Initial data commit

Task/History-variables/Tcl/history-variables-1.tcl

@@ -0,0 +1,33 @@
+# 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]
+}

Task/History-variables/Tcl/history-variables-2.tcl

@@ -0,0 +1,13 @@
+# 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