I think, an explicit type would make it even easier to grok: ISet<Author> authorsOfLongBooks = books .filter(book => book.pageCount > 1000) .map(

I think, an explicit type would make it even easier to grok: ISet<Author> authorsOfLongBooks = books .filter(book => book.pageCount > 1000) .map(book => book.author) .distinct() .toHashset() Or whatever the equivalent for ISet<Author> is in the respective language. Or IReadonlySet<Author> if the set should be immutable.