History variables

History variables is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

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.

"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.

Protium

<lang protium>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 protium>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>

Sample 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