Mutable REPL

Would be great to have a REPL with mutable definitions in another pane. The definitions pane lists all state, in the form:

<expr num>  <name> = <original evaled expression>

And it reassigns variables as the expressions are edited, on every keystroke (if it evaluates. if not, display eval error in a gutter below).

I hope LightTable already does this. And, I wish a REPL-UI lib existed to simplify this and other common aspects of writing REPLs. And repl <lang> should also exist.