What are the big advancements between the major K versions?
APL -> A+ : opinionated (vector indices always start at 0, for example) and “electric” GUI (deserves a whole post to describe)
A+ -> K2 : significantly simplified - only ascii, no multidimensional arrays (uses nested vectors instead), much smaller vocabulary but just as effective in the basics department (e.g. the Whitney where/replicate monastic operator replaces 6 or 7 APL operators), added simple dictionaries, workspaces, dropped a lot of math from language. Simpler electric GUI. Time and date native types (as floating point)
K2 -> K3 mostly dropped GUI afaik. Added integral time and date.
k3 -> K4/kdb+: simplifies language more, comprehensive dictionary, TSDB rolled deeply into language (was previously a product written in K), native 64 bit support.
K4->K6: still more simplification, I think k6 is the kOS language. Never left dev state.
K6->k7->k9: continuous simplification, now at shakti rather than kx/firstderivatives - and with much more community involvement.
If I were to explore K today, where do I start and what version would I pick?
k9 (Shakti) for a simpler product that's in the active language design phase and with not as much integrations - yet.
ngn/k for an actively developed version that's free and targets k6-era (for now).
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[1] qSQL is actually a DSL implemented in K on top of kdb functional queries
1 Nigerian naira / Boltzmann constant =
1.96791577 × 1020 m-2 kg-1 s2 K U.S. dollars
APL has a few expand/compress operators that take a boolean vector; all of these in K use monadic & (where/replicate), either on the at-index e.g. +/a[&x>3] which is read “sum over a where x is greater than 3”, or at the amend-index, e.g. @[&7 9 15;i:&x>3;:;7-x@|i] which is read “in a vector with seven 0s, followed by nine 1s and fifteen 2s, in the indices where x is greater than 3 (call those indices i), place 7 minus those values from the same indices in reverse order.
Obviously contrived examples, but in APL you’d have to compress/expand a couple of times instead. (Sorry, don’t remember the operator names, my APL is rusty, last wrote a real program in 1992....)
But I have gotten lazy and too comfortable with K.... oh well.