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Continuations

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Continuations are a generalization of control constructs: from a common 
primitive, you can build such things as loops (with exits), exceptions, 
and even coroutines.

The way I have implemented this in GXScript is with the “cexec” operator: 
this takes a procedure as an argument, like “exec”, but before calling it, 
it pushes a continuation object on the stack. Invoking this object will 
transfer control to the point immediately following the cexec call that 
created it.

Example:

        {
            /Count 5 ldef
            /Top null ldef
            {/Top exch lstore} cexec
                {
                    /Break exch ldef
                    Count =
                    /Count Count 1 sub lstore
                    Count 0 eq {Break} if
                    Top
                }
            cexec
        }
    exec

Output:

    5
    4
    3
    2
    1

But that’s a clunky way to have to write a loop every time. So why not 
package up the creation of the continuations in a reusable procedure:

    /DoLoop
        {
            /Proc exch ldef
            /Top null ldef
            {/Top exch lstore} cexec
            {Proc Top} cexec
        }
    ddef

DoLoop will take your loop-body procedure as argument, and execute it 
repeatedly, each time pushing a continuation that can be used to exit the 
loop, until you invoke that continuation. Example use:

    /Count 5 ddef
        {
            /Break exch ldef # always pop arg even if not used
            Count =
            /Count Count 1 sub lstore
            Count 0 eq {Break} if
        }
    DoLoop

The output is the same as before.

The source is here <https://bitbucket.org/ldo17/gxscript/> if you want to 
have a play.

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Continuations Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-06-05 07:05 +0000

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