Problem Statement

The   Look and say sequence   is a recursively defined sequence of numbers studied most notably by   John Conway.

The   look-and-say sequence   is also known as the   Morris Number Sequence,   after cryptographer Robert Morris,   and the puzzle   What is the next number in the sequence 1,   11,   21,   1211,   111221?   is sometimes referred to as the Cuckoo's Egg,   from a description of Morris in Clifford Stoll's book   The Cuckoo's Egg.

Sequence Definition

An example:

Write a program to generate successive members of the look-and-say sequence.

Let's start with the solution:

  [ stack ] is instances 

  [ 1 instances put
    $ "" swap
    behead swap space join
    witheach
      [ 2dup != iff
        [ rot instances share
          number$ join 
          rot join swap
          1 instances replace ]
        else 
          [ drop 
            1 instances tally ] ]
    drop instances release ]       is lookandsay ( $ --> $ )

  $ "1"
  15 times 
    [ dup echo$ cr
      lookandsay ]
  echo$ cr