Echo3

Now, let's try a variation; program Echo3 writes Hello!\n to stdout once a second. The user can change this message by typing a line on stdin; after the line is finished with a return, the line typed will be the new message. Here is the program:

module Echo3 where

import POSIX

echo3 env = class

   current := "Hello!\n"

   save str = action
      current := str

   tick = action
      env.stdout.write current
      after (sec 1) tick

   result action
      env.stdin.installR save
      tick
   
root = newRoot echo3

• The message to be printed once a second is maintained in state variable current.
• Method save changes current.
• Method tick prints current and schedules itself for later execution.
• The returned initialization method not only installs save as an input handler for stdin, it also makes an initial invocation of tick so that the self-triggering execution pattern can get started.
• The scheduled execution time (the baseline) for each instance of tick is one second after the baseline of the previous instance. Thus there is no accumulating drift in the program, even if a particular tick may be delayed in execution.

When running Echo3 one notices a peculiarity of the standard implementation of the POSIX environment: the screen doubles as both the output device stdout and as user feedback part of the input device stdin. Thus, output is intertwined with input characters in a rather unsatisfactory way.