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Storing the history of objects in a program is a common task. Maintaining the history of an object in a program has traditionally required programmers either to write specific code for handling the historical data, or to use a library which supports history logging.

History variables are variables in a programming language which store not only their current value, but also the values they have contained in the past. Some existing languages do provide support for history variables. However these languages typically have many limits and restrictions on use of history variables.

[http://www.bod.com/index.php?id=3435&objk_id=148050 "History Variables: The Semantics, Formal Correctness, and Implementation of History Variables in an Imperative Programming Language" by Mallon and Takaoka]

Concept also discussed on LtU and Patents.com.

Task

Demonstrate History variable support:

enable history variable support (if needed)
define a history variable
assign three values
non-destructively display the history
recall the three values.

For extra points, if the language of choice does not support history variables, demonstrate how this might be implemented.

Ada

Ada does not natively support history variables -- we have to implement them.

Furthermore, Ada is a strongly typed language -- that means, we would need to write a history variable type for every basic item type. Instead, we write a single generic package "History_Variables" that works for any item type.

Generic Package "History_Variables"

Specification:

<lang Ada>private with Ada.Containers.Indefinite_Vectors; generic

  type Item_Type (<>) is private;

package History_Variables is

  type Variable is tagged limited private;

  -- set and get current value
  procedure Set(V: in out Variable; Item: Item_Type);
  function Get(V: Variable) return Item_Type;

  -- number of items in history (including the current one)
  function Defined(V: Variable) return Natural;

  -- non-destructively search for old values
  function Peek(V: Variable; Generation: Natural := 1) return Item_Type;
  -- V.Peek(0) returns current value; V.Peek(1) the previous value, ect.
  -- when calling V.Peek(i), i must be in 0 .. V.Defined-1, else Constraint_Error is raised

  -- destructively restore previous value
  procedure Undo(V: in out Variable);
  -- old V.Peek(0) is forgotten, old V.Peek(i) is new V.Peek(i-1), ect.
  -- accordingly, V.Defined decrements by 1
  -- special case: if V.Defined=0 then V.Undo does not change V

private

  package Vectors is new Ada.Containers.Indefinite_Vectors
    (Index_Type   => Positive,
     Element_Type => Item_Type);

  type Variable is tagged limited record
     History: Vectors.Vector;
  end record;

end History_Variables;</lang>

The implementation of "History_Variables":

<lang Ada>package body History_Variables is

  -- set and get
  procedure Set(V: in out Variable; Item: Item_Type) is
  begin
     V.History.Prepend(Item);
  end Set;

  function Get(V: Variable) return Item_Type is
  begin
     return V.History.First_Element;
  end Get;

  -- number of items in history (including the current one)
  function Defined(V: Variable) return Natural is
  begin
     return (1 + V.History.Last_Index) - V.History.First_Index;
  end Defined;

  -- non-destructively search
  function Peek(V: Variable; Generation: Natural := 1) return Item_Type is
     Index: Positive  := V.History.First_Index + Generation;
  begin
     if Index > V.History.Last_Index then
        raise Constraint_Error;
     end if;
     return V.History.Element(Index);
  end Peek;

  procedure  Undo(V: in out Variable) is
  begin
     V.History.Delete_First;
  end Undo;

end History_Variables;</lang>

Sample 1: The History of an Integer Variable

<lang Ada>with Ada.Text_IO, History_Variables;

procedure Test_History is

  package Int_With_Hist is new History_Variables(Integer);

  -- define a history variable
  I: Int_With_Hist.Variable;

  Sum: Integer := 0;

begin

  -- assign three values
  I.Set(3);
  I.Set(I.Get + 4);
  I.Set(9);

  -- non-destructively display the history
  for N in reverse 0 .. I.Defined-1 loop
     Ada.Text_IO.Put(Integer'Image(I.Peek(N)));
  end loop;
  Ada.Text_IO.New_Line;

  -- recall the three values
  while I.Defined > 0 loop
     Sum := Sum + I.Get;
     I.Undo;
  end loop;
  Ada.Text_IO.Put_Line(Integer'Image(Sum));

end Test_History;</lang>

Output:

 3 7 9
 19

Sample 2: The History of a String

<lang Ada>with Ada.Text_IO, History_Variables;

procedure Test_History is

  package Str_With_Hist is new History_Variables(String);

  -- define a history variable
  S: Str_With_Hist.Variable;

  Sum: Integer := 0;

begin

  -- assign three values
  S.Set("one");
  S.Set(S.Get & S.Get); --"oneone"
  S.Set("three");

  -- non-destructively display the history
  for N in reverse 0 .. S.Defined-1 loop
     Ada.Text_IO.Put(S.Peek(Generation => N) &" ");
  end loop;
  Ada.Text_IO.New_Line;

  -- recall the three values
  while S.Defined > 0 loop
     Sum := Sum + S.Get'Length;
     S.Undo;
  end loop;
  Ada.Text_IO.Put_Line(Integer'Image(Sum));

end Test_History;</lang>

Output:

one oneone three 
 14

AspectJ

AspectJ implementation for Java 7. Type of the history variable (Java class): <lang java>public class HistoryVariable {

   private Object value;

   public HistoryVariable(Object v)
   {
       value = v;
   }

   public void update(Object v)
   {
       value = v;
   }

   public Object undo()
   {
       return value;
   }

   @Override
   public String toString()
   {
       return value.toString();
   }

   public void dispose()
   {
   }

}</lang> Aspect (HistoryHandling.aj): <lang java>import java.util.Deque; import java.util.HashMap; import java.util.LinkedList; import java.util.Map;

public privileged aspect HistoryHandling {

   before() : execution(HistoryVariable.new(..))
   {
       history.put((HistoryVariable) thisJoinPoint.getTarget(), new LinkedList<>());
   }

   after() : execution(void HistoryVariable.dispose())
   {
       history.remove(thisJoinPoint.getTarget());
   }

   before(Object v) : execution(void HistoryVariable.update(Object)) && args(v)
   {
       final HistoryVariable hv = (HistoryVariable) thisJoinPoint.getThis();
       history.get(hv).add(hv.value);
   }

   after() : execution(Object HistoryVariable.undo())
   {
       final HistoryVariable hv = (HistoryVariable) thisJoinPoint.getThis();
       final Deque<Object> q = history.get(hv);
       if (!q.isEmpty())
           hv.value = q.pollLast();
   }

   String around() : this(HistoryVariable) && execution(String toString())
   {
       final HistoryVariable hv = (HistoryVariable) thisJoinPoint.getThis();
       final Deque<Object> q = history.get(hv);
       if (q == null)
           return "<disposed>";
       else
           return "current: "+ hv.value + ", previous: " + q.toString();
   }

   private Map<HistoryVariable, Deque<Object>> history = new HashMap<>();

}</lang> Usage: <lang java>public final class Main {

   public static void main(final String[] args)
   {
       HistoryVariable hv = new HistoryVariable("a");
       hv.update(90);
       hv.update(12.1D);
       System.out.println(hv.toString());
       System.out.println(hv.undo());
       System.out.println(hv.undo());
       System.out.println(hv.undo());
       System.out.println(hv.undo());
       System.out.println(hv.toString());
       hv.dispose();
       System.out.println(hv.toString());
   }

