Imagine that we are in a lovely cafe in Paris.
With us we have a genie who takes photographs of the things that are happening.
Here we are ordering our food.
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And in this one we are eating a delicious dessert.
Now we are payig the bill.
Later, when we are remembering that wonderful night, we can ask the
genie to rub a magic liquid on one of the protographs, and we will be
back there in that cafe.
The waiter may be a bit older, we may be a lot heavier, but otherwise
things are as they were before.
When we have finished our meal, we can spend the rest of the eveing
wandering through the streets of Paris as we did before, or
we can ask the genie to rub another photograph
to take us somewhere else.
What if the Genie rubbed the wrong prhotograph and took us back to
when we were paying the bill and leaving the restaurant?
What a shame to have to pay for a meal that we never got to enjoy?
A Scheme interpreter is like a genie. It can remember where it is
and go back there. It doesn’t photograph everything, but only the
things we tell it to photograph.
It has a special “take a photograph” procedure
named call-with-current-continuatin.”
Applying call/cc produces a photograph (continuation) that we
can rub (apply) whenever we want to escape to that point
in the computation.
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