Is it possible to achieve pattern test inside a pure function? I have tried

f = Function[{x_?NumericQ},

but the error Function::flpar occured.

  • 3
    $\begingroup$ No, this is not possible. f = Replace[x_?NumericQ :> x^2] is the closest thing you get. $\endgroup$ Apr 20, 2023 at 12:53
  • $\begingroup$ Welcome to the community. You can achieve a similar effect with If[MatchQ[x, _?NumericQ], ...], the only difference is when it doesn't match, you'll get an unevaluated expression normally but with If you can control what to return. $\endgroup$
    – Ben Izd
    Apr 20, 2023 at 12:54
  • $\begingroup$ What is your larger objective? Without any other context, I'd say don't set f equal to a Function but instead give it DownValues. Is there a reason why you can't do that? $\endgroup$
    – lericr
    Apr 20, 2023 at 14:50

1 Answer 1


You may do the pattern test inside the function and eventually return immediately without further evaluation. E.g.

f = (If[! NumericQ[#], "Argument not numeric", #^2]) &;

Argument nnot numeric


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