tasks a-s

Task/Null-object/0DESCRIPTION

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+'''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.
+
+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.''

Task/Null-object/1META.yaml

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+---
+note: Basic language learning

Task/Null-object/ALGOL-68/null-object.alg

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+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

Task/Null-object/ActionScript/null-object.as

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+if (object == null)
+    trace("object is null");

Task/Null-object/Ada/null-object.ada

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+with Ada.Text_Io;
+
+if Object = null then
+   Ada.Text_Io.Put_line("object is null");
+end if;

Task/Null-object/AmigaE/null-object.amiga

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+DEF x : PTR TO object
+-> ...
+IF object <> NIL
+  -> ...
+ENDIF

Task/Null-object/AppleScript/null-object.applescript

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+if x is missing value then
+  display dialog "x is missing value"
+end if
+
+if x is null then
+  display dialog "x is null"
+end if

Task/Null-object/AutoHotkey/null-object.ahk

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+If (object == null)
+  MsgBox, object is null

Task/Null-object/BBC-BASIC/null-object.bbc

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+      PROCtestobjects
+      END
+
+      DEF PROCtestobjects
+      PRIVATE a(), b(), s{}, t{}
+      DIM a(123)
+      DIM s{a%, b#, c$}
+
+      IF !^a() <= 1 PRINT "a() is null" ELSE PRINT "a() is not null"
+      IF !^b() <= 1 PRINT "b() is null" ELSE PRINT "b() is not null"
+      IF !^s{} <= 1 PRINT "s{} is null" ELSE PRINT "s{} is not null"
+      IF !^t{} <= 1 PRINT "t{} is null" ELSE PRINT "t{} is not null"
+      ENDPROC

Task/Null-object/Babel/null-object.pb

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+{ nil { nil? } { "Whew!\n" } { "Something is terribly wrong!\n" } ifte << }

Task/Null-object/C++/null-object-1.cpp

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+#include <iostream>
+#include <cstdlib>
+if (object == 0) {
+   std::cout << "object is null";
+}

Task/Null-object/C++/null-object-2.cpp

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+#include <boost/optional.hpp>
+#include <iostream>
+
+boost::optional<int> maybeInt()
+
+int main()
+{
+  boost::optional<int> maybe = maybeInt();
+
+  if(!maybe)
+    std::cout << "object is null\n";
+}

Task/Null-object/C/null-object.c

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+#include <stdio.h>
+
+int main()
+{
+	char *object = 0;
+
+	if (object == NULL) {
+		puts("object is null");
+	}
+	return 0;
+}

Task/Null-object/Clojure/null-object-1.clj

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+(let [x nil]
+ (println "Object is" (if (nil? x) "nil" "not nil")))

Task/Null-object/Clojure/null-object-2.clj

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+(find (ns-interns *ns*) 'foo)

Task/Null-object/Clojure/null-object-3.clj

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+(ns-unmap *ns* 'foo)

Task/Null-object/Common-Lisp/null-object-1.lisp

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+(if (condition) (do-this))

Task/Null-object/Common-Lisp/null-object-2.lisp

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+(defmethod car* ((arg cons))
+  (car arg))
+
+(defmethod car* ((arg null))
+  nil)

Task/Null-object/Common-Lisp/null-object-3.lisp

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+(defmethod car* ((arg t))  ;; can just be written (defmethod car* (arg) ...)
+  (error "CAR*: ~s is neither a cons nor nil" arg))

Task/Null-object/Component-Pascal/null-object.component

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+MODULE ObjectNil;
+IMPORT StdLog;
+TYPE
+	Object = POINTER TO ObjectDesc;
+	ObjectDesc = RECORD
+	END;
+VAR	
+	x: Object; (* default initialization to NIL *)
+	
+PROCEDURE DoIt*;
+BEGIN
+	IF x = NIL THEN
+		StdLog.String("x is NIL");StdLog.Ln
+	END
+END DoIt;
+
+END ObjectNil.

Task/Null-object/D/null-object.d

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+import std.stdio;
+
+class K {}
+
+void main() {
+    K k;
+    if (k is null)
+        writeln("k is null");
+    k = new K;
+    if (k !is null)
+        writeln("Now k is not null");
+}

Task/Null-object/Delphi/null-object.delphi

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+  // the following are equivalent
+  if lObject = nil then
+  ...
+
+  if not Assigned(lObject) then
+  ...

