How do I test for integers

[nvingo]

More specifically, I would suggest that a function which can return either a string or a numeric is never a sensible approach, unless the result is used in a context when both types can be accepted, which practically means only in a PRINT list.

Thanks - let's hope the OP stops at my first implementation of FNinput() which maybe I ought to rename FNinput_positive_integer() for clarity
As presented it would still fall over if too many digits are entered (number too big error) so maybe another function parameter to limit that should be added.
I wouldn't have even contemplated the possibility that different data types be returned had you not mentioned it.

[Hated Moron]

I wouldn't have even contemplated the possibility that different data types be returned had you not mentioned it.

The OP referred to the Python type() function, which evidently takes as a parameter an object which may be a numeric or a string (or probably several other object types too). One could easily conclude, from reading the BBC BASIC documentation, that the language has no similar concept, but in order to support user-defined functions in fact it does, internally.

In some ways it's a shame that there is no BBC BASIC variable type that can hold such a 'true variant', especially as it would be very easy to create one (given a suitable type suffix being identified), but it's not something that one expects from a BASIC dialect and the possible applications for it would be few and far between.

Amusingly, in an early incarnation of BBC BASIC for SDL 2.0 a type-mismatch test was inadvertently omitted and you could write var = "string" without it throwing an error!