Conway’s Game of Life¶

To showcase that Egel can be used to write real programs I’ll walk you through an example of a small Conway’s life application. I’ll assume you know some functional programming, some territory not covered yet comes along too.

Conway’s game of life plays on a grid. At any point a cell on the grid may be dead or alive. Any life cell with fewer than two, or more than three, neighbours dies. Any dead cell with exactly three neighbours comes alive.

Preamble¶

It’s good practice to start every file with some comment on what it implements.

```####
# Conway's Game of Life.
#
```

We’ll rely on combinators defined in prelude.eg and io.ego. A .ego file is an object file, a binary on your system.

```import "prelude.eg"
import "io.ego"
```

We’ll open up the different namespaces we need from those files.

```using System
using List
using IO
```

The board¶

The board size of the two-dimensional grid is defined with a constant.

```def boardsize = 5
```

So, now the real programming starts. The grid is implemented as a stencil. A stencil is a function mapping coordinates to cells. A cell 0 is dead, any other value means it’s alive.

The empty grid maps all coordinates to dead cells.

```def empty = [ X Y -> 0 ]
```

To insert an alive cell, we update the stencil with a clause mapping two matching coordinates to an alive cell.

```def insert =
[ X Y BOARD ->
[ X0 Y0 -> if and (X0 == X) (Y0 == Y) then 1
else BOARD X0 Y0 ] ]
```

To get to all coordinates we map multiple times on the list {0,..,boardsize-1} to retrieve the pairs {{0 0, 0 1, ..},..}. Note that we don’t need to tuple explicitly.

```def coords =
let R = fromto 0 (boardsize - 1) in
[ XX YY -> map (\X -> map (\Y -> X Y) YY) XX ] R R
```

Printing¶

To print, we just apply IO.print for a dead or alive cell. Though Egel is a mostly pure term rewrite system, combinators loaded may have side effects.

```def printcell =
[ 0 -> print ". "
| _ -> print "* " ]
```

A wildcard pattern _ is used to match against any value.

Printing a board is done by going over all coordinates and printing the cell for that coordinate.

```def printboard =
[ BOARD ->
foldl [_ XX -> map [(X Y) -> printcell (BOARD X Y)] XX; print "\n" ] nop coords ]
```

Note

Though Egel combinators may be side-effecting, they must reduce to a value. IO:print will print all its arguments but will reduce to the uninformative value System:nop. Often, with side-effecting calculations these values are simply discarded. The semicolon separates such statements.

Generations¶

The neighbour count of a coordinate on a board can be calculated by just looking around.

```def count =
[ BOARD X Y ->
(BOARD (X - 1) (Y - 1)) + (BOARD (X) (Y - 1)) + (BOARD (X+1) (Y - 1)) +
(BOARD (X - 1) Y) + (BOARD (X+1) Y) +
(BOARD (X - 1) (Y+1)) + (BOARD (X) (Y+1)) + (BOARD (X+1) (Y+1)) ]
```

The status of the next cell is calculated from whether the current cell is alive or dead and the number of neighbours.

```def next =
[ 0 N -> if N == 3 then 1 else 0
| _ N -> if or (N == 2) (N == 3) then 1 else 0 ]
```

A board is updated by applying the above function next to every coordinate on the board.

```def updateboard =
[ BOARD ->
let XX = map (\(X Y) -> X Y (BOARD X Y) (count BOARD X Y)) (flatten coords) in
let YY = map (\(X Y C N) -> X Y (next C N)) XX in
foldr [(X Y 0) BOARD -> BOARD | (X Y _) BOARD -> insert X Y BOARD ] empty YY ]
```

A blinker consists of three alive cells next to each other.

```def blinker =
(insert 1 2) . (insert 2 2) . (insert 3 2)
```

We print three generations of a board with a blinker.

```def main =
let GEN0 = blinker empty in
let GEN1 = updateboard GEN0 in
let GEN2 = updateboard GEN1 in
foldl [_ G -> print "generation:\n"; printboard G ] nop {GEN0, GEN1, GEN2}
```

And that wraps it up. A real Egel application.