Echo2

Let us make the Echo program just a little bit more interesting: We add an initial welcome phrase and a prompt with a line number. Here is the program:

module Echo2 where

import POSIX

echo2 env = class

   count := 1

   prompt = do
      env.stdout.write (show count++"> ")
      count := count+1

   echo str = action
      env.stdout.write str
      prompt

   result
      action
         env.stdin.installR echo
         env.stdout.write "Welcome to Echo2!\n"
         prompt

root = newRoot echo2

The class definition now has two more components:

• The (implicit) declaration and initialisation of the state variable count, which will maintain line numbers.
• The local procedure prompt, which expresses the common task of writing a prompt to the user and increasing the line count (used both during initialization and subsequent handling of user input).

In addition, the interface returned upon instantiation is a dedicated method that takes care of all program-specific initialization. Such a class allows for a particularly simple root definition, as shown in the last line of the example. The wrapper function newRoot has the following definition in module POSIX:

newRoot :: (Env -> Class Action) -> World -> Cmd () ()
newRoot cl world = do
   env = new posix world
   init = new cl env
   init

Some additional details:

• show is a function that converts the integer value count to a string; ++ denotes string concatenation. Both these are defined in the Prelude, a library module which is implicitly imported by all modules.

• The user input string given to echo will include the newline; hence so will the string echoed to stdout. But to write the welcome phrase on a separate line, it will have to end with an explicit newline.

• The main object maintains state variable count, which is initialised to 1 and increased for each prompt written. Obviously, it would be possible to use a counter object here instead, but in this simple program that would seem an overkill.