The REPL exists to read expressions, evaluate them, and then print their results. But sometimes one wants to tell the REPL to evaluate an expression in a different way, or to do something else altogether. A user can affect the way the REPL works with a REPL command.
The previous section had an example of a command, in the form of
,option.
scheme@(guile-user)> ,option value-history #t
Commands are distinguished from expressions by their initial comma (‘,’). Since a comma cannot begin an expression in most languages, it is an effective indicator to the REPL that the following text forms a command, not an expression.
REPL commands are convenient because they are always there. Even if the
current module doesn't have a binding for pretty-print, one can
always ,pretty-print.
The following sections document the various commands, grouped together
by functionality. Many of the commands have abbreviations; see the
online help (,help) for more information.