summary of point lambdafication survey

[Stuart Marks]

On 9/23/11 1:39 PM, Yuval Shavit wrote:
> Collections.toMap sounds like a convenience function for a fold, is that about
> right?

I don't think so. This might be because of how heavily I had condensed the 
original submission. Let me explain.

As it stood, the original submission was interesting but not quite right in my 
view. It seemed to lead in either of two different directions, both of which 
are potentially very useful in their own right. Perhaps the submitter could 
post here to clarify, in case I've misinterpreted something.

1) The first direction is, given a Set and a Mapper, return a Map:

     <K,V> Map<K,V> toMap(Set<K> set, Mapper<K,V> mapper)

Conceptually this applies the mapper function to every element K of the set, 
giving a value V; these (K,V) pairs are stored into a map which is then 
returned. It's an open question whether this is done immediately, done lazily, 
whether results are cached, etc.

2) The other direction is, given a collection of values and a Mapper that 
generates keys, return a Map that maps each key to the values from which it was 
generated.

I've seen this called groupBy elsewhere. The problem is, how to represent 
multiple values for a single key. Other systems use tuples or multi-valued 
maps. For this example I'll use List<V> as the value of each map entry:

     <K,V> Map<K,List<V>> groupBy(Collection<V> coll, Mapper<V,K> mapper)

(Proper wildcards elided.)

For each value in the collection, run the mapper on it to get a key; then put 
this key-value pair into a map. If there are multiple values corresponding to 
the same key, the values get put into a list. For uniformity you'd always put 
the values into a list, even if there's only one value.

For example,

     List<String> list = Arrays.asList("Beulah", "Maude", "Gertrude",
         "Beatrice", "Gladys", "Georgia", "Mildred", "Mabel");
     Map<String, String> map = groupBy(list, s -> s.substring(0, 1));

would result in map containing

     "B" => ["Beulah", "Beatrice"]
     "G" => ["Gertrude", "Gladys", "Georgia"]
     "M" => ["Maude", "Mildred", "Mabel"]

s'marks