Pure function with pattern test

Question

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

f = Function[{x_?NumericQ},
    x
]

but the error Function::flpar occured.

No, this is not possible. f = Replace[x_?NumericQ :> x^2] is the closest thing you get. — Sjoerd Smit, Apr 20 at 12:53
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. — Ben Izd, Apr 20 at 12:54
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? — lericr, Apr 20 at 14:50

Daniel Huber · Accepted Answer · 2023-04-20 14:19:17Z

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]) &;

f[a]
Argument nnot numeric

f[2]
4

answered Apr 20 at 14:19

Daniel Huber

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