Wednesday, June 9, 2010

An illustrative problem (part II)

Mike Coffin took the bait. (Thanks, Mike!) Unfortunately, pasting code into the blog comments is a very poorly supported feature. Blogs about Python must be interesting. I'll recopy some of Mike's code here.

Mike said: While I prefer this:
int minimumIntegerInBox (Collection<Box> boxes) {
    int result = Integer.MAX_VALUE;
    for (Box box : boxes) {
        if (box.get() instanceof Integer) {
            result = Math.min (result, Integer.class.cast (box.get()));
    return result;
that would be cheating.

It isn't cheating, but it isn't answering the question I posed, either. It's more or less the obvious thing to do in Java or Lisp. But conceptually it is pretty primitive. I don't care about the machinery involved in traversing a list (the looping construct). I want to take a binary operation like Math.min and make it n-ary.

Mike gave a very complete solution, but I'm only going to reformat the relevant part:
public static int minimumIntegerInBox(Collection<Box> boxes) {
    Iterable<Box> intBoxes = 
        filter (boxes,
                new Function<Box, Boolean>() {
                    public Boolean of(Box domainItem) {
                        return domainItem.get() instanceof Integer;

    Iterable ints = 
        map (intBoxes,
             new Function<Box, Integer>() {
                 public Integer of(Box domainItem) {
                     return Integer.class.cast(domainItem.get());

    return minimumElement(ints);

public static int minimumElement(Iterable items) {
    return best(
        new Function<Pair, Boolean>() {
            public Boolean of(Pair domainItem) {
                return domainItem.second() < domainItem.first();
That's a lot closer to what I had in mind, but these are untyped Boxes. I want to use parameterized Boxes.

1 comment:

Mike Coffin said...

Actually, my boxes were typed, but it looks like blogger decided that all the generic types were HTML tags and dropped them. Since I can't figure out how to post formatted code, I'll mail it to you.