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