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[?] Define a function that operates on patterns?

Pavel Perikov
Posted 5 years ago

Hello everybody.

I'm trying to define a function that accepts a pattern as a parameter. After playing with HoldPattern and HoldAll attributes I'm out of ideas.

Is this at all possible? Here's an example of what I'd like to get:

Remove[f];
f[HoldPattern[Blank[Real]]] := 0; 
f[a_] := 1; SetAttributes[f, HoldFirst];
f[_Real] (* expecting to get 0 *)

Pure function will receive a pattern as expected.

Any ideas are welcome.
POSTED BY: Pavel Perikov
4 Replies
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How about Verbatim?

Clear[f];
f[Verbatim[_Real]] := 0;
f[_] := 1;
{f[_Integer] (*1*), f[_Real] (*0*)}
POSTED BY: Gerli Jõgeva
Pavel Perikov
Pavel Perikov
Posted 5 years ago

Exactly! This is THE solution. Thank you!
POSTED BY: Pavel Perikov

Hi Pavel, see if any of these examples are useful for you:

Clear[f];
a := {f[a_] := 1};
f[HoldPattern[_]] := 0;

f[_Real]
a;
f[_Real]

i1

Clear[f];
f[a_] := a /. x_ /; x === pat -> 0;

f[pat]
f[3]
f[1]

i2

Clear[f];
f[a_: 10] := a;

f[]
f[2]
f[3]

i3

Clear[f];
f[a_] := a /. {pat -> 0, _ -> 1};

f[pat]
f[2]
f[3]

i4

Clear[f];
f[a_] := If[a \[Element] NegativeIntegers, 0, a];

f[2]
f[-2]
f[2.37]

i5
POSTED BY: Claudio Chaib
Pavel Perikov
Pavel Perikov
Posted 5 years ago

Thanks! Playing with those now.
POSTED BY: Pavel Perikov
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