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Mon Aug 31 09:34:24 CEST 2009

Using Partial Continuations

Let's try the classical inversion of control problem: convert a
traversal function into a cursor.

First example: convert a list into a stream by inverting `each'.

    (list->stream . ((((cons) shift) each '()) reset))

A `stream' is a pair of a value and a quotation or '().  


> '(1 2 3) list->stream
> > ps
<1> {1 . <,[each-next] '{2 3} '[[cons] shift] ,['()] ,[prompt-tag]>}
> uncons
> ps
<2> 1 <,[each-next] '{2 3} '[[cons] shift] ,['()] ,[prompt-tag]>
> run ps
> <2> 1 {2 . <,[each-next] '{3} '[[cons] shift] ,['()] ,[prompt-tag]>}
> uncons run ps
> > <3> 1 2 {3 . <,[each-next] '() '[[cons] shift] ,['()] ,[prompt-tag]>}
> uncons run ps
> > <4> 1 2 3 ()


This also makes it pretty clear that a partial continuation shouldn't
take the data stack, otherwise `list->stream' wouldn't work
correctly.  I.e. in the following you really want everything
underneath the literal list to be left intact.

> 123 123 123 '(1 2 3) list->stream
> > > > > ps
<4> 123 123 123 {1 . <,[each-next] '{2 3} '[[cons] shift] ,['()] ,[prompt-tag]>}

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