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 not so well-documented. Your best bet might be to read the oK Manual as a general introduction to K, and then click "help" here to see an overview of ngn/k specifically.

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

• The K tree, a StackExchange chatroom about K.