
Null (or nil) is the computer science concept of an undefined or unbound object.
Some languages have an explicit way to access the null object, and some don't.
Some languages distinguish the null object from undefined values, and some don't.



Task

Show how to access null in your language by checking to see if an object is equivalent to the null object.


This task is not about whether a variable is defined. The task is about "null"-like values in various languages, which may or may not be related to the defined-ness of variables in your language.

REF STRING no result = NIL;
STRING result := "";
 
IF no result :=: NIL THEN print(("no result :=: NIL", new line)) FI;
IF result :/=: NIL THEN print(("result :/=: NIL", new line)) FI;
 
IF no result IS NIL THEN print(("no result IS NIL", new line)) FI;
IF result ISNT NIL THEN print(("result ISNT NIL", new line)) FI;
 
COMMENT using the UNESCO/IFIP/WG2.1 ALGOL 68 character set
  result := °;
  IF REF STRING(result) :≠: ° THEN print(("result ≠ °", new line)) FI;
END COMMENT
 
# Note the following gotcha: #
 
REF STRING var := NIL;
IF var ISNT NIL THEN print(("The address of var ISNT NIL",new line)) FI;
IF var IS REF STRING(NIL) THEN print(("The address of var IS REF STRING(NIL)",new line)) FI