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HoldPattern

HoldPattern(expr)

HoldPattern doesn’t evaluate expr for pattern-matching.

Examples

One might be very surprised that the following line evaluates to True!

>> MatchQ(And(x, y, z), Times(p__))
True

When the line above is evaluated Times(p__) evaluates to (p__) before Symja checks to see if the pattern matches. MatchQ then determines if And(x,y,z) matches the pattern (p__) and it does because And(x,y,z) is itself a sequence of one.

Now the next line also evaluates to True because both ( And(p__) ) and ( Times(p__) ) evaluate to ( p__ ).

>> Times(p__)===And(p__)
True

In the examples above prevent the patterns from evaluating, by wrapping them with HoldPattern as in the following lines.

>> MatchQ(And(x, y, z), HoldPattern(Times(p__)))
False


>> HoldPattern(Times(p__))===HoldPattern(And(p__))
False

In the next lines HoldPattern is used to ensure the head (And) is changed to (List).
The two examples that follow have the same effect, but the use of HoldPattern isn’t needed.

>> And(x, y, z)/.HoldPattern(And(a__)) ->List(a)
{x,y,z}


>> And(x, y, z)/.And->List
{x,y,z}


>> And(x, y, z)/.And(a_,b___)->List(a,b)
{x,y,z}

Hold, HoldComplete, HoldForm, ReleaseHold, Unevaluated

Implementation status

  • ✅ - full supported

Github