I want to write an iterator that traverses a data-structure:

my $iter = getIter( $structRef );

while( my $next = $iter->() ) {
    ## use $next;
}
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Problem: elements of the data structure can legitimately return undef. And 0. And -1. And ''. And any other value a scalar can hold.

It is an age old problem. As soon as you define [sic] an undefined value -- or null or nil or bottom -- it become useful to be able to store it.

And once you can store it, it is no longer useful for detecting exceptional, non-error conditions.

What is needed is a 'nothing' place-holder.

A non-storable, non-value that if detected in a while or for loop causes the loop to terminate -- without error.

But if you try to assign it to any variable, terminates with an exception.