}</lang>

Output:

current: 12.1, previous: [a, 90]
12.1
90
a
a
current: a, previous: []
<disposed>

AutoHotkey

AutoHotkey records a history of your keypresses, but not your variables. The closest you can come is with a class:

Works with: AutoHotkey 1.1

<lang ahk>HV := new HistoryVariable HV.var := 1 HV.var := 2 HV.var := "Latest value" Msgbox % HV.var "`n" HV[2] "`n" HV[3]

class HistoryVariable { __Set(aName, aValue) { this.Insert(1,aValue) Return aValue } __Get(aName) { Return this[1] } }</lang>

C#

<lang c sharp>using System; using System.Collections; using System.Collections.Generic; using System.Linq;

namespace History {

   class Program
   {
       static void Main(string[] args)
       {
           var h = new HistoryObject();
           h.Value = 5;
           h.Value = "foo";
           h.Value += "bar";

           var history = h.ToArray();

           for (int i = 0; i < history.Length; i++)
           {
               Console.Write("{0}{1}", history[i], ((i >= history.Length - 1) ? "\n" : " <- "));
           }

           h.Undo();
           h.Undo();
           h.Undo();

           Console.WriteLine(h.Value);
       }

       private class HistoryObject : IEnumerable<object>
       {
           public HistoryObject()
           {
               _history = new Stack<object>(); // Initiates the history stack.
           }

           public object Value
           {
               get // Returns the top value from the history if there is one. Otherwise null.
               {
                   if (_history.Count > 0)
                       return _history.Peek();
                   return null;
               }
               set { _history.Push(value); } // Adds the specified value to the history.
           }

           public void Undo()
           {
               if (_history.Count > 0)
                   _history.Pop(); // Removes the current value from the history.
           }

           // History stack that will hold all previous values of the object.
           private readonly Stack<object> _history;

           public IEnumerator<object> GetEnumerator()
           {
               return _history.GetEnumerator();
           }

           IEnumerator IEnumerable.GetEnumerator()
           {
               return GetEnumerator();
           }
       }
   }

}</lang>

Output:

foobar <- foo <- 5

Clojure

Clojure does not have history variables, but it can be accomplished via a watcher function that can track changes on a variable.

<lang clojure> (def a (ref 0)) (def a-history (atom [@a])) ; define a history vector to act as a stack for changes on variable a (add-watch a :hist (fn [key ref old new] (swap! a-history conj new))) </lang>

Sample Output:

user=> (dosync (ref-set a 1))
1
user=> (dosync (ref-set a 2))
2
user=> (dosync (ref-set a 3))
3
user=> @a-history
[0 1 2 3]
user=> (dotimes [n 3] (println (peek @a-history)) (swap! a-history pop))
3
2
1
nil
user=> @a-history
[0]

Common Lisp

In Lisp the list is a natural and simple data structure for representing variables with history. By adding some simple macros we can make a small DSL that hides the list and makes it look like a history variable is it's own special thing.

<lang lisp>(defmacro make-hvar (value)

 `(list ,value))

(defmacro get-hvar (hvar)

 `(car ,hvar))

(defmacro set-hvar (hvar value)

 `(push ,value ,hvar))

Make sure that setf macro can be used

(defsetf get-hvar set-hvar)

(defmacro undo-hvar (hvar)

 `(pop ,hvar))

(let ((v (make-hvar 1)))

 (format t "Initial value = ~a~%" (get-hvar v))
 (set-hvar v 2)
 (setf (get-hvar v) 3) ;; Alternative using setf
 (format t "Current value = ~a~%" (get-hvar v))
 (undo-hvar v)
 (undo-hvar v)
 (format t "Restored value = ~a~%" (get-hvar v)))

</lang>

Output:

Initial value = 1
Current value = 3
Restored value = 1

D

D does not have history variables. The following implementation provides a generic HistoryVariable that protects the historical values by defining them as 'const'.

<lang d>import std.stdio, std.array, std.string, std.datetime, std.traits;

/// A history variable. struct HistoryVariable(T) {

   /// A value in a point in time.
   static struct HistoryValue {
       SysTime time;
       T value;

       // Alternative to the more common toString.
       //void toString(scope void delegate(string) output) const {
       void toString(scope void delegate(const(char)[]) output)const {
           output(format("%s; %s", time, value));
       }
   }

   const(HistoryValue)[] values;

   private void addValue(T value) {
       values ~= HistoryValue(Clock.currTime(), value);
   }

   void opAssign(T value) {
       addValue(value);
   }

   @property T currentValue() const pure nothrow {
       return values.back.value;
   }

   alias currentValue this;

   @property auto history() const pure nothrow {
       return values;
   }

   /**
   Demonstrating D's compile-time reflection features. The member
   functions that are in this 'static if' block would be added for
   types T that are arrays (including strings). */
   static if (isArray!T) {
       // Append-with-assign operator.
       void opOpAssign(string op : "~")(T element) {
           addValue(currentValue ~ element);
       }

       // Similar implementation for other array operators...
   }

}

void main() {

   HistoryVariable!int x;
   x = 1;
   x = 2;
   x = 3;
   writefln("%(%s\n%)\n", x.history);

   HistoryVariable!string s;
   s = "hello";
   s ~= " world";
   s = "goodby";
   writefln("%(%s\n%)", s.history);

}</lang>

Sample Output:

2013-Jan-19 23:04:55.1660302; 1
2013-Jan-19 23:04:55.1660407; 2
2013-Jan-19 23:04:55.1660424; 3

2013-Jan-19 23:04:55.1662551; hello
2013-Jan-19 23:04:55.1662581; hello world
2013-Jan-19 23:04:55.1662596; goodby

EchoLisp

No native support. We implement an anonymous stack associated with the variable, and a few syntax rules to define the needed operations. <lang lisp> (define-syntax-rule (make-h-var name) (define name (stack (gensym)))) (define-syntax-rule (h-get name) (stack-top name)) (define-syntax-rule (h-set name value) (push name value)) (define-syntax-rule (h-undo name) (begin (pop name) (when ( stack-empty? name) (error "no more values" 'name)) (stack-top name)))

(define-syntax-rule (h-values name) (stack->list name))

usage

(make-h-var x)→ x

(h-set x 42) → 42 (h-set x 666)→ 666 (h-set x 'elvis)→ elvis (h-values x) → (42 666 elvis) ;; historized values

(h-get x) → elvis (h-undo x)→ 666 (h-undo x) → 42 (h-undo x) → ❌ error: no more values x </lang>

Elena

ELENA 3.4 : <lang elena>import extensions. import system'collections. import system'routines. import extensions'routines.

class HistoryVariable {

   T<Stack> _previous := Stack new.

   object prop Value :: _value.
   
   set Value:v
   [
       if (nil != _value)
       [
           _previous push(_value)
       ].

       _value := v
   ]    

   undo
   [
       ifnot (_previous isEmpty)
       [
           _value := _previous pop
       ];
       [
           _value := nilValue
       ]
   ]

   enumerator => _previous.

   T<literal> literal => _value.

}

public program [

   var o := HistoryVariable new.
   o Value := 5.
   o Value := "foo".
   o Value := o Value + "bar".

   console printLine(o).

   o forEach:printingLn.

   o undo; undo; undo.

   console printLine(o Value).

