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HoldPattern

Status: Stable

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

Description

HoldPattern[expr]

is equivalent to expr for pattern matching, but maintains expr in an unevaluated form.

Notes HoldPattern has attributes {HoldAll, Protected}. The left-hand sides of rules and assignments are normally evaluated before being used for matching; wrap the LHS in HoldPattern to stop that evaluation (e.g. HoldPattern\[\_+\_\] -\> 0 matches any two-term sum, whereas \_+\_ -\> 0 would match a pattern simplified by Plus before the rule is applied). HoldPattern is removed by one layer of ReleaseHold.

Examples (8)

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

Basic examples (4)

In[1]:= HoldPattern[_+_] -> 0
Out[1]= HoldPattern[_ + _] -> 0

In[2]:= a + b /. HoldPattern[_+_] -> 0
Out[2]= 0

In[3]:= Cases[{a -> b, c -> d}, HoldPattern[a -> _]]
Out[3]= {a -> b}

In[4]:= MatchQ[a + b, HoldPattern[_+_]]
Out[4]= True

Applications (4)

In[5]:= HoldPattern[1 + 1]
Out[5]= HoldPattern[1 + 1]

In[6]:= {a + b, c + d} /. HoldPattern[p_ + q_] -> p*q
Out[6]= {a b, c d}

In[7]:= Integrate[f[x], x] /. HoldPattern[Integrate[a_, b_]] -> done
Out[7]= done

In[8]:= ReleaseHold[HoldPattern[2 + 3]]
Out[8]= 5

Implementation notes

HoldPattern has no C handler; the attribute table in src/attr.c gives it ATTR_HOLDALL | ATTR_PROTECTED so its argument is not evaluated. In pattern matching it is transparent ā€” HoldPattern[p] matches exactly as p does, letting p contain otherwise-evaluating constructs on a rule's left-hand side. ReleaseHold strips it.

  • Attributes: {HoldAll, Protected}.
  • HoldPattern[p] is equivalent to p in the pattern matcher; the matcher transparently unwraps a single-argument HoldPattern before matching.
  • Useful on the left-hand side of rules and assignments, because those positions are normally evaluated before being used for matching. Wrapping in HoldPattern stops that evaluation and preserves the literal pattern shape.
  • HoldPattern is removed by one layer of ReleaseHold.

Attributes: HoldAll, Protected.

References

See also: ReleaseHold

Notes & additional examples

Notes

HoldPattern[expr] is equivalent to expr for matching purposes but keeps expr unevaluated, so the LHS of a rule or assignment is not simplified before it is used as a pattern. It has attributes {HoldAll, Protected} and is removed by one layer of ReleaseHold.