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schemecontinuations

Continuation in Scheme with lambda with no arguments

So I am dealing with continuations and have something like this:

(or
    (call/cc (lambda (cont)
           ...
          (if ( ... )
              (cons randomList (lambda() (cont #f)))
              #f)})}
     ( do something else)

I was wondering what the difference between (lambda() (cont #f)) and (cont #f) are. I get the answer I want with the lambda and something wrong without. Could someone explain the difference? Thanks.

Solution

  • A nullary (zero arguments) lambda used in this way is called a thunk.

    Thunks are used in Scheme to defer the execution of some piece of code. Suppose, for example, that we're talking about (display #f) instead of (cont #f). If you wrote (display #f) directly, then when the code execution reached that point, it'd display #f straight away, whereas if you wrapped it in a thunk ((lambda () (display #f))), it wouldn't display the #f until you invoked the thunk.

    Back to your code, a (cont #f) in the code would cause an immediate jump at the point where the continuation is invoked. Wrapping it in a thunk delays the invocation of the continuation until you invoke the thunk.