]</lang>

Output:

foobar
foo
5

Erlang

If we consider the usage of history variables, like not losing the old value by mistake or having a way to get an old value, then Erlang can really help. Being single assignment Erlang will not allow you to lose an old value, and it is certain that the previous values are always there. The downside is that you must handle this manually.

Output:

1> V1 = "123".
"123"
2> V1 = V1 ++ "qwe".
** exception error: no match of right hand side value "123qwe"
3> V2 = V1 ++ "qwe".
"123qwe"
4> V3 = V2 ++ "ASD".
"123qweASD"
5> V1. 
"123"
6> V2.
"123qwe"
7> V3.
"123qweASD"

Forth

Forth does not have history variables, but it is trivial to define them. This one uses a linked list. The history variable is initialized to zero to avoid expensive sanity checks in H@. H-- undoes the assignment of a value and effectively returns a history variable to its former state. <lang forth>: history create here cell+ , 0 , -1 , ;

h@ @ @ ;

h! swap here >r , dup @ , r> swap ! ;

.history @ begin dup cell+ @ -1 <> while dup ? cell+ @ repeat drop ;

h-- dup @ cell+ @ dup -1 = if abort" End of history" then swap ! ;</lang>

A sample session:

history z  ok
23 z h!  ok
z h@ . 23  ok
34 z h!  ok
z h@ . 34  ok
45 z h!  ok
z h@ . 45  ok
z .history 45 34 23  ok
z dup h-- h@ . 34  ok
z dup h-- h@ . 23  ok
z dup h-- h@ . 0  ok
z dup h-- h@ .
:86: End of history

Go

We're all in this for the extra points. Mallon and Takota seem happy with sequences, but time and timestamps were mentioned on LtU. Beyond a separate sequence for each history variable, timestamps enable multiple variables to be seen in a common temporal sequence. In Go, with it's attention to concurrency, this might be done with flexibility for proper handling of shared variables, and efficient handling of variables limited to a single thread. <lang go>package main

import (

   "fmt"
   "sort"
   "sync"
   "time"

)

// data type for history variable (its an int) type history struct {

   timestamp tsFunc
   hs        []hset

}

// data type for timestamp generator type tsFunc func() time.Time

// data type for a "set" event type hset struct {

   int           // new value
   t   time.Time // timestamp

}

// newHistory creates a history variable func newHistory(ts tsFunc) history {

   return history{ts, []hsetTemplate:T: ts()}

}

// int returns the current value func (h history) int() int {

   return h.hs[len(h.hs)-1].int

}

// set does what you expect and returns the timestamp recorded for the event func (h *history) set(x int) time.Time {

   t := h.timestamp()
   h.hs = append(h.hs, hset{x, t})
   return t

}

// dump displays a complete history func (h history) dump() {

   for _, hs := range h.hs {
       fmt.Println(hs.t.Format(time.StampNano), hs.int)
   }

}

// recall recalls the value stored in the history variable at time t. // if the variable had not been created yet, ok is false. func (h history) recall(t time.Time) (int, /*ok*/ bool) {

   i := sort.Search(len(h.hs), func(i int) bool {
       return h.hs[i].t.After(t)
   })
   if i > 0 {
       return h.hs[i-1].int, true
   }
   return 0, false

}

// newTimestamper returns a function that generates unique timestamps. // Use a single timestamper for multiple history variables to preserve // an unambiguous sequence of assignments across the multiple history // variables within a single goroutine. func newTimestamper() tsFunc {

   var last time.Time
   return func() time.Time {
       if t := time.Now(); t.After(last) {
           last = t
       } else {
           last.Add(1)
       }
       return last
   }

}

// newProtectedTimestamper generates unique timestamps for concurrent // goroutines. func newProtectedTimestamper() tsFunc {

   var last time.Time
   var m sync.Mutex
   return func() (t time.Time) {
       t = time.Now()
       m.Lock() // m protects last
       if t.After(last) {
           last = t
       } else {
           last.Add(1)
           t = last
       }
       m.Unlock()
       return
   }

}

func main() {

   // enable history variable support appropriate for single goroutine.
   ts := newTimestamper()
   // define a history variable
   h := newHistory(ts)
   // assign three values.  (timestamps kept for future reference.)
   ref := []time.Time{h.set(3), h.set(1), h.set(4)}
   // non-destructively display history
   fmt.Println("History of variable h:")
   h.dump() 
   // recall the three values.  (this is non-destructive as well, but
   // different than the dump in that values are recalled by time.)
   fmt.Println("Recalling values:")
   for _, t := range ref {
       rv, _ := h.recall(t)
       fmt.Println(rv)
   }

}</lang>

Output:

History of variable h:
Dec  3 18:51:17.292260000 0
Dec  3 18:51:17.292262000 3
Dec  3 18:51:17.292264000 1
Dec  3 18:51:17.292270000 4
Recalling values:
3
1
4

Haskell

There are no native Haskell history variables, but they are simple to implement.

<lang haskell>import Data.IORef

newtype HVar a = HVar (IORef [a])

newHVar :: a -> IO (HVar a) newHVar value = fmap HVar (newIORef [value])

readHVar :: HVar a -> IO a readHVar (HVar ref) = fmap head (readIORef ref)

writeHVar :: a -> HVar a -> IO () writeHVar value (HVar ref) = modifyIORef ref (value:)

undoHVar :: HVar a -> IO () undoHVar (HVar ref) = do

   (_ : history) <- readIORef ref
   writeIORef ref history

getHistory :: HVar a -> IO [a] getHistory (HVar ref) = readIORef ref

-- Testing main :: IO () main = do

   var <- newHVar 0
   writeHVar 1 var
   writeHVar 2 var
   writeHVar 3 var
   getHistory var >>= print
   undoHVar var
   undoHVar var
   undoHVar var</lang>

J

J does not natively support "history variables", but the functionality is easy to add:

<lang j>varref_hist_=:'VAR','_hist_',~] set_hist_=:4 :0

 V=.varref x
 if.0>nc<V do.(<V)=:end.
 (<V)=.V~,<y
 y

) getall_hist_=:3 :0

 (varref y)~

) length_hist_=: #@getall get_hist_=: _1 {:: getall</lang>

Example use:

<lang j> 'x' set_hist_ 9 9

  'x' set_hist_ 10

10

  'x' set_hist_ 11

11

  get_hist_ 'x'

11

  length_hist_ 'x'

3

  getall_hist_ 'x'

┌─┬──┬──┐ │9│10│11│ └─┴──┴──┘</lang>

Note that each value is contained in a box, so different values do not need to be type compatible with each other. If this is considered a defect then assertions could be added to enforce type compatibility across assignments.

Note that only nouns are supported here: If you want to store verbs using this mechanism you will need to use their gerunds.

Finally, note that the distinction between relevant history and irrelevant history depend on the application, and perhaps on the user. Also, necessity suggests that a lot of the history detail will be irrelevant for many purposes.

Java

Java does not support history variables, but they are easy to implement using the lists that come with Java's Collections framework.

Java Integer with History

This implementation does not allow the History Variable to be "empty". It can be assigned one or multiple "nulls", but it can never have an undefined value (such as being created and not initialized). <lang Java>import java.util.Collections; import java.util.LinkedList; import java.util.List;

/**

* A class for an "Integer with a history".

