LISP was special because it had only 2 data types: atoms and lists.
Both lists and atoms could appear in any place in any function or special form.
NIL was a special atom, which was used to stand for an empty list. Because it could appear in any place in a LISP program, it could be used anywhere where one had to write that something does not exist.
In a programming language with a more complicated and also extensible system of data types the handling of "nothing" values must also be more complex.
Any optional type can be viewed as a variable-length array of its base type, whose maximum length is 1 and a null length indicates a non-existent value.
This is equivalent with the use of NIL in LISP.
However, it is better to consider optional types as a distinct method of deriving types from base types than arrays, structures or unions, because in most cases more efficient implementations are possible for optional types than implementing them as variable-length arrays that may have a null length or as tagged unions where one of the alternative types is the empty set (a.k.a. void).
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LISP was special because it had only 2 data types: atoms and lists.
Both lists and atoms could appear in any place in any function or special form.
NIL was a special atom, which was used to stand for an empty list. Because it could appear in any place in a LISP program, it could be used anywhere where one had to write that something does not exist.
In a programming language with a more complicated and also extensible system of data types the handling of "nothing" values must also be more complex.
Any optional type can be viewed as a variable-length array of its base type, whose maximum length is 1 and a null length indicates a non-existent value.
This is equivalent with the use of NIL in LISP.
However, it is better to consider optional types as a distinct method of deriving types from base types than arrays, structures or unions, because in most cases more efficient implementations are possible for optional types than implementing them as variable-length arrays that may have a null length or as tagged unions where one of the alternative types is the empty set (a.k.a. void).