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4.4.3 Value History
Just as Readline helps you to reuse a previous input line, value
history allows you to use the result of a previous evaluation in
a new expression. When value history is enabled, each evaluation result
is automatically assigned to the next in the sequence of variables
$1, $2, …. You can then use these variables in
subsequent expressions.
scheme@(guile-user)> (iota 10) $1 = (0 1 2 3 4 5 6 7 8 9) scheme@(guile-user)> (apply * (cdr $1)) $2 = 362880 scheme@(guile-user)> (sqrt $2) $3 = 602.3952191045344 scheme@(guile-user)> (cons $2 $1) $4 = (362880 0 1 2 3 4 5 6 7 8 9)
Value history is enabled by default, because Guile’s REPL imports the
(ice-9 history) module. Value history may be turned off or on within the
repl, using the options interface:
scheme@(guile-user)> ,option value-history #f scheme@(guile-user)> 'foo foo scheme@(guile-user)> ,option value-history #t scheme@(guile-user)> 'bar $5 = bar
Note that previously recorded values are still accessible, even if value history
is off. In rare cases, these references to past computations can cause Guile to
use too much memory. One may clear these values, possibly enabling garbage
collection, via the clear-value-history! procedure, described below.
The programmatic interface to value history is in a module:
(use-modules (ice-9 history))
- Scheme Procedure: value-history-enabled?
Return true if value history is enabled, or false otherwise.
- Scheme Procedure: clear-value-history!
Clear the value history. If the stored values are not captured by some other data structure or closure, they may then be reclaimed by the garbage collector.
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