persistentsummary
Creates persistent summaries for various collection types.
Creates persistent summaries for various collection types. The return value of these methods must be stored somewhere for the summaries calculated by it to be persistent. The underlying implementation uses a guava cache with weak keys.
https://code.google.com/p/guava-libraries/wiki/CachesExplained
Typeclass for summaries.
The empty element and the combine operation must form a monoid.
the element type
the summary type