1 Subject: Clojure's sequences and nil punning

2 Time-stamp: <2010-04-16 17:25:01 tonyg>

3 From: tonyg

4

5 http://clojure.org/lazy

6

7 namespace s = "http://eighty-twenty.org/etng/r1/ns/stream#";

8 define s:foldr stream knil kons ->

9 stream .s:case {

10 .s:empty -> knil;

11 .s:next(head, tail) -> kons(head, s:foldr tail knil kons);

12 };

13 define s:map stream fn -> s:foldr stream s:empty {elt, acc -> s:cons(fn(elt), acc)};

14

15 Expand through `s:foldr`, and we get

16

17 define s:map stream fn ->

18 stream .s:case {

19 .s:empty -> s:empty;

20 .s:next(head, tail) -> s:cons(fn(head), s:map tail fn);

21 };

22

23 Delay:

24

25 define nothing = { .case v -> v .nothing };

26 define just x = { .case v -> v .just x };

27

28 define delay v ->

29 -- Carefully written to avoid holding onto v for too long.

30 -- Needs let rec cell = etc. to avoid the initial cell.set!

31 let cell = ref ();

32 do cell.set { let forced = v ();

33 do cell.set { forced };

34 forced };

35 { msg -> cell.get () msg };

36

37 Lazy map:

38

39 define s:lazymap stream fn ->

40 delay {stream .s:case {

41 .s:empty -> s:empty;

42 .s:next(head, tail) = s:cons(delay {fn(head)}, delay {s:map tail fn});

43 }};