Null object: Difference between revisions

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In ALGOL 68 the NIL yields a name that does not refer to any value. NIL can never be

naturally coerced and can only appear where the context is [[ALGOL 68#strong|strong]].

{{works with|ALGOL 68|Revision 1 - no extensions to language used}}

{{works with|ALGOL 68G|Any - tested with release [http://sourceforge.net/projects/algol68/files/algol68g/algol68g-1.18.0/algol68g-1.18.0-9h.tiny.el5.centos.fc11.i386.rpm/download 1.18.0-9h.tiny]}}

{{works with|ELLA ALGOL 68|Any (with appropriate job cards) - tested with release [http://sourceforge.net/projects/algol68/files/algol68toc/algol68toc-1.8.8d/algol68toc-1.8-8d.fc9.i386.rpm/download 1.8-8d]}}

<lang algol68>REF STRING no result = NIL;

STRING result := "";

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# 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

IF var IS REF STRING(NIL) THEN print(("The address of var IS REF STRING(NIL)",new line)) FI</lang>

</lang>

Output:

<pre>

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The address of var IS REF STRING(NIL)

</pre>

NIL basically is an ~~untypes~~ ~~REF~~ (pointer) that does not refer anywhere.

NIL basically is an untyped '''ref''' (pointer) that does not refer anywhere.

ALGOL 68 also has ~~EMPTY~~. This is a "constant" of size 0 and type ~~VOID~~.

ALGOL 68 also has '''empty'''. This is a "constant" of size 0 and type '''void'''.

c.f. [[Roots_of_a_function#ALGOL_68|Roots of a function]] for two different

examples of usage.

* ~~EMPTY~~ as an undefined argument to a routine.

* '''empty''' as an undefined argument to a routine.

* ~~EMPTY~~ as a routine return if no result is found.

* '''empty''' as a routine return if no result is found.

~~EMPTY~~ is typically used to refer to am empty leaf in a tree structure.

'''empty''' is typically used to refer to am empty leaf in a tree structure.

Basically:

* ALGOL 68's ~~EMPTY~~ is python's <code>None</code>,

* ALGOL 68's '''empty''' is python's <code>None</code>,

* ALGOL 68's ~~VOID~~ is python's <code>NoneType</code>, and

* ALGOL 68's '''void''' is python's <code>NoneType</code>, and

* ALGOL 68's ~~NIL~~ is python's <code>hash(None)</code>

* ALGOL 68's '''nil''' is python's <code>hash(None)</code>

=={{header|AmigaE}}==