The Game of Life, also known simply as Life, is a cellular automaton devised by the British mathematician John Horton Conway in 1970.

The universe of the Game of Life is an infinite, two-dimensional orthogonal grid of square cells, each of which is in one of two possible states, alive or dead, (or populated and unpopulated, respectively). Every cell interacts with its eight neighbours, which are the cells that are horizontally, vertically, or diagonally adjacent. At each step in time, the following transitions occur:

• Any live cell with fewer than two live neighbours dies, as if by
underpopulation. • Any live cell with two or three live neighbours lives on to the
next generation. • Any live cell with more than three live neighbours dies, as if
by overpopulation. • Any dead cell with exactly three live neighbours becomes a live
cell, as if by reproduction.

The initial pattern constitutes the seed of the system. The first generation is created by applying the above rules simultaneously to every cell in the seed; births and deaths occur simultaneously, and the discrete moment at which this happens is sometimes called a tick. Each generation is a pure function of the preceding one. The rules continue to be applied repeatedly to create further generations.

Taken from https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life

This is my take on implementing this in RETRO.

I first wrote a quick helper to inline a representation of part of the world.

~~~:w/l [ $. eq? [ #0 ] [ #1 ] choose , ] s:for-each ; ~~~

I then define the intitial world. This is a 20x20 grid.

~~~'World d:create   '.................... w/l   '.................... w/l   '.................... w/l   '..ooo............... w/l   '....o............... w/l   '...o................ w/l   '.................... w/l   '.................... w/l   '.................... w/l   '.................... w/l   '.................... w/l   '....ooo............. w/l   '.................... w/l   '.................... w/l   '.................... w/l   '........ooo......... w/l   '.......ooo.......... w/l   '.................... w/l   '.................... w/l   '.................... w/l ~~~

Space is also reserved for the next generation.

~~~'Next d:create   #20 #20 * allot ~~~

~~~{{   'Surrounding var   :get (rc-v)     dup-pair [ #0 #19 n:between? ] bi@ and     [ &World + [ #20 * ] dip + fetch ] [ drop-pair #0 ] choose ;   :neighbor?  (rc-) get &Surrounding v:inc-by ;   :NW (rc-rc) dup-pair [ n:dec ] bi@       neighbor? ;   :NN (rc-rc) dup-pair [ n:dec ] dip       neighbor? ;   :NE (rc-rc) dup-pair [ n:dec ] dip n:inc neighbor? ;     :WW (rc-rc) dup-pair   n:dec             neighbor? ;   :EE (rc-rc) dup-pair   n:inc             neighbor? ;   :SW (rc-rc) dup-pair [ n:inc ] dip n:dec neighbor? ;   :SS (rc-rc) dup-pair [ n:inc ] dip       neighbor? ;   :SE (rc-rc) dup-pair [ n:inc ] bi@       neighbor? ;   :count (rc-rcn)     #0 !Surrounding NW NN NE                     WW    EE                     SW SS SE @Surrounding ;   :alive (rc-n)     count #2 #3 n:between? [ #1 ] if; #0 ;   :dead (rc-n)     count #3 eq? [ #1 ] if; #0 ;   :new-state (rc-n)     dup-pair get #1 eq? &alive &dead choose ;   :set   (nrc-)  &Next + [ #20 * ] dip + store ;   :cols (r-)     #20 [ I over swap new-state rot rot set ] indexed-times drop ;   :output (n-) n:-zero? [ $o ] [ $. ] choose c:put sp ; ---reveal---   :display (-)     nl &World #20 [ #20 [ fetch-next output ] times nl ] times drop ;   :gen (-)     #20 [ I cols ] indexed-times &Next &World #20 #20 * copy ; }}   {{   :divide #20 [ $- c:put ] times sp 'Gen:_ s:put dup n:put nl ; ---reveal---   :gens (n-)  #0 swap [ display divide  n:inc gen ] times drop ; }}   #12 gens