*

* Note that it is not possible to create an empty Variable (so there is no "null") with this type. This is a design * choice, because if "empty" variables were allowed, reading of empty variables must return a value. Null is a * bad idea, and Java 8's Optional<T> (which is somewhat like the the official fix for the null-bad-idea) would * make things more complicated than an example should be. */ public class IntegerWithHistory { /** * The "storage Backend" is a list of all values that have been ever assigned to this variable. The List is * populated front to back, so a new value is inserted at the start (position 0), and older values move toward the end. */ private final List<Integer> history; /** * Creates this variable and assigns the initial value * * @param value initial value */ public IntegerWithHistory(Integer value) { history = new LinkedList<>(); history.add(value); } /** * Sets a new value, pushing the older ones back in the history * * @param value the new value to be assigned */ public void set(Integer value) { //History is populated from the front to the back, so the freshest value is stored a position 0 history.add(0, value); } /** * Gets the current value. Since history is populuated front to back, the current value is the first element * of the history. * * @return the current value */ public Integer get() { return history.get(0); } /** * Gets the entire history all values that have been assigned to this variable. * * @return a List of all values, including the current one, ordered new to old */ public List<Integer> getHistory() { return Collections.unmodifiableList(this.history); } /** * Rolls back the history one step, so the current value is removed from the history and replaced by it's predecessor. * This is a destructive operation! It is not possible to rollback() beyond the initial value! * * @return the value that had been the current value until history was rolled back. */ public Integer rollback() { if (history.size() > 1) { return history.remove(0); } else { return history.get(0); } } } </lang> Test Program: <lang Java>public class TestIntegerWithHistory { public static void main(String[] args) { //creating and setting three different values IntegerWithHistory i = new IntegerWithHistory(3); i.set(42); i.set(7); //looking at current value and history System.out.println("The current value of i is :" + i.get()); System.out.println("The history of i is :" + i.getHistory()); //demonstrating rollback System.out.println("Rolling back:"); System.out.println("returns what was the current value: " + i.rollback()); System.out.println("after rollback: " + i.get()); System.out.println("returns what was the current value: " + i.rollback()); System.out.println("after rollback: " + i.get()); System.out.println("Rolling back only works to the original value: " + i.rollback()); System.out.println("Rolling back only works to the original value: " + i.rollback()); System.out.println("So there is no way to 'null' the variable: " + i.get()); } }</lang>

Output:

The current value of i is :7
The history of i is :[7, 42, 3]
Rolling back:
returns what was the current value: 7
after rollback: 42
returns what was the current value: 42
after rollback: 3
Rolling back only works to the original value: 3
Rolling back only works to the original value: 3
So there is no way to 'null' the variable: 3

Java History for "any class" using Generics

This implementation does not allow the History Variable to be "empty". It can be assigned one or multiple "nulls", but it can never have an undefined value (such as being created and not initialized). <lang java></lang> Test Program using a "String with History": <lang java>import java.util.Collections; import java.util.LinkedList; import java.util.List;

/**

* A Generic class for an "Anything with a history".

*

* Note that it is not possible to create an empty Variable (so there is no "null") with this type. This is a design * choice, because if "empty" variables were allowed, reading of empty variables must return a value. Null is a * bad idea, and Java 8's Optional<T> (which is somewhat like the the official fix for the null-bad-idea) would * make things more complicated than an example should be. *

* Also note that this "really works" only with constant Ts. If somebody keeps a reference to an assigned value, * and is able to modify the state of this value through the reference , this will not be reflected in the history! */ public class WithHistory<T> { /** * The "storage Backend" is a list of all values that have been ever assigned to this variable. The List is * populated front to back, so a new value is inserted at the start (position 0), and older values move toward the end. */ private final List<T> history; /** * Creates this variable and assigns the initial value * * @param value initial value */ public WithHistory(T value) { history = new LinkedList<>(); history.add(value); } /** * Sets a new value, pushing the older ones back in the history * * @param value the new value to be assigned */ public void set(T value) { //History is populated from the front to the back, so the freshest value is stored a position 0 history.add(0, value); } /** * Gets the current value. Since history is populuated front to back, the current value is the first element * of the history. * * @return the current value */ public T get() { return history.get(0); } /** * Gets the entire history all values that have been assigned to this variable. * * @return a List of all values, including the current one, ordered new to old */ public List<T> getHistory() { return Collections.unmodifiableList(this.history); } /** * Rolls back the history one step, so the current value is removed from the history and replaced by it's predecessor. * This is a destructive operation! It is not possible to rollback() beyond the initial value! * * @return the value that had been the cueent value until history was rolled back. */ public T rollback() { if (history.size() > 1) { return history.remove(0); } else { return history.get(0); } } } </lang>

Output:

The current value is :And now to something completely different.
The history is:[And now to something completely different., See you later History!, Hello History!]
Rolling back:
returns what was the current value: And now to something completely different.
after rollback: See you later History!
returns what was the current value: See you later History!
after rollback: Hello History!
Rolling back only works to the original value: Hello History!
Rolling back only works to the original value: Hello History!
So there is no way to 'null' the variable: Hello History!

Kotlin

<lang scala>// version 1.1.4

class HistoryVariable<T>(initialValue: T) {

   private val history = mutableListOf<T>()

   var currentValue: T
       get() = history[history.size - 1]
       set(value) {
          history.add(value)
       }

   init {
       currentValue = initialValue
   }

   fun showHistory() {
       println("The variable's history, oldest values first, is:")
       for (item in history) println(item)
   }

}

fun main(args: Array<String>) {

   val v = HistoryVariable(1)
   v.currentValue = 2
   v.currentValue = 3
   v.showHistory()
   println("\nCurrentvalue is ${v.currentValue}")

}</lang>

Output:

The variable's history, oldest values first, is:
1
2
3

Currentvalue is 3

M2000 Interpreter

We can make objects type of Group to act as History Variables. Each group has a pointer to a stack object (a linked list). First we see how we work with Stack (each module/function see a "current stack", we can make pointers to new stacks, but we can't get pointer of current stack). There are some statement no demonstrate here, such as Shift and ShiftBack (move top to a place, and move back to top), and Over (make new items by copying items).

<lang M2000 Interpreter> Flush ' empty curtrent stack \\ a is a pointer to a new stack object a=stack:=1,2@,3% Print Len(A)=3 For i=1 to Len(a) {

     Print StackType$(a, i)="Number"

} b=stack Print len(b)=0 Push 1, 2 \\ to current stack Stack b {

     \\ make b the current stack
     Data 1, 2, 3

} ' Data add to bottom \\ now current stack get the old object Stack b {Push 1, 2, 3 } ' Push add to top, so 3 is at top Stack b {

     While not empty {
           Read k  ' Read used to pop value to a variable
           \\ number pop value in an expression
           Print  k, number,  ' 3 2 1 1 2 3
     }
     Print

} z=[] ' [] pop all values from current stack to a new stack, and return a pointer Print z ' 2 1 z=Stack(a, z,a, z) ' merge stack objects, to a new one, preserve a and z Print Len(z) \\ empty a using a new object a=stack b=stack:=1,2,3 \\ z has two stack objects z=stack:= a, b Stack b { data 1000} b=stack ' b point to a new stack \\ we get pointer back b=stackitem(z, 2) Print stackitem(b, 4)=1000 </lang>

