GNU/Andrew’s Blog

I’ve just been playing with the first fruits of the Lambda project. It takes a bit of messing around to get going; you need to downgrade the version of HotSpot in the lambda repository to b93 as so:

$ hg fclone http://hg.openjdk.java.net/lambda/lamdba
$ cd lambda/hotspot
$ hg update -r jdk7-b93

Once done, you can build code using closures like so:

$/mnt/builder/lambda/j2sdk-image/bin/javac -XDallowFunctionTypes Closures.java

I found that it’s pretty easy to implement a simple version of map which takes a collection of objects and a function to produce a collection of the results:

public static <S,T> List<S> map(List<T> s, #S(T) func)
{
    List<S> list = new ArrayList<S>();
    for (T t : s)
      list.add(func.(t));
    return list;
}

public static void main(String[] args)
{
  #Integer(Integer) square = #(Integer x)(x * x);
  List<Integer> list = Arrays.asList(new Integer[]{4,5,6});
  List<Integer> squares = map(list, square);
  System.out.println(list + " squared is " + squares);
}

This can be run as follows:

$ /mnt/builder/lambda/j2sdk-image/bin/java Closures
[4, 5, 6] squared is [16, 25, 36]

Sadly, the current implementation doesn’t like autoboxing too much. javac crashes if you pass an Integer into a function requiring an int, and you need an #Integer(Integer) function for the above; javac throws an error if you try to use a #int(int).

Ron

can you pass a closure to a method/closure and get back a closure?
http://nabeelalimemon.blogspot.com Nabeel Ali Memon

A nice post. But please correct the error in the example code:
… = map(set, square);
should be
… = map(list, square);
Andy Tripp

List squares = map(set, square);

should be

List squares = map(list, square);
http://mulletron.uwcs.co.uk/blog/ Richard Warbrton

where you have ‘set’ as the first argument to ‘map’ in your example, do you mean ‘list’?
mike

A small typo:

map(set, square);

should be

map(list, square);
http://www.gnarc.com/sphere/higher-level-functions%e2%80%a6-in-java Higher Level Functions… in Java! | Gnu Architecture

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http://developer.db4o.com German Viscuso

Hi!

In your code

List squares = map(set, square);

should be corrected to:

List squares = map(list, square);

Best!

German
http://fuseyism.com gnu_andrew

Typo corrected; thanks.

Ron, I assume you mean something like:

public static #Integer(Integer) partial(#Integer(Integer, Integer) func, int arg1)
{
return func.(Integer.valueOf(arg1));
}

which in a functional language would return a new function created by using arg1 as the first argument of func.

Sadly it doesn’t compile:

TestClosures.java:5: lambda expression cannot be applied to given types
return func.(Integer.valueOf(arg1));
^
required: Integer,Integer
found: Integer

Higher Level Functions… in Java!

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