Scheme has the usual control constructs that most languages have--conditionals
(if statements), loops, and recursion--but it also has a very special
control structure called call-with-current-continuation.
(Warning: call-with-current-continuation is weird.)
call-with-current-continuation allows you to save the state of
a computation, package it up as a data structure, and go off and do
something else. Whenever you want, you can restore the old saved state,
abandoning the current computation and and picking up where the saved
computation left off.
This is far more powerful than normal procedure calling and returning, and allows you to implement advanced control structures such as backtracking, cooperative multitasking, and custom exception-handling.
You won't use call-with-current-continuation most of the time, because
more conventional control structures are usually sufficient. But if
you need to customize Scheme with a special control structure to
solve a particular kind of problem, you can do it with
call-with-current-continuation.
================================================================== This is the end of Hunk O. TIME TO TRY IT OUT At this point, you should go read Hunk P of the next chapter and work through the examples using a running Scheme system. Then return here and resume this chapter. ==================================================================
(Go to Hunk P, which starts at section Basic Programming Examples (Hunk P).)