So now lets see the code:

<lang M2000 Interpreter> Module CheckHistoryVariables {

     Class History {
     Private:
           myvalue=stack
     Public:
           Group Count {
                 Value {
                       link parent myvalue to m 
                       =Len(m)
                 }
           }
           Function CopyMe {
                 m=This
                 .myvalue<=stack(.myvalue)
                 =Group(m)
           }
           Group CopyValues {
                 Value {
                       link parent myvalue to m
                       =Stack(m)
                 }
           }
           Function Values (x as long) {
                 if x>=0 and x<=len(.myvalue) then  =StackItem(.myvalue, x)
                 Else Error "Fault index"
           }
           Module Back {
                 if len(.myvalue)<2 then exit
                Stack .myvalue {Drop}
           }
           Operator "++" {
                 Stack .myvalue {
                       Push stackitem()+1
                 }
           }
           Operator "--" {
                 Stack .myvalue {
                       Push stackitem()-1
                 }
           }
           Operator "+="  (x) {
                 Stack .myvalue {
                       Push stackitem()+x
                 }       }
           Operator "-=" (x) {
                 Stack .myvalue {
                       Push stackitem()-x
                 }       
           }
           Operator "/=" (x){
                 if x==0 then error "division by zero"
                 Stack .myvalue {
                       Push stackitem()/x
                 } 
           }
           Operator "*=" (x){
                 Stack .myvalue {
                       Push stackitem()*x
                 }             
           }
           Value {
                 =StackItem(.myvalue)
           }
           Set {
                  Read x : Stack .myvalue { Push x}
           }
     Class:
           Module History {
                 If Match("N") then Read x : Stack .myvalue { Push x}
           }
     }
     
     N=History()
     N=1
     N=2
     N=3
     Print N, N.Values(2), N.Values(3), N.Count
     \\ Get a copy of history
     m=N.CopyValues
     Print len(m)=3
     \\ just show all
     Print m  ' 3 2 1
     \\ or iterate from last to first
     k=each(m, -1, 1)
     While k {
           Print Stackitem(k)  \\  1 NL 2 NL 3  (NL = new line)
     }
     N1=N.CopyMe()
     N=5
     N1=4
     N=6
     Print N
     Print N1.Count=4, N.Count=5
     Print N1, N
     Print N.CopyValues ' 6 5 3 2 1
     Print N1.CopyValues ' 4 3 2 1
     Print N1<N
     N=N+1
     Print N.Count=6
     Print N.CopyValues ' 6 5 3 2 1
     Print "Go Back"
     While N.Count>1 {
           N.Back
           Print N
     }
     N+=10
     N*=2
     Print N.CopyValues  ' 22 11 1

} CheckHistoryVariables

\\ for strings Module CheckStringHistoryVariables {

     Class History$ {
     Private:
           myvalue=stack
     Public:
           Group Count {
                 Value {
                       link parent myvalue to m 
                       =Len(m)
                 }
           }
           Function CopyMe {
                 m=This
                 .myvalue<=stack(.myvalue)
                 =Group(m)
           }
           Group CopyValues {
                 Value {
                       link parent myvalue to m
                       =Stack(m)
                 }
           }
           Function Values$ (x as long) {
                 if x>=0 and x<=len(.myvalue) then  =StackItem$(.myvalue, x)
                 Else Error "Fault index"
           }
           Module Back {
                 if len(.myvalue)<2 then exit
                Stack .myvalue {Drop}
           }
           Operator "+="  (x$) {
                 Stack .myvalue {
                       Push stackitem$()+x$
                 }       }
           Value {
                 =StackItem$(.myvalue)
           }
           Set {
                  Read x$ : Stack .myvalue { Push x$}
           }
     Class:
           Module History {
                 If Match("S") then Read x$ : Stack .myvalue { Push x$}
           }
     }
     N$=History$("First")
     N$="Second"
     N$="Third"
     Print N.Count=3
     M=N.CopyValues
     K=Each(M, -1, 1)
     While K {
           Print StackItem$(K)
     }
     N$+=" and this"
     Print N.Count=4
     Print N.CopyValues
     N.Back
     Print N.Count=3
     Print N.CopyValues
     Print N.Values$(2)="Second"
     Input "New value:", N$
     Print N$
     Print N.Count=4
     Print N.CopyValues

} CheckStringHistoryVariables </lang>

Oberon-2

<lang oberon2> MODULE HVar; IMPORT Out, Conv; TYPE (* Generic Object *) Object* = POINTER TO ObjectDesc; ObjectDesc = RECORD Show: PROCEDURE(o:Object); AsString: PROCEDURE(o: Object; VAR str: ARRAY OF CHAR); END;

(* Integers *) Integer = POINTER TO IntegerDesc; IntegerDesc = RECORD (ObjectDesc) val: INTEGER; END;

(* Chars *) Char = POINTER TO CharDesc; CharDesc = RECORD (ObjectDesc) val: CHAR; END;

Node = POINTER TO NodeDesc; NodeDesc = RECORD val: Object; next: Node; END;

(* History Variable *) HVar* = POINTER TO HVarDesc; HVarDesc = RECORD first, last: Node; size-: INTEGER; END;

PROCEDURE CharAsString(o:Object; VAR dst: ARRAY OF CHAR); BEGIN IF LEN(dst) >= 2 THEN dst[1] := 0X; END; dst[0] := o(Char)^.val END CharAsString;

PROCEDURE IntAsString(o:Object; VAR dst: ARRAY OF CHAR); BEGIN Conv.ConvInt(o(Integer)^.val,dst); END IntAsString;

PROCEDURE ShowInt(o:Object); BEGIN Out.Int(o(Integer)^.val,10); END ShowInt;

PROCEDURE ShowChar(o:Object); BEGIN Out.Char(o(Char)^.val); END ShowChar;

PROCEDURE BoxChar(val: CHAR): Char; VAR c: Char; BEGIN NEW(c); c^.val := val; c^.Show := ShowChar; c^.AsString := CharAsString; RETURN c; END BoxChar;

PROCEDURE BoxInt(val:INTEGER): Integer; VAR i: Integer; BEGIN NEW(i); i^.val := val; i^.Show := ShowInt; i^.AsString := IntAsString; RETURN i; END BoxInt;

PROCEDURE InitNode(): Node; VAR l: Node; BEGIN NEW(l); l.val := NIL; l.next := NIL; RETURN l; END InitNode;

PROCEDURE InitHVar(): HVar; VAR hv: HVar; BEGIN NEW(hv); hv.first := NIL; hv.last := NIL; hv.size := 0; RETURN hv; END InitHVar;

PROCEDURE (v: HVar) Value(): Object; BEGIN RETURN v^.first^.val; END Value;

PROCEDURE (v: HVar) Set(o: Object); VAR l: Node; BEGIN l := InitNode(); l^.val := o; IF (v^.first = v^.last) & (v^.first = NIL) THEN v^.first := l; v^.last := l; INC(v^.size); ELSIF (v^.first = v^.last) & (v^.first # NIL) THEN v^.first^.next := l; v^.last := v^.first.next; INC(v^.size); ELSIF (v^.first # v^.last) THEN v^.last^.next := l; v^.last := l; INC(v^.size); END END Set;

