It is a problem with this community. Until recently kdb+ was like this with barely any documentation. They've come a long way in recent years though on the website and there is at least one book published.
With regards to Kerf, I don't know much of the history. I think the Kona developer (Lawler?) and Scott Locklin tried to make a company like kdb+, but it never worked out.
With regards to Kerf, I don't know much of the history. I think the Kona developer (Lawler?) and Scott Locklin tried to make a company like kdb+, but it never worked out.
Dyalog APL and J both have decent doc.