If it fails, edit/1 uses its internal defaults, which are defined by the Prolog flag editor and/or the environment variable EDITOR. The following rules apply. If the Prolog flag editor is of the format $<name>, the editor is determined by the environment variable <name>. Else, if this flag is pce_emacs or built_in and XPCE is loaded or can be loaded, the built-in Emacs clone is used. Else, if the environment EDITOR is set, this editor is used. Finally, vi is used as default on Unix systems and notepad on Windows.

See the default user preferences file customize/init.pl for examples.