streams.pl

   1 /*******************************************************************
   2  *
   3  * A Lisp compiler, written in Prolog
   4  *
   5  * (streams_test.pl)
   6  *
   7  * (c) Neil Smith, 2001
   8  *
   9  * The Lisp compiler supports lexical closures and other higher-
  10  * order programming techniques.  To demonsrate this, here is a
  11  * simple and naive implementation of streams, and some examples
  12  * of integer stream generators, including an infinite stream.
  13  * Code for this program was taken from 'Structure and Interpretation
  14  * of Computer Programs' by Abelson et al., and from 'Lisp' by
  15  * Winston and Horn.
  16  *
  17  *******************************************************************/
  18
  19 stream_first(stream) <<==
  20         first(stream).
  21
  22 stream_rest(stream) <<==
  23         lisp_apply(second(stream), []).
  24
  25 stream_cons(a, b) <<==
  26         list_2( a, b).
  27
  28 stream_null(stream) <<==
  29         null(stream).
  30
  31
  32 % take the first n items from a stream, placing them in a normal list
  33 stream_take(n, stream) <<==
  34   if( or( equalp(n, 0), stream_null( stream)),
  35       [],
  36       cons(stream_first(stream),
  37            stream_take(minus(n, 1), stream_rest( stream)))).
  38
  39 % remove the first n items from a stream
  40 stream_drop(n, stream) <<==
  41   if( or( equalp( n, 0), stream_null( stream)),
  42       stream,
  43       stream_drop( minus( n, 1), stream_rest( stream))).
  44
  45
  46 % creates a stream of integers from low to high
  47 stream_interval(low, high) <<==
  48   if( equalp(low, high),
  49     [],
  50     stream_cons( low, function(lambda([], stream_interval( plus( low, 1), high))))).
  51
  52
  53 % creates an infinite stream of integers, starting at n
  54 stream_ints_from(n) <<==
  55   stream_cons( n, function(lambda([], stream_ints_from( plus( n, 1))))).
  56
  57
  58 % tests on streams
  59 t1 <<==
  60         stream_take(3, stream_interval(1,5)).
  61
  62 t2 <<==
  63         stream_take(5, stream_drop(10, stream_ints_from(1))).