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liststreamprologlazy-evaluationlazy-sequences

Lazy lists in Prolog?


Is it possible to have lazy lists in Prolog? Something like the following:

ones([1 | Y]) :- ones(Y).

Although this obviously doesn't work as it's written.


Solution

  • Markus Triska placed here in public domain some code worth to study:

    /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
       Prolog stream/lazy list demonstration
    
       Written 2005 by Markus Triska (triska@gmx.at)
       Public domain code.
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
    

    The title of the document (prost, for Prolog streams) maybe make the document a bit difficult to find, but make sense. Quoting from the above:

    Here, "stream" is used in the sense of "sequence", "delayed list", "lazy list" etc. as in Structure and Interpretation of Computer Programs, not in the sense of a Prolog input/output stream.