Currently, as it stands, this system will trigger a warning if the $input isn't a number of some sort. That's because of the >= in the non_negative validation.

Also, what if I did something like

    validate($input, as => ['blah']);
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Since $validate{'blah'} doesn't exist, you end up with a fatal error. A simple (if uninformative) change would be to

for (@{$args{as}}) {
    my $func = $validate{$_} || next;
    my $r = $func->($input);
    die "$_ validation failed" unless $r;
}
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Better would be to throw some exception or warning.

Furthermore, you do know that this will be less efficient, execution-wise, than just putting the checks into the program. This methdology is best if you plan on doing the same checks on a large variety of inputs. (Of course, you're just playing, which is cool.)

One possibility is to, instead of forcing the user to validate the input, have the function validate its own input using these functions. Or, maybe a wrapper around factorial(). That way, everything is encapsulated.

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