PROCEDURE (v: HVar) Undo(): Object; VAR o: Object; BEGIN IF (v^.first = v^.last) & (v^.first = NIL) THEN o := NIL; ELSIF (v^.first = v^.last) & (v^.first # NIL) THEN o := v^.first^.val; v^.first := NIL; v^.last := NIL; DEC(v^.size); ELSE o := v^.first^.val; v^.first := v^.first^.next; DEC(v^.size); END; RETURN o; END Undo;

PROCEDURE (v: HVar) Print(); VAR iter : Node; BEGIN iter := v.first; WHILE(iter # NIL) DO iter^.val^.Show(iter^.val); iter := iter^.next; END; Out.Ln; END Print;

PROCEDURE ShowVal(val: Object); VAR s: ARRAY 128 OF CHAR; BEGIN val^.AsString(val,s); Out.String("> ");Out.String(s);Out.Ln; END ShowVal; VAR history: HVar;

BEGIN history := InitHVar(); history.Set(BoxChar(64X)); history.Set(BoxInt(10)); history.Set(BoxInt(15)); history.Set(BoxInt(20)); history.Print(); ShowVal(history.Undo()); ShowVal(history.Undo()); ShowVal(history.Undo()); END HVar. </lang>

OCaml

The easiest solution is to use the Stack module coming with OCaml's standard library: <lang ocaml> open Stack (* The following line is only for convenience when typing code *) module H = Stack

let show_entry e =

   Printf.printf "History entry: %5d\n" e

let () =

 let  hs = H.create() in
 H.push 111 hs ;
 H.push 4 hs ;
 H.push 42 hs ;
 H.iter show_entry hs;
 hs |> H.pop |> Printf.printf "%d\n";
 hs |> H.pop |> Printf.printf "%d\n";
 hs |> H.pop |> Printf.printf "%d\n"

</lang>

Output:

History entry:    42
History entry:     4
History entry:   111
42
4
111

OxygenBasic

Simple history class for fixed length types that do not contain volatile pointer members. <lang oxygenbasic> '============ class History '============

indexbase 0

string buf sys ii,ld,pb

method constructor(sys n=1000, l=sizeof sys) {buf=nuls n*l : pb=strptr buf : ld=l : ii=0} method destructor () {clear} ' method setup(sys n=1000, l=sizeof sys) {buf=nuls n*l : pb=strptr buf : ld=l : ii=0} method clear() {buf="" : pb=0 : ld=0 : ii=0} method max (sys i) {if i>ii{ii=i}} method count() as sys {return ii} method size () as sys {return ld} ' method get (any*p,i) {copy @p, pb+i*ld,ld } 'out method add (any*p) {copy pb+ii*ld,@p,ld : ii++} 'in method put (any*p,sys i) {copy pb+i*ld,@p,ld : max i} 'in ' end class

'==== 'TEST '====

'this works for fixed length types

'it will not work for types containing 'volatile pointers. eg: string members

type vector double x,y,z

vector v

new History hv(1000,sizeof v) 'give number of records and variable size

sys i for i=0 to 9

 v<=i,i*10,i*100 'assign new values to vector
 hv.add v      'add to history

 hv.get sv,i
 pr+=i tab sv.x tab sv.y tab sv.z cr

del hv </lang>

PARI/GP

<lang parigp>default(histsize, 1000) \\ or some other positive number to suit 1+7 sin(Pi) 2^100 \a1 \\ display history item #1, etc. % \\ alternate syntax %1 \\ alternate syntax \a2 \a3 [%1, %2, %3] \\ or any other command using these values</lang>

Peloton

<lang sgml>Turn history on <@ DEFHST>__on</@> Notify Protium we are interested in the variable mv <@ DEFHST>mv</@> Assign a value: <@ LETVARLIT>mv|first value</@><@ SAYVAR>mv</@> Reassign the value: <@ LETVARLIT>mv|second value</@><@ SAYVAR>mv</@> Reassign the value: <@ LETVARLIT>mv|third value</@><@ SAYVAR>mv</@> Dump history <@ SAYDMPHSTVAR>mv</@> Current value: <@ SAYVAR>mv</@> Undo once: <@ ACTUNDVAR>mv</@><@ SAYVAR>mv</@> Undo twice: <@ ACTUNDVAR>mv</@><@ SAYVAR>mv</@> Turn history off <@ DEFHST>__off</@></lang>

Same code, Simplified Chinese dialect <lang sgml>Turn history on <# 定义变量史>__on</#> Notify Protium we are interested in the variable mv <# 定义变量史>mv</#> Assign a value: <# 指定变量字串>mv|first value</#><# 显示变量>mv</#> Reassign the value: <# 指定变量字串>mv|second value</#><# 显示变量>mv</#> Reassign the value: <# 指定变量字串>mv|third value</#><# 显示变量>mv</#> Dump history <# 显示全内容变量史变量>mv</#> Current value: <# 显示变量>mv</#> Undo once: <# 运行撤消变量>mv</#><# 显示变量>mv</#> Undo twice: <# 运行撤消变量>mv</#><# 显示变量>mv</#> Turn history off <# 定义变量史>__off</#> </lang>

Output:

Turn history on  
Notify Protium we are interested in the variable mv 
 
Assign a value: first value 
Reassign the value: second value 
Reassign the value: third value 
Dump history third value^second value^first value^ 
Current value: third value
Undo once: second value 
Undo twice: first value 
Turn history off

Perl

Implemented via tie (and what's the usefulness of this?) <lang Perl>package History;

sub TIESCALAR { my $cls = shift; my $cur_val = shift; return bless []; }

sub FETCH { return shift->[-1] }

sub STORE { my ($var, $val) = @_; push @$var, $val; return $val; }

sub get(\$) { @{tied ${+shift}} } sub on(\$) { tie ${+shift}, __PACKAGE__ } sub off(\$) { untie ${+shift} } sub undo(\$) { pop @{tied ${+shift}} }

package main;

my $x = 0; History::on($x);

for ("a" .. "d") { $x = $_ }

print "History: @{[History::get($x)]}\n";

for (1 .. 3) { print "undo $_, "; History::undo($x); print "current value: $x\n"; }

History::off($x); print "\$x is: $x\n";</lang>

Output:

<lang>History

a b c d

undo 1, current value: c undo 2, current value: b undo 3, current value: a $x is: a</lang>

Perl 6

Works with: Rakudo version 2018.03

<lang perl6>class HistoryVar {

   has @.history;
   has $!var handles <Str gist FETCH Numeric>;
   method STORE($val) is rw {
       push @.history, [now, $!var];
       $!var = $val;
   }

}

my \foo = HistoryVar.new;

foo = 1; foo = 2; foo += 3; foo = 42;

.say for foo.history; say "Current value: {foo}";</lang>

Output:

[Instant:1523396079.685629 (Any)]
[Instant:1523396079.686844 1]
[Instant:1523396079.687130 2]
[Instant:1523396079.687302 5]
Current value: 42

Phix

No native support, but trivial to implement.
If you only need the history for a single variable, you can just do this: <lang Phix>sequence history = {}

type hvt(object o)

   history = append(history,o)
   return true

end type

hvt test = 1 test = 2 test = 3 ?{"current",test} ?{"history",history}</lang>

Output:

{"current",3}
{"history",{1,2,3}}

Multiple history variables would require that routines must be invoked to create, update, and inspect them.
Writing this as a separate reusable component (but omitting destroy/freelist handling for simplicity): <lang Phix>-- history.e sequence histories = {}

global function new_history_id(object v)

   histories = append(histories,{v})
   return length(histories)

end function

global procedure set_hv(integer hv, object v)

   histories[hv] = append(histories[hv],v)

end procedure

global function get_hv(integer hv)

   return histories[hv][$]

end function

global function get_hv_full_history(integer hv)

   return histories[hv]

end function</lang> And use it like this <lang Phix>include history.e

constant test2 = new_history_id(1) set_hv(test2, 2) set_hv(test2, 3) ?{"current",get_hv(test2)} ?{"history",get_hv_full_history(test2)}</lang> Same output. Of course test2 does not have to be a constant, but it may help.

