FlightGear provides a interpreter for debugging short pieces of Nasal code, the so called "Nasal REPL interpreter" is available in FlightGear's menu (Debug > Nasal REPL interpreter).
Usage
ESC —— Exit/close the dialog.
Ctrl-d —— Same as ESC.
Ctrl-v —— Insert text (at the end of the current line).
Ctrl-c —— Copy the current line of text.
Ctrl-x —— Copy and delete the current line of text.
Up —— Previous line in history.
Down —— Next line in history.
Gallery
Philosopher's interactive Nasal console REPL.
Now at KSFO with transparency enabled (no title bar/styling).
Don't look, it's bad color choices! This shows how colors can be changed while running.
Showing a simple example of functional closures, with white styling.
Canvas-based Interactive Nasal REPL running in a heavily down-stripped FlightGear version that only provides the most essential subsystems
REPL-based subsystem-reinitialization in FGCanvas mode