Skip to content

OneIdentity

Status: Experimental

present and registered, but lightly documented and not yet covered by dedicated tests.

Description

OneIdentity is an attribute that can be assigned to a symbol f to indicate that f[x], f[f[x]], etc. are all equivalent to x for the purpose of pattern matching.

Examples

No verified examples yet for this function.

Implementation notes

OneIdentity is an attribute marker symbol, not a function. attr.c maps the name to the ATTR_ONEIDENTITY bitflag. The bit is consumed by the pattern matcher: in match.c, when matching a single argument against a sequence/optional pattern under a head carrying ATTR_ONEIDENTITY, f[x] is treated as equivalent to x (and an optional/defaulted argument lets x match f[x, default]). This is what lets a + b_. match a (with b bound to the identity 0) for Plus, or a x_. match a for Times. The matcher checks def->attributes & ATTR_ONEIDENTITY at the relevant decision points. purefunc.c maps SYM_OneIdentity → ATTR_ONEIDENTITY for pure-function attribute specs.

Attributes: none registered.

Implementation status

Experimental — present and registered, but lightly documented and not yet covered by dedicated tests.

References

Notes & additional examples

Worked examples

In[1]:= SetAttributes[r, OneIdentity]
Out[1]= Null

In[2]:= Attributes[r]
Out[2]= {OneIdentity}

In[3]:= MatchQ[a, r[a]]
Out[3]= True

Notes

OneIdentity makes r[x], r[r[x]], etc. equivalent to x for pattern matching, so a bare a matches the pattern r[a]. It does not change evaluation — only how patterns unify against the head.