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Annotation of researchv10no/cmd/sml/doc/examples/iterators.sml, revision 1.1.1.1

1.1       root        1: (* Generators and Iterators *)
                      2: 
                      3: exception DONE
                      4: 
                      5: (* An arithmetic range generator: numbers from n upto (or downto) m *)
                      6: infix to;
                      7: fun (n:int) to (m:int) =
                      8:     let val dir = if n<=m then 1 else ~1;
                      9:        val pos = ref(n-dir);
                     10:        val test = if n<=m then (fn () => !pos>m) else (fn () => !pos<m)
                     11:      in (fn () => (pos := !pos+dir; 
                     12:                   if test() then raise DONE else !pos))
                     13:     end
                     14: 
                     15: (* Exercise: generalize this to take a step parameter. *)
                     16: 
                     17: 
                     18: (* A list generator: generates elements of a list in succession *)
                     19: fun over l =
                     20:     let val rest = ref l
                     21:      in fn () => case !rest
                     22:                   of []  => raise DONE
                     23:                    | x::l  => (rest:=l; x)
                     24:     end
                     25: 
                     26: 
                     27: (* A loop iterator *)
                     28: fun loop gen f = (f(gen ()); loop gen f) handle DONE => ()
                     29: 
                     30: (* A collect iterator -- collects results in a list *)
                     31: fun collect gen f = f(gen())::(collect gen f) handle DONE => [];
                     32: 
                     33: 
                     34: (* examples *)
                     35: loop (3 to 5) (fn (n:int) => print((makestring n)^"\n"));
                     36: 
                     37: collect (3 to 5) (fn n => n*n);
                     38: 
                     39: fun sq (x:int) = x*x;
                     40: collect (over [1,2,3]) sq;
                     41: 
                     42: 
                     43: (* Exercises:
                     44: 1. Define iterators over pairs of generators.
                     45: 2. Define a generator that produces the first n elements of a stream.
                     46: 3. Define a function that maps generators to streams.
                     47: 4. Define a binary tree type and generators that traverse trees.
                     48: 5. Try to think of some other interesting iterators beside "loop" and
                     49:    "collect".
                     50: *)

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