matzo/examples/peano.matzo

isZero := { [Z] => True; [<S,_>] => False };
add := { [Z]     => { [y] => y }
       ; [<S,x>] => { [y] => add[x][<S,y>] } };
incr := add[<S,Z>];

two  := <S,<S,Z>>;
four := <S,<S,<S,<S,Z>>>>;

if := { [True]  => { [x] => { [_] => x } }
      ; [False] => { [_] => { [y] => y } }
      };
minusOne := { [Z] => Z; [<S,x>] => x };

puts "S is " S;
puts "Z is " Z;

puts "True is " True;
puts "False is " False;

puts "Is 0 zero?";
puts isZero[Z];

puts "Is 4 zero?";
puts isZero[four];

puts "2 + 4 = ";
puts add[four][two];

puts "Is zero zero?";
puts if[isZero[Z]]["yes"]["no"];

puts "Is 0+1 zero?";
puts if[isZero[incr[Z]]]["yes"]["no"];