reduceByKey() and foldByKey()

xxxxByKey.ceylon

shared Map<Key,Item> reduceByKey<Key,Item>(
	{<Key->Item>+} stream, 
	Item accumulating(Item x, Item y)) 
		given Key satisfies Object 
		given Item satisfies Object
		=> stream.summarize(Entry.key, 
			(Item? partial, entry) 
					=> let (_->item = entry) 
					if (exists partial) 
					then accumulating(partial, item) 
					else item);

shared Map<Key,Result> foldByKey<Key,Item,Result>
	({<Key->Item>+} stream, Result initial)
	(Result accumulating(Result x, Item y)) 
		given Key satisfies Object
		given Result satisfies Object
		=> stream.summarize(Entry.key, 
			(Result? partial, entry) 
					=> let (_->item = entry)
					if (exists partial) 
					then accumulating(partial, item) 
					else initial);