PicoLisp

<lang PicoLisp>(de setH ("Var" Val)

  (when (val "Var")
     (with "Var"
        (=: history (cons @ (: history))) ) )
  (set "Var" Val) )

(de restoreH ("Var")

  (set "Var" (pop (prop "Var" 'history))) )</lang>

Test:

: (setH 'A "Hello world")
-> "Hello world"

: (setH 'A '(a b c d))
-> (a b c d)

: (setH 'A 123)
-> 123

: A
-> 123

: (get 'A 'history)
-> ((a b c d) "Hello world")

: (restoreH 'A)
-> (a b c d)

: (restoreH 'A)
-> "Hello world"

: A
-> "Hello world"

: (restoreH 'A)
-> NIL

PL/I

<lang PL/I> declare t float controlled;

do i = 1 to 5; /* a loop to read in and save five values. */ allocate t; get (t); end;

do while (allocation(t) > 0); /* a loop to retrieve the values. */ put (t); free t; end; </lang>

PureBasic

<lang PureBasic>; integer history variable

Structure historyint

   List value.i()

EndStructure

Procedure SetInt (*var.historyint, val.i)

   AddElement(*var\value())
   
   If (ListSize(*var\value()) = 1)
       *var\value() = 0
       AddElement(*var\value())

   EndIf
   
   *var\value() = val

EndProcedure

Procedure ShowHistory (*var.historyint)

   ForEach *var\value()
       Debug *var\value()
   Next

EndProcedure

Procedure UndoInt (*var.historyint)

   If (ListSize(*var\value()) = 1)
       ProcedureReturn
   EndIf
   
   DeleteElement(*var\value())

EndProcedure

----------------------------------------------

Define x.historyint

For i = 0 To 3

   setint(x, Random(50)+100 )

Debug ""

For i = 0 To 3

   UndoInt(x)
   Debug "undo, x = "+Str(x\value())

Next </lang>

Output:

x history:
0
131
109
143
108

undo, x = 143
undo, x = 109
undo, x = 131
undo, x = 0

Python

<lang Python>import sys

HIST = {}

def trace(frame, event, arg):

   for name,val in frame.f_locals.items():
       if name not in HIST:
           HIST[name] = []
       else:
           if HIST[name][-1] is val:
               continue
       HIST[name].append(val)
   return trace

def undo(name):

   HIST[name].pop(-1)
   return HIST[name][-1]

def main():

   a = 10
   a = 20

   for i in range(5):
       c = i

   print "c:", c, "-> undo x3 ->",
   c = undo('c')
   c = undo('c')
   c = undo('c')
   print c
   print 'HIST:', HIST

sys.settrace(trace) main()</lang>

Output:

<lang>c

4 -> undo x3 -> 1

HIST: {'a': [10, 20], 'i': [0, 1, 2, 3, 4], 'c': [0, 1], 'name': ['c']}</lang>

Racket

Racket does not come with history variables, but they can be provided as a library as follows:

lang racket

(require rackunit)

(struct hvar (current history) #:mutable)

(define (make-hvar v) (hvar v empty))

(define (hvar-set! hv new)

 (match-define (hvar cur hist) hv)
 (set-hvar-history! hv (cons cur hist))
 (set-hvar-current! hv new))

(define (hvar-undo! hv)

 (match-define (hvar cur (cons old hist)) hv)
 (set-hvar-current! hv old)
 (set-hvar-history! hv hist))

unit tests

(define hv (make-hvar 0)) (hvar-set! hv 1) (check-equal? (hvar-current hv) 1) (hvar-set! hv 2) (hvar-set! hv 3) (check-equal? (hvar-history hv) '(2 1 0)) (hvar-undo! hv) (hvar-undo! hv) (hvar-undo! hv) (check-equal? (hvar-current hv) 0) </lang>

REXX

Version 1

The REXX language doesn't support histories, but can be coded with little trouble.

The history list part of the &nbsp' varSet subroutine could be separated into its
own if you wanted to keep the subroutine's function pure. <lang rexx>/*REXX program demonstrates a method to track history of assignments to a REXX variable.*/ varSet!.=0 /*initialize the all of the VARSET!'s. */ call varSet 'fluid',min(0,-5/2,-1) ; say 'fluid=' fluid call varSet 'fluid',3.14159 ; say 'fluid=' fluid call varSet 'fluid',' Santa Claus' ; say 'fluid=' fluid call varSet 'fluid',,999 say 'There were' result "assignments (sets) for the FLUID variable." exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ varSet: arg ?x; parse arg ?z, ?v, ?L /*obtain varName, value, optional─List.*/ if ?L== then do /*not la ist, so set the X variable.*/

              ?_=varSet!.0.?x+1                 /*bump the history count  (# of SETs). */
              varSet!.0.?x=?_                   /*   ... and store it in the "database"*/
              varSet!.?_.?x=?v                  /*   ... and store the  SET  value.    */
              call value(?x),?v                 /*now,  set the real  X  variable.     */
              return ?v                         /*also, return the value for function. */
              end

say /*show a blank line for readability. */

           do ?j=1 to ?L while ?j<=varSet!.0.?x /*display the list of  "set"  history. */
           say 'history entry' ?j "for var" ?z":" varSet!.?J.?x
           end   /*?j*/

return ?j-1 /*return the number of assignments. */</lang> output

fluid= -2.5
fluid= 3.14159
fluid=  Santa  Claus

history entry 1 for var fluid: -2.5
history entry 2 for var fluid: 3.14159
history entry 3 for var fluid:  Santa  Claus
There were 3 assignments (sets) for the FLUID variable.

