replace

function fw()
 turtle.digDown()
 turtle.forward()
end
function turn(x)
 if x % 2 == 0 then
  right = true
  left = false
 else
  right = false
  left = true
 end
 turtle.digDown()
 if right then
  turtle.turnRight()
 else if left then
  thurtle.turnLeft()
 end
end

while true do
 if turtle.detect() then
  turn()
 else
  fw()
 end
end