table[If[test, val, Nothing], ...] the idiom for conditionally building a list: the iterator produces a plain List whose unwanted slots hold Nothing, and the evaluator strips them on the next step.

Data structures / limits. O(n) over the list's arguments, one reallocation when a Nothing is present (none otherwise). Nothing carries ATTR_PROTECTED (src/attr.c). No numeric surface — it is a structural identity element.

Attributes: Protected.

References¶

Source: src/eval.c
Specification: docs/spec/builtins/lists-and-iteration.md
Tests: tests/test_sequence.c

Notes & additional examples¶

Notes¶

Nothing is a symbol that is automatically removed from any list in which it appears. It is the identity element for list construction, which makes Table[If[test, val, Nothing], ...] the standard idiom for building a list of just the values for which a test holds — no Select or DeleteCases pass is needed. Any Nothing[...] form is stripped likewise.

The removal is list-specific: for a non-List head, Nothing is an ordinary symbol and is left in place.