Ocaml stdin interface [duplicate] - user-interface

Question

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How to make an interactive program?
(1 answer)
Closed 5 years ago.
I need to process user input from stdin with ocaml. User will enter commands until he types quit and then the program finishes. How to do this? I know how to program in imperative, but I want to learn functional. The user is supposed to manipulate data from a stack based on his commands. Also I want to do like a parser to process user commands.
Really appreciate your help!

Answer 1

Here is a sketch of something you could write using OCaml stack library. It's far from perfect and it could be improved in many ways, but the general structure is here.
The most important part as far as your question is concerned is the loop function. It reads a line from the standard input, and uses a pattern-matching to either end the program, or evaluate the given and command, and recursively call itself to wait for another command.
The eval function uses a pattern-matching on the given arguments to do the right thing. You can find documentation for the Stack module here.
let stack = Stack.create ()
let eval args =
match args with
| ["push"; v] -> Stack.push v stack
| ["pop"] -> begin try print_endline (Stack.pop stack) with
| Stack.Empty -> print_endline "Stack is empty"
end
| ["show"] -> Stack.iter print_endline stack
| _ -> print_endline "Unrecognized command"
let rec loop () =
match read_line () with
| "quit" -> print_endline "Bye"
| _ as command -> eval (String.split_on_char ' ' command); loop ()
let () =
loop ()
Note: I usually don't really like the idea of giving a full solution to a question that doesn't show a lot of research, but hey, you have to start somewhere when you're new to functional programming.
Note 2: This code will only work for string stacks. If you intend to store a different type, say ints, or if you want it to be polymorphic, you'll need to tweak that code a little bit.
EDIT: According to a remark made in the comments, below is an improved version of the above code, which doesn't use a global variable stack.
let eval s args =
match args with
| ["push"; v] -> Stack.push v s
| ["pop"] -> begin try print_endline (Stack.pop s) with
| Stack.Empty -> print_endline "Stack is empty"
end
| ["show"] -> Stack.iter print_endline s
| _ -> print_endline "Unrecognized command"
let rec loop s =
match read_line () with
| "quit" -> print_endline "Bye"
| _ as command -> eval s (String.split_on_char ' ' command); loop s
let () =
loop (Stack.create ())