Re^2: Existing module for PerlX::Maybe except for hash existence?

[cavac]

Both are saying "The caller does not provide a defined value", but there is no hint why you need two different ways to say that

My guess is that it's the difference "the caller does not have/want a value for this" and "the caller forgot to specify the value"

Take one example: A program called SchrödingersCat needs a command line flag to specify if the cat is alive or dead. But when called with a value of "undef", the caller provides a valid value of "we don't know if the cat is alive or dead". See Tom Scott's video on Null Island: The Busiest Place That Doesn't Exist

[haj]

Take one example: A program called SchrödingersCat needs a command line flag to specify if the cat is alive or dead. But when called with a value of "undef", ...

A nice example, indeed. Just one question: How do you pass an undefined value as a command line flag?

See Tom Scott's video....

Thanks: Another example which underpins my point. The video explains the difference between NULL, "NULL" and 0 when read from a database. So yes, a database has NULL as the equivalent of an undefined value. But a database has no notion of "this field does not exist": If a field is in a table definition, then it exists for every single record. So, again, it has only one special value, not two.

[ikegami]

Your example fails.

There are three states: Alive, dead, superposition.

There's no need to differentiate between not provided and undef. Both would be considered superposition.

And you mention command-line arguments where it's not even possible to pass undef, completely defying your point.

Your example supports haj, not you.