Netstrings

A netstring is a simple, human-readable protocol for sending sequences of strings. The format is just the length of the string, followed by the string itself:

<byte length>:<string>,

["Hello" "World"] would encode to:

5:Hello,5:World,

Note that the length specified does not include the comma. This somewhat complicates the frame, but the definition is still straightforward:

(finite-frame
  (prefix (string-integer :ascii :delimiters [":"])
    inc
    dec)
  (string :utf-8 :suffix ","))

The prefix describes the byte-length, so we use (finite-frame ...) to describe the entire frame. The prefix is a string-integer, with the slight twist that the number is off-by-one from the actual byte-length that follows. To decode the length we must increment the value, and to get the value to encode we must decrement the length. The inner frame is just a string, terminated by a comma.