Task/Null-object/E/null-object.e

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+object == null

Task/Null-object/Factor/null-object.factor

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+: is-f? ( obj -- ? ) f = ;

Task/Null-object/Fantom/null-object.fantom

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+fansh> x := null
+fansh> x == null
+true
+fansh> x = 1
+1
+fansh> x == null
+false

Task/Null-object/Go/null-object.go

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+package main
+
+import "fmt"
+
+var (
+    s []int       // slice type
+    p *int        // pointer type
+    f func()      // function type
+    i interface{} // interface type
+    m map[int]int // map type
+    c chan int    // channel type
+)
+
+func main() {
+    fmt.Println(s == nil)
+    fmt.Println(p == nil)
+    fmt.Println(f == nil)
+    fmt.Println(i == nil)
+    fmt.Println(m == nil)
+    fmt.Println(c == nil)
+}

Task/Null-object/Haskell/null-object-1.hs

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+undefined      -- undefined value provided by the standard library
+error "oops"   -- another undefined value
+head []        -- undefined, you can't take the head of an empty list

Task/Null-object/Haskell/null-object-2.hs

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+ data Maybe a = Nothing | Just a

Task/Null-object/Haskell/null-object-3.hs

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+case thing of
+ Nothing -> "It's Nothing. Or null, whatever."
+ Just v  -> "It's not Nothing; it is " ++ show v ++ "."

Task/Null-object/Icon/null-object.icon

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+procedure main()
+nulltest("a",a)                # unassigned variables are null by default
+nulltest("b",b := &null)       # explicit assignment is possible
+nulltest("c",c := "anything")
+nulltest("c",c := &null)       # varibables can't be undefined
+end
+
+procedure nulltest(name,var)
+return write(name, if /var then " is" else " is not"," null.")
+end

Task/Null-object/Io/null-object.io

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+if(object == nil, "object is nil" println)

Task/Null-object/J/null-object-1.j

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+isUndefined=: _1 = nc@boxxopen

Task/Null-object/J/null-object-2.j

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+   isUndefined 'foo'
+1
+   foo=:9
+   isUndefined 'foo'
+0

Task/Null-object/J/null-object-3.j

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+   1 1 0 1#3 4 _ 5
+3 4 5
+   I.1 1 0 1
+0 1 3
+   0 1 3 { 3 4 _ 5
+3 4 5
+   1 1 0 1 #inv 3 4 5
+3 4 0 5
+   1 1 0 1 #!._ inv 3 4 5
+3 4 _ 5

Task/Null-object/Java/null-object.java

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+// here "object" is a reference
+if (object == null) {
+   System.out.println("object is null");
+}

Task/Null-object/JavaScript/null-object.js

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+if (object === null) {
+  alert("object is null");
+  // The object is nothing
+}
+
+typeof null === "object"; // This stands since the beginning of JavaScript

Task/Null-object/K/null-object.k

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+  (1;;2) ~ (1 ; _n ; 2)    /  ~  is ''identical to'' or ''match'' .
+1
+  _n ~' ( 1 ; ; 2 )        /  ''match each''
+0 1 0
+
+additional properties :  _n@i and _n?i are i; _n`v is _n

Task/Null-object/Logo/null-object.logo

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+to test :thing
+if empty? :thing [print [list or word is empty]]
+end
+
+print empty? []  ; true
+print empty? "|| ; true

Task/Null-object/Lua/null-object.lua

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+isnil = (object == nil)
+print(isnil)

Task/Null-object/MAXScript/null-object.maxscript

@@ -0,0 +1 @@
+if obj == undefined then print "Obj is undefined"

Task/Null-object/MUMPS/null-object.mumps

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+CACHE>WRITE $DATA(VARI)
+0
+CACHE>SET VARI="HELLO" WRITE $DATA(VARI)
+1
+CACHE>NEW VARI WRITE $DATA(VARI) ;Change to a new scope
+0
+CACHE 1S1>SET VARI(1,2)="DOWN" WRITE $DATA(VARI)
+10
+CACHE 1S1>WRITE $DATA(VARI(1))
+10
+CACHE 1S1>WRITE $D(VARI(1,2))
+1
+CACHE 1S1>SET VARI(1)="UP" WRITE $DATA(VARI(1))
+11
+<CACHE 1S1>QUIT ;Leave the scope
+
+<CACHE>W $DATA(VARI)," ",VARI
+1 HELLO

