2.1.3 Restarting

Another way to exit a REPL is to use the restart procedure:

procedure: restart [k]

This procedure selects and invokes a restart method. The list of restart methods is different for each REPL and for each error; in the case of an error REPL, this list is printed when the REPL is started:

;Unbound variable: foo
;To continue, call RESTART with an option number:
; (RESTART 3) => Specify a value to use instead of foo.
; (RESTART 2) => Define foo to a given value.
; (RESTART 1) => Return to read-eval-print level 1.

2 error>

If the k argument is given, it must be a positive integer index into the list (in the example it must be between one and three inclusive). The integer k selects an item from the list and invokes it. If k is not given, restart prints the list and prompts for the integer index:

2 error> (restart)
;Choose an option by number:
;  3: Specify a value to use instead of foo.
;  2: Define foo to a given value.
;  1: Return to read-eval-print level 1.

Option number:

The simplest restart methods just perform their actions. For example:

2 error> (restart 1)
;Abort!

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Other methods will prompt for more input before continuing:

2 error> (restart)
;Choose an option by number:
;  3: Specify a value to use instead of foo.
;  2: Define foo to a given value.
;  1: Return to read-eval-print level 1.

Option number: 3

Value to use instead of foo: '(a b)
;Value: (a b)

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