Identity¶

Status: Stable

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

Description¶

Identity[expr]

gives expr unchanged (the identity function).

Examples (6)¶

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

Basic examples (3)¶

In[1]:= Identity[x]
Out[1]= x

In[2]:= Identity[1 + 2]
Out[2]= 3

In[3]:= Map[Identity, {a, b, c}]
Out[3]= {a, b, c}

Applications (3)¶

Returns its argument unchanged

In[4]:= Identity[x]
Out[4]= x

Its argument is evaluated normally first

In[5]:= Identity[1 + 1]
Out[5]= 2

The identity function is handy as a default callback

In[6]:= Map[Identity, {1, 2, 3}]
Out[6]= {1, 2, 3}

Implementation notes¶

builtin_identity (src/core.c) is the one-argument identity: it returns a copy of its single argument unchanged, or NULL for any other arity.

Attributes: Protected.

References¶

Source: src/core.c
Specification: docs/spec/builtins/expression-information.md
Tests: tests/test_eval.c
Tests: tests/test_repl_hooks.c
Tests: tests/test_sequence.c

Notes & additional examples¶

Notes¶

Identity[expr] returns expr unchanged. It takes exactly one argument; any other arity is left unevaluated. Having no held attributes, it evaluates its argument through the normal pipeline before returning it, so Identity[1 + 1] is 2.

It is most useful as a neutral function argument — a do-nothing callback where some transformation is expected.