Task/Null-object/Mathematica/null-object-1.math

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+x=Null;

Task/Null-object/Mathematica/null-object-2.math

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+x =.
+x = (1 + 2;)
+FullForm[x]

Task/Null-object/Mathematica/null-object-3.math

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+SameQ[x,Null]

Task/Null-object/Mathematica/null-object-4.math

@@ -0,0 +1 @@
+x===Null

Task/Null-object/Mathematica/null-object-5.math

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+x =.;
+ValueQ[x]
+x = 3;
+ValueQ[x]

Task/Null-object/Mathematica/null-object-6.math

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+False
+True

Task/Null-object/Modula-3/null-object-1.mod3

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+VAR foo := NIL

Task/Null-object/Modula-3/null-object-2.mod3

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+VAR foo: REF INTEGER := NIL;

Task/Null-object/Modula-3/null-object-3.mod3

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+IF foo = NIL THEN
+  IO.Put("Object is nil.\n");
+END;

Task/Null-object/NetRexx/null-object.netrexx

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+/* NetRexx */
+options replace format comments java crossref symbols binary
+
+robject = Rexx -- create an object for which the value is undefined
+say String.valueOf(robject) -- will report the text "null"
+if robject = null then say 'Really, it''s "null"!'

Task/Null-object/OCaml/null-object-1.ocaml

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+type 'a option = None | Some of 'a

Task/Null-object/OCaml/null-object-2.ocaml

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+match v with
+| None -> "unbound value"
+| Some _ -> "bounded value"

Task/Null-object/Objeck/null-object.objeck

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+# here "object" is a reference
+if(object = Nil) {
+   "object is null"->PrintLine();
+};

Task/Null-object/Objective-C/null-object-1.m

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+// here "object" is an object pointer
+if (object == nil) {
+   NSLog("object is nil");
+}

Task/Null-object/Objective-C/null-object-2.m

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+[nil fooBar];

Task/Null-object/Oz/null-object-1.oz

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+declare
+  X
+in
+  {Show X+2}  %% blocks

Task/Null-object/Oz/null-object-2.oz

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+declare
+  X = {Value.failed dontTouchMe}
+in
+  {Wait X}  %% throws dontTouchMe

Task/Null-object/Oz/null-object-3.oz

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+declare
+  X = just("Data")
+in
+  case X of nothing then skip
+  [] just(Result) then {Show Result}
+  end

Task/Null-object/PARI-GP/null-object.pari

@@ -0,0 +1 @@
+foo!='foo

Task/Null-object/PHP/null-object.php

@@ -0,0 +1,3 @@
+$x = NULL;
+if (is_null($x))
+  echo "\$x is null\n";

Task/Null-object/PL-I/null-object.pli

@@ -0,0 +1,7 @@
+declare x fixed decimal (10);
+...
+if ^valid(x) then signal error;
+
+declare y picture 'A9XAAA9';
+...
+if ^valid(y) then signal error;

Task/Null-object/Perl-6/null-object-1.pl6

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+my $var;
+say $var.WHAT;      # Any()
+$var = 42;
+say $var.WHAT;      # Int()
+say $var.defined;   # True
+$var = Nil;
+say $var.WHAT;      # Any()
+say $var.defined    # False

Task/Null-object/Perl-6/null-object-2.pl6

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+my Mu $junction;
+say $junction.WHAT;      # Mu()
+$junction = 1 | 2 | 3;
+say $junction.WHAT;      # Junction()

Task/Null-object/Perl-6/null-object-3.pl6

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+my Str $str;
+say $str.WHAT;      # Str()
+$str = "I am a string.";
+say $str.WHAT;      # Str()
+$str = 42;          # (fails)

Task/Null-object/Perl/null-object-1.pl

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+print defined($x) ? 'Defined' : 'Undefined', ".\n";

Task/Null-object/Perl/null-object-2.pl

@@ -0,0 +1 @@
+say $number // "unknown";

Task/Null-object/PicoLisp/null-object-1.l

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+(if (not MyNewVariable)
+   (handle value-is-NIL) )

Task/Null-object/PicoLisp/null-object-2.l

@@ -0,0 +1,2 @@
+(unless MyNewVariable
+   (handle value-is-NIL) )

