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Say I define a function f[x_, y_]:= ..., the concrete definition of which is not important now.

What is important though is, that I want x to be a real number, and y to be a boolean variable, such that it can only have value True or False.

The former I can do by f[x_Real, y_]:= ..., such as described here in the help for "patterns", under "Specifying Types of Expression in Patterns".

However there does not seem to be an equivalent boolean type. Or at least, it is not simply y_Boolean.

Does anyone know how to do this? Thanks for suggestions!

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    $\begingroup$ f[x_?NumericQ, y_?BooleanQ]:=... $\endgroup$
    – Bob Hanlon
    Apr 8, 2020 at 17:00
  • $\begingroup$ Looks like a good alternative to the answer of Henrik, thanks! $\endgroup$
    – Britzel
    Apr 9, 2020 at 13:10

1 Answer 1

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Should be doable with 

f[y : True | False] := ...

Beware that the pattern x_Real checks only whether the Head of x is equal to Real; it does not check whether x is a real number or not. For example, MatchQ[1, x_Real] returns False.

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  • $\begingroup$ @Hendrik Schumacher Works like a charm, thanks! Also thanks for the warning. I see what you mean. So how would I then for instance restrict x to be in the interval (0, 3), but where ideally I would also integers allowed to be entered? If this is not possible, then the restriction to the interval as float would do. $\endgroup$
    – Britzel
    Apr 8, 2020 at 11:47
  • $\begingroup$ Then try x_?(0<#<3)&. The _? pattern means that the pattern matches only if the function following it (here: (0<#<3)&) evaluates to True. $\endgroup$
    – Henrik Schumacher
    Apr 8, 2020 at 11:50
  • $\begingroup$ This doesn't seem to work. However it is not the most pressing issue for now. The other one was more important. Thanks again! $\endgroup$
    – Britzel
    Apr 8, 2020 at 14:33
  • $\begingroup$ Oh, it was the closing parenthesis. Please try x_?(0<#<3&). $\endgroup$
    – Henrik Schumacher
    Apr 8, 2020 at 15:05
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    $\begingroup$ @Britzel "how would I then for instance restrict x to be in the interval $(0, 3)$, but where ideally I would also integers allowed to be entered" - you can use Condition for this: f[x_Integer /; 0 < x < 3] := (* stuff *). $\endgroup$
    – J. M.'s lack of A.I. ♦
    May 16, 2020 at 8:16

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