Forth is a little different, compared with the usual suspects that are JAVA, C, Shellscripts and even PHP. It's stack-oriented!

The code for simple bounds-checking:

: check-range ( u lower upper -- )
  within ;

That's it!

It would be called like 1 0 2 check-range and return -1, if in bounds, 0 otherwise.

Exceptions in Forth are a purist's dream. if you call 'throw' with 0 on top of stack, nothing happens. if you call it with a number, an exception is thrown.

my exception would be -24: invalid numeric argument, if out of bounds.

in code: -24 throw

So, now we need to combine those two ideas of bounds checking and exceptions. it's kind of very easy!

the magic behind it happens via the and-instruction. but let's have a look at the definition first:

: check-range ( u lower upper -- )
  within 0= -24 and throw ;

We know about the within, we know the throw part. But what about the rest?

0= 'inverts' a logical value on the stack. if the value is 0, it's replaced by -1. if it isn't, it's replaced by 0.

Now, remember the and-instruction. -1 has all bits set, thus something and -1 is something. 0 has no bit set, thus something and 0 is 0.

Put together: if(in bounds) throw(0) else throw (-24);