Note: This is taken from the Chicken Wiki, where a more recent version could be available.
The advice extensions allows modifying the behaviour of existing procedures by specifying code that should run before, after or instead of the original procedure.
[procedure] (advise MODE PROCEDURE ADVISOR [ID])
Adds a "piece of advice" to PROCEDURE. Depending on MODE, do any of the following:
before advice-procedures are invoked in the reverse order: the most recently added advisor runs first. after advice-procedures are invoked in the order in which they are added, the most recent one running last. The most recently added around advice-procedures is called with the next "inner" advice procedure passed as argument.
The procedure passed as first argument to around advisors takes its arguments in the same manner as the original procedure.
advise returns an ID that can be used to identify a particular piece of advice. If no ID was given, one will be generated.
[procedure] (unadvise PROCEDURE [ID [MODE]])
Removes advice from an advised procedure. Advice can be removed completely (no ID or mode given). Specific pieces of advice can be removed by passing the advice-ID (as returned by advice). Alternatively advice for a given mode (before, after or around) can be removed (by passing #f as the ID argument).
Copyright (c) 2007, Felix L. Winkelmann All rights reserved.
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