When I declare a function f in the following form:

f[a|b] := 1

then I can use it like:

f[a] (* gives 1 *)

But when I try to use the form:


I get the following error, and I can't use it:

Function::flpar: Parameter specification a|b in Function[a|b,1] should be a symbol or a list of symbols.

Why does Alternatives[a,b] not "work" like a list {a,b} in this case?

  • 1
    $\begingroup$ Function does not allow you to use patterns in the variable specification. You can't do things like Function[n_?NumericQ, ...] either. All you can do, is list the names of the variables that the function takes. $\endgroup$ Jul 11, 2021 at 21:46
  • $\begingroup$ Do you know if there is a logical reason or it is a technical limitation? $\endgroup$ Jul 12, 2021 at 0:04
  • 2
    $\begingroup$ := creates a definition based on pattern matching. so, on the lhs f[a|b] := the pattern in the argument of f is the literal expression a|b. It's actually a totally different mechanism of substituting in values than the Function head! the Function head is for anonymous functions, and the first argument is just a list of the formal parameters used in the functions body, which are filled from the argument(s) sequentially—it's not a pattern that is matched against the argument. $\endgroup$
    – thorimur
    Jul 12, 2021 at 1:11
  • 1
    $\begingroup$ To get an equivalent definition of f to the := definition, consider f=Replace[{(a|b) :> 1}] $\endgroup$
    – thorimur
    Jul 12, 2021 at 1:13
  • $\begingroup$ You can also use f[a | b] = 1; instead. $\endgroup$
    – Somos
    Jul 12, 2021 at 16:41

1 Answer 1


Function does not do pattern matching. If you need pattern matching, you can do it manually:

    a|b, 1,
    _, 0 (* this is the default choice for input that don't match preceding patterns *)

Note that it is not possible to keep Function[...] unevaluated when there is no match.

  • $\begingroup$ Why not replace 0 with x? Or Switch[#, a | b, 1, _, #] &; $\endgroup$
    – Somos
    Jul 12, 2021 at 16:39
  • $\begingroup$ @Somos There are many possibilities. My point was that one must choose something. It's up to the OP to make the appropriate choice for their application. $\endgroup$
    – Szabolcs
    Jul 12, 2021 at 19:41
  • $\begingroup$ Agreed. I just wanted to present some more possibilities. $\endgroup$
    – Somos
    Jul 12, 2021 at 19:46

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