Why doesn't PatternTest work with Composition?

Question

While playing around with the solutions to this question, I've found some very strange behaviour:

MatchQ[3,_?Composition[Not,OptionQ]]
(*
==> False
*)
MatchQ[3,_?(Not[OptionQ[#]]&)]
(*
==> True
*)
Composition[Not,OptionQ][3]
(*
==> True
*)

So what's wrong with the first pattern? Or did I just find some bug?

Leonid Shifrin · Accepted Answer · 2012-02-13 16:40:58Z

31

Because PatternTest binds very tightly. You need extra parentheses:

MatchQ[3, _?(Composition[Not, OptionQ])]

answered Feb 13, 2012 at 16:40

Leonid Shifrin

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$\begingroup$ Thanks. I should have thought of that possibility myself ... $\endgroup$
– celtschk
Commented Feb 13, 2012 at 16:44
7

$\begingroup$ @celtschk This is not at all obvious - it binds stronger than a function call: a?f[b] // FullForm gives PatternTest[a,f][b], which is not something we are used to. I just happened to have been bitten by this a few times. The same story with the Function: have to wrap it in parentheses as well. I actually mentioned that in my answer to the question you linked. $\endgroup$
– Leonid Shifrin
Commented Feb 13, 2012 at 16:46
8

$\begingroup$ This is one reason I tend to use Condition for anything that isn't completely trivial... $\endgroup$
– Brett Champion
Commented Feb 13, 2012 at 16:46
$\begingroup$ @Brett I use PatternTest in this form with parentheses quite a bit as well, but I agree that there are less chances for errors with Condition. $\endgroup$
– Leonid Shifrin
Commented Feb 13, 2012 at 16:50
5

$\begingroup$ It's unusual for something to have a higher precedence than [...] function call ... $\endgroup$
– Szabolcs
Commented Feb 13, 2012 at 16:50

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