i totally agree with you wrt the value of a REPL. i've done a fair amount of work in matlab, and i would mock up a method and work in the command line interface while stopped in the debugger until i got something working and then paste it back into the editor
beanshell2 for java isn't bad, but it's not integrated into the debugger so it doesn't help much. i use netbeans as my IDE and the "evaluate expression" debugger tool is decent for java, as is gdb for c. but all 3 pale in comparison to the matlab environment. someday, i aspire to have the time to make a command line repl plugin for netbeans
So I'm wondering what else can one get with the new approach that is substantially better than a REPL-based development environment.
the only thing new that i see McDirmid proposing is that the output is continuously updated as you edit. that doesn't sound very useful. i'd prefer a REPL that's integrated with the debugger