[kazinator]

There is no difference between "no value" and "the fact that there is no value". "The fact that" is just rhetorical verbiage.

There is an ambiguity in a polymorphic container between a present entry indicating a null value, and a null return indicating there is no entry.

E.g. hash table being used to represent global variables. We'd like to diagnose it when a nonexistent variable is accessed, while allowing access to variables that exist, but have a null value.

This is because the variables are polymorphic and can hold anything.

When we are dealing with a typed situation (all entries are expected to be of a certain type, and null is not a member of that type's domain) then there is no ambiguity: if a null appears in the search for a value that must be a string, it doesn't matter whether "not found" is being represented by an explicit entry with a null value, or absence of an entry.

[tedunangst]

This rhetorical verbiage matters a lot in a language like Lua where you can pack an array full of dbnull sentinel types but filling it with nil results in an empty array.