Version 2

<lang rexx> /* REXX ***************************************************************

Demonstrate how the history of assignments can be kept and shown
13.07.2012 Walter Pachl Rewrite of Version 1 for (my) readability
varset.i.varu contains the ith value assigned to var
varset.0.varu contains the number of assignments so far
- - - - /

varset.=0 /*initialize the assignment count */

call varset 'fluid',min(0,-5/2,-1) ; say 'fluid=' fluid call varset 'fluid',3.14159 ; say 'fluid=' fluid call varset 'fluid',3.14159 ; say 'fluid=' fluid call varset 'fluid',' Santa Claus' ; say 'fluid=' fluid call varset 'FLUID',' Easter Rabbit' ; say 'fluid=' fluid

say 'There were' varset('fluid',,'L'),

   'assignments (sets) for the FLUID variable.'

exit

varset: Procedure Expose varset. /**********************************************************************

record values assigned to var (=varu as Rexx is case insensitive)
Invoke with list<> to list the sequence of assigned values
and return the number of assignments made (using this routine)
- - - - /

 Parse Upper Arg varu              /* name of variable in uppercase */
 Parse arg var,value,list          /*varName, value, optional-List. */

 if list= then do                /*not list, so set the X variable*/
   z=varset.0.varu+1               /*bump the history count (SETs). */
   varset.z.varu=value             /* ... and store it in "database"*/
   varset.0.varu=z                 /*the new history count          */
   call value var,value            /*now assign the value to var    */
   end
 else Do
   Say                           /*show blank line for readability*/
   do i=1 to varset.0.varu         /*list the assignment history    */
     say 'history entry' i "for var" var":" varset.i.varu
     end
   end
 Return varset.0.varu           /*return the number of assignments. */

</lang>

Ruby

This uses trace_var, which only works for global variables: <lang ruby>foo_hist = [] trace_var(:$foo){|v| foo_hist.unshift(v)}

$foo = "apple" $foo = "pear" $foo = "banana"

p foo_hist # => ["banana", "pear", "apple"] </lang>

Scala

Scala doesn't have a native support for history variables, but it's quite easy to implement them. The following class uses same conventions as ML's mutable reference cells. (i.e. ! as accessor, and := as mutator.) <lang scala>class HVar[A](initialValue: A) extends Proxy {

 override def self = !this
 override def toString = "HVar(" + !this + ")"
 
 def history = _history

 private var _history = List(initialValue)
 def unary_! = _history.head
 def :=(newValue: A): Unit = {
   _history = newValue :: _history
 }
 def modify(f: A => A): Unit = {
   _history = f(!this) :: _history
 }
 def undo: A = {
   val v = !this
   _history = _history.tail
   v
 }

}</lang> Usage: <lang scala>scala> val h = new HVar(3) h: HVar[Int] = HVar(3)

scala> h := 11

scala> h := 90

scala> !h res32: Int = 90

scala> h.history res33: List[Int] = List(90, 11, 3)

scala> h.undo res34: Int = 90

scala> h.undo res35: Int = 11

scala> h.undo res36: Int = 3</lang>

Sidef

Implemented as a class: <lang ruby>class HistoryVar(v) {

   has history = []
   has variable = v

   method ≔(value) {
       history << variable
       variable = value
   }

   method to_s {
       "#{variable}"
   }

   method AUTOLOAD(_, name, *args) {
       variable.(name)(args...)
   }

}

var foo = HistoryVar(0)

foo ≔ 1 foo ≔ 2 foo ≔ foo+3 foo ≔ 42

say "History: #{foo.history}" say "Current value: #{foo}"</lang>

Output:

History: [0, 1, 2, 5]
Current value: 42

Smalltalk

Smalltalk doesn't have native support for history variables. It could be implemented using (implementation dependent) tracing facilities, or by changing the code generator (which is part of the library and open for change). The following implementation is portable and in the spirit of the Lisp implementation above, and defines a new "assignment operator": <lang smalltalk>Object subclass:'HVar'

   instanceVariableNames:'values' 
   classVariableNames: 
   poolDictionaries: 
   category:'example'.

!

!HVar methodsFor:'accessing'!

<-- value

   (values ifNil:[values := OrderedCollection new]) add:value.
   ^  value

!

value

   ^ values last

!

undo

   values removeLast.

!

history

   ^ history

! !

|x|

x := HVar new. x <-- 1. x value. x <-- 2. x <-- (x value + 1). x value. x history. </lang>

Swift

Swift does not support history variables. However, you can add a watcher that can track when the variable will change. <lang Swift>var historyOfHistory = [Int]() var history:Int = 0 {

   willSet {
       historyOfHistory.append(history)
   }

}

history = 2 history = 3 history = 4 println(historyOfHistory) </lang>

another approach, using generics: plug this code into a Playground to see the output

Works with: Swift version 2.x+

struct History <T> {

   private var _history = [T]()
   var history : [T] {
       return _history
   }
   
   var now : T {
       return history.last!
   }
   
   init(_ firstValue:T) {
       _history = [firstValue]
   }
   
   mutating func set(newValue:T) {
       _history.append(newValue)
   }
   
   mutating func undo() -> T {
       guard _history.count > 1 else { return _history.first! }
       _history.removeLast()
       return _history.last!
   }

}

var h = History("First") h.set("Next") h.set("Then") h.set("Finally") h.history // output ["First", "Next", "Then", "Finally"]

h.now // outputs "Finally" h.undo() // outputs "Then" h.undo() // outputs "Next" h.undo() // outputs "First" h.undo() // outputs "First", since it can't roll back any further h.undo() // outputs "First"

</lang>

Tcl

Though Tcl's variables don't have history by default, it can be added easily through the use of traces: <lang tcl># Define the history machinery proc histvar {varName operation} {

   upvar 1 $varName v ___history($varName) history
   switch -- $operation {

start { set history {} if {[info exist v]} { lappend history $v } trace add variable v write [list histvar.write $varName] trace add variable v read [list histvar.read $varName] } list { return $history } undo { set history [lrange $history 0 end-1] } stop { unset history trace remove variable v write [list histvar.write $varName] trace remove variable v read [list histvar.read $varName] }

} proc histvar.write {key varName args} {

   upvar 1 $varName v ___history($key) history
   lappend history $v

} proc histvar.read {key varName args} {

   upvar 1 $varName v ___history($key) history
   set v [lindex $history end]

}</lang> Demonstrating how to use it: <lang tcl># Enable history for foo histvar foo start set foo {a b c d} set foo 123 set foo "quick brown fox" puts $foo puts foo-history=[join [histvar foo list] ", "] puts $foo histvar foo undo puts $foo histvar foo undo puts $foo histvar foo stop</lang>

Output:

quick brown fox
foo-history=a b c d, 123, quick brown fox
quick brown fox
123
a b c d

zkl

No native support, here is a something that can be done with a class: <lang zkl>class HistoryVar{

  var [private] _v, _history=List(), maxSz;
  fcn init(v,maxEntries=3){ maxSz=maxEntries; set(v) }
  fcn set(v){ 
     _v=v; _history.append(T(Time.Clock.time,v));
     if(_history.len()>maxSz) _history.del(0);
     self
  }
  fcn get(n=0){  // look back into history
     z:=_history.len();
     n=(if(n>=z) 0 else z-n-1);
     _history[n][1] 
  }
  var [proxy] v=fcn{ _v };
  var [proxy] history=
     fcn{ _history.pump(List,fcn([(t,v)]){ T(Time.Date.ctime(t),v) }) };
  fcn __opAdd(x){ set(_v + x); self }

}</lang> <lang zkl>hv:=HistoryVar(123); hv+4; hv.set("Shuttle prepared for liftoff"); hv+": orbit achived"; hv.history.concat("\n").println(); hv.get(3).println("<-- value two changes ago");</lang>

Output:

L("Thu Oct 13 13:48:37 2016",127)
L("Thu Oct 13 13:48:37 2016","Shuttle prepared for liftoff")
L("Thu Oct 13 13:48:37 2016","Shuttle prepared for liftoff: orbit achived")
127<-- value two changes ago