Blank¶

Status: Stable

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

Description¶

_ or Blank[] represents any single expression. _h or Blank[h] represents any single expression with head h.

Examples (3)¶

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

Applications (3)¶

In[1]:= MatchQ[5, _]
Out[1]= True

In[2]:= MatchQ[g[a], _g]
Out[2]= True

In[3]:= FullForm[x_]
Out[3]= Pattern[x, Blank[]]

Implementation notes¶

Blank is a pattern object head, not an evaluator builtin: _ parses to Blank[] and _h to Blank[h], and x_ to Pattern[x, Blank[...]]. The matcher recognises it in src/match.c via is_blank, which reports whether a pattern node is Blank[] (matches any single expression — head_out = NULL) or Blank[h] (constrains the head). When a head is present, match_internal checks it with get_expr_head_borrowed + expr_eq, with a special case mapping atomic expression types to their symbolic heads (Integer, Real, Symbol, String). A successful single match consumes exactly one argument and continues via call_parent. Blank and its sequence relatives are also among the pattern heads the matcher treats structurally (e.g. excluded from OneIdentity/literal-head handling).

Attributes: none registered.

References¶

Source: src/match.c
Specification: docs/spec/builtins/pattern-matching.md
Tests: tests/test_assoc_forms.c
Tests: tests/test_backtrack.c
Tests: tests/test_compile.c
Tests: tests/test_eval.c

Notes & additional examples¶

Notes¶

_ (Blank[]) matches any single expression; _h (Blank[h]) matches any single expression whose head is h (Out[2]). A named blank x_ is Pattern[x, Blank[]] (Out[3]), binding the match to x.