Wiki: K

K is like APL with fewer built-ins. The syntax is ASCII-only, and most built-in functions are heavily overloaded single bytes of ASCII punctuation.

There are several implementations with subtly varying behavior. code.golf uses ngn/k, which is documented with the reference card found under "help" at the online interpreter. If you're new to K (and J, APL, etc...), this style of documentation might not be very helpful, but other learning material is available:

• Razetime has put together an extensive tutorial which introduces the language in multiple chapters, and then spends some time on solving larger tasks (including a Sudoku solver!).
• If you're looking for something in between the ref-card and a full blown tutorial, Stefan Kruger's kbook might be a good a fit.
• Another implementation of a similar K version is oK which comes with a more extensive manual

Example

Let's look at a program that outputs all of the input arguments with their length appended to them: `0:{x,$#x}'x

(Try it on United States: it outputs Nebraska8 Rhode Island12…)

`0:{x,$#x}'x
           x    Get the list of inputs.
   {     }'     Apply some lambda function to each input.
                ({} makes a lambda, and the adverb ' means each.)
    x   x       Inside a monad lambda, "x" refers to the single argument.
       #        Monad # means "length"
      $         Monad $ means "convert to string"
     ,          Dyad , means "concatenate"
`0:             Output the list of strings line by line.
                0: is a dyadic writing-lines-to-a-file verb,
                and the left argument ` (the empty symbol) means "write to STDOUT".

You can see that K is essentially read and written "right-to-left", much like J and APL. x,$#x means (x,($(#x))). There's no operator precedence.

Golf tips

• \ preceded by a space is "trace" (an identity function for debugging that prints its argument to STDOUT). This can be shorter than using `0:. Trace however passes its argument through the K formatter before printing, so this may only be useful for numbers.
• $[x;T;F] can become x{T}/F when x is a non-negative integer.
• x in the global scope is a list of the command line args.

External resources

• Matrix chat room specifically for ngn/k,
• The APL Farm has a room for K discussion,
• The K tree, a StackExchange chatroom about K. (inactive)