Task/Null-object/Pike/null-object.pike

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+> mapping bar;
+> bar;
+Result: 0
+> bar = ([ "foo":0 ]);
+> bar->foo;
+Result 0;
+> zero_type(bar->foo);
+Result: 0
+> bar->baz;
+Result: 0
+> zero_type(bar->baz);
+Result: 1

Task/Null-object/PowerShell/null-object.psh

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+if ($null -eq $object) {
+    ...
+}

Task/Null-object/PureBasic/null-object.purebasic

@@ -0,0 +1,3 @@
+If variable = #Null
+  Debug "Variable has no value"
+EndIf

Task/Null-object/Python/null-object.py

@@ -0,0 +1,5 @@
+x = None
+if x is None:
+  print "x is None"
+else:
+  print "x is not None"

Task/Null-object/R/null-object.r

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+is.null(NULL)       # TRUE
+is.null(123)        # FALSE
+is.null(NA)         # FALSE
+123==NULL           # Empty logical value, with a warning
+foo <- function(){} # function that does nothing
+foo()               # returns NULL

Task/Null-object/REBOL/null-object.rebol

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+x: none
+
+print ["x"  either none? x ["is"]["isn't"]  "none."]

Task/Null-object/REXX/null-object.rexx

@@ -0,0 +1,13 @@
+/*REXX program demonstrates null strings, and also undefined values. */
+
+if symbol('ABC')=="VAR" then say 'variable ABC is defined, value='abc"<<<"
+                        else say "variable ABC isn't defined."
+xyz=47
+if symbol('XYZ')=="VAR" then say 'variable XYZ is defined, value='xyz"<<<"
+                        else say "variable XYZ isn't defined."
+drop xyz
+if symbol('XYZ')=="VAR" then say 'variable XYZ is defined, value='xyz"<<<"
+                        else say "variable XYZ isn't defined."
+cat=''
+if symbol('CAT')=="VAR" then say 'variable CAT is defined, value='cat"<<<"
+                        else say "variable CAT isn't defined."

Task/Null-object/Ruby/null-object.rb

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+   puts "object is null" if object.nil?

Task/Null-object/Scala/null-object.scala

@@ -0,0 +1,22 @@
+scala> Nil
+res0: scala.collection.immutable.Nil.type = List()
+
+scala> Nil == List()
+res1: Boolean = true
+
+scala> Null
+<console>:8: error: not found: value Null
+              Null
+              ^
+
+scala> null
+res3: Null = null
+
+scala> None
+res4: None.type = None
+
+scala> Unit
+res5: Unit.type = object scala.Unit
+
+scala> val a = println()
+a: Unit = ()

Task/Null-object/Scheme/null-object.ss

@@ -0,0 +1 @@
+(null? object)

Task/Null-object/Slate/null-object.slate

@@ -0,0 +1 @@
+Nil isNil = True.

Task/Null-object/Smalltalk/null-object-1.st

@@ -0,0 +1,6 @@
+object isNil ifTrue: [ "true block" ]
+             ifFalse: [ "false block" ].
+nil isNil ifTrue: [ 'true!' displayNl ]. "output: true!"
+foo isNil ifTrue: [ 'ouch' displayNl ].
+x := (foo == nil).
+x := foo isNil

Task/Null-object/Smalltalk/null-object-2.st

@@ -0,0 +1,6 @@
+foo := nil.
+foo class. "-> UndefinedObject"
+foo respondsTo: #'bar'. "asking if a message is implemented"
+
+foo class compile:'fancyOperation ^ 123'.
+foo fancyOperation "->123"

Task/Null-object/Standard-ML/null-object-1.ml

@@ -0,0 +1 @@
+datatype 'a option = NONE | SOME of 'a

Task/Null-object/Standard-ML/null-object-2.ml

@@ -0,0 +1,2 @@
+case v of NONE => "unbound value"
+        | SOME _ => "bounded value"

Task/Null-object/Tcl/null-object-1.tcl

@@ -0,0 +1 @@
+if {$value eq ""} ...

Task/Null-object/Tcl/null-object-2.tcl

@@ -0,0 +1,3 @@
+if {![info exist nullvar]} ...
+if {![info exists arr(nullval)]} ...
+if {![dict exists $dic nullval]} ...