I agree that map reduce does not solve parallel computing. However, that is just one example of how closures/lambdas can help you design software. It helps in almost any system in which you have two different functions that do almost the same thing, but a little different.
For example, say you are writing a double linked list. The push to head and push to tail functions may be completely the same, except one uses the head and one uses the tail, so you could just write one function and pass in the head or tail, right?
But you can't. The problem is that when you add something to one side, the links are the opposite. That is a new head goes to the right of the old head, while a new tail goes to the left of the old tail. In order to really do this, you also have to pass in a function.
Now, I've written a few doubly linked lists in Java (don't get mad, there _are_ good to do this), and in the end I just copy the function and make the inline changes. Trying to create the right interface and then pass in anonymous classes is almost impossible. And even if in my supreme cleverness I could figure it out, the code would be so confusing as to be unmaintainable.
Anyway, I would say that closures are immensely useful, not _just_ because of map reduce, but because of a whole class of examples that work in this manner.
For example, say you are writing a double linked list. The push to head and push to tail functions may be completely the same, except one uses the head and one uses the tail, so you could just write one function and pass in the head or tail, right?
But you can't. The problem is that when you add something to one side, the links are the opposite. That is a new head goes to the right of the old head, while a new tail goes to the left of the old tail. In order to really do this, you also have to pass in a function.
Now, I've written a few doubly linked lists in Java (don't get mad, there _are_ good to do this), and in the end I just copy the function and make the inline changes. Trying to create the right interface and then pass in anonymous classes is almost impossible. And even if in my supreme cleverness I could figure it out, the code would be so confusing as to be unmaintainable.
Anyway, I would say that closures are immensely useful, not _just_ because of map reduce, but because of a whole class of examples that work in this manner.