construct _Iter {
    ;
    foreach { | with callable this ;
        def itm
        while { this:next dup =itm null eq not } {
            itm callable call
        }
    }
    collect { array | with this ;
        [ { any | } this:foreach ]
    }
    map { MapIter | with map-function this ;
        map-function this MapIter:new
    }
    reduce { ReduceIter | with reduce-function this ;
        reduce-function this ReduceIter:new
    }
    fold { FoldIter | with accumulator fold-function this ;
        accumulator fold-function this FoldIter:new
    }
    sum { mega | with this ; 
        { mega | with accum item ;
            accum item +
        } this:reduce:calculate
    }
    product { mega | with this ; 
        { mega | with accum item ;
            accum item *
        } this:reduce:calculate
    }
    join { str | with separator this ;
        { str | with accum item ;
            accum _str separator item _str strconcat strconcat
        } this:reduce:calculate
    }
    filter { FilterIter | with filter this ;
        filter this FilterIter:new
    }
    skip { this | with amount this ;
        { | 
            this:next;
        } amount:foreach
    }
    count { mega | with this ;
        def n 0 =n
        while { this:next null eq not } {
            n ++ =n
        }
        n
    }
    last { any | with this ;
        def last 
        def cur
        while { this:next dup =cur null eq not } {
            cur =last
        }
        last
    }
    chain { ChainIter | with other this ;
        other this ChainIter:new
    }
    enumerate { EnumerationIter | with this ;
        this EnumerationIter:new
    }
}

construct MapIter {
    origin
    map-function
    ;
    construct { this | with map-function origin this ;
        origin this:=origin
        map-function this:=map-function
        this
    }
    next { any | with this ;
        this:origin:next dup null eq if {
            2 stop
        }
        this:map-function call
    }
}

include _Iter in MapIter 

construct ReduceIter {
    origin
    accumulator
    reduce-function
    ;
    construct { this | with reduce-function origin this ;
        origin this:=origin
        reduce-function this:=reduce-function
        this
    }
    next { any | with this ;
        def itm 
        this:origin:next dup null eq if {
            2 stop
        } =itm
        this:accumulator null eq if {
            itm dup this:=accumulator
            2 stop
        }
        this:accumulator itm this:reduce-function call dup this:=accumulator
    }
    calculate { any | with this ;
        { | pop } this:foreach
        this:accumulator
    }
}

include _Iter in ReduceIter

construct FoldIter {
    origin
    accumulator
    reduce-function
    ;
    construct { this | with accumulator fold-function origin this ;
        accumulator this:=accumulator
        origin this:=origin
        fold-function this:=fold-function
        this
    }
    next { any | with this ;
        def itm 
        this:origin:next dup null eq if {
            2 stop
        } =itm
        this:accumulator itm this:fold-function call dup this:=accumulator
    }
}

include _Iter in FoldIter

construct FilterIter {
    origin 
    filter
    ;
    construct { this | with filter origin this ;
        origin this:=origin
        filter this:=filter
        this
    }
    next { any | with this ;
        while { 1 } {
            def next this:origin:next =next 
            next null eq if {
                null
                3 stop
            }
            next this:filter call if { 
                next 3 stop
            }
        }
    }
}

include _Iter in FilterIter

construct ChainIter {
    current
    next-iters
    ;
    construct { this | with other origin this ;
        [ other ] List:new:from this:=next-iters
        origin this:=current
        this
    }
    next { any | with this ;
        def item this:current:next =item
        while { item null eq } {
            this:next-iters:pop-front dup null eq not if {
                this:=current
                this:current:next =item
            } 2 stop
        }
        item
    }
    chain { this | with other this ;
        other this:next-iters:push
    }
}

include _Iter in ChainIter

construct EnumerationIter {
    origin
    idx
    ;
    construct { this | with origin this ;
        origin this:=origin
        this
    }
    next { [mega,any]|null | with this ;
        this:origin:next dup null eq not if {
            [ swap this:idx swap ]
            this:idx ++ this:=idx
        }
    }
}

include _Iter in EnumerationIter