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 26.7.5 How Guile Implements Exceptions
 --------------------------------------
 
 It is traditional in Scheme to implement exception systems using
 Continuations::) are such a powerful concept that any other control
 mechanism -- including `catch' and `throw' -- can be implemented in
 terms of them.
 
    Guile does not implement `catch' and `throw' like this, though.  Why
 not?  Because Guile is specifically designed to be easy to integrate
 with applications written in C.  In a mixed Scheme/C environment, the
 concept of "continuation" must logically include "what happens next" in
 the C parts of the application as well as the Scheme parts, and it
 turns out that the only reasonable way of implementing continuations
 like this is to save and restore the complete C stack.
 
    So Guile's implementation of `call-with-current-continuation' is a
 stack copying one.  This allows it to interact well with ordinary C
 code, but means that creating and calling a continuation is slowed down
 by the time that it takes to copy the C stack.
 
    The more targeted mechanism provided by `catch' and `throw' does not
 need to save and restore the C stack because the `throw' always jumps
 to a location higher up the stack of the code that executes the
 `throw'.  Therefore Guile implements the `catch' and `throw' primitives
 independently of `call-with-current-continuation', in a way that takes
 advantage of this _upwards only_ nature of exceptions.
 

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(guile.info.gz) Lazy Catch

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