Function call stack growth for recursive IO? · beginners

Stream: beginners

Topic: Function call stack growth for recursive IO?

Austin Davis (Jun 19 2025 at 02:31):

So far I haven’t had any issues with this, but I wanted to understand how this works in Roc.

Let’s say I am writing an interactive terminal program (e.g. a REPL or a game). In order to capture and maintain some state, I need to pass a function to the platform API to handle inputs, and then I need that function to call the same platform API, passing a call to itself inside a closure with the updated state. At least this is my understanding. Please correct me if I’m wrong.

Given this fact, should I be worried about the function call stack growing indefinitely and potentially crashing the program? Is there some kind of optimization for that? Or is there another way to handle persistent state without writing to a file/database?

Brendan Hansknecht (Jun 19 2025 at 06:56):

Can you be more concrete in the specifications here? I don't understand why a function needs to be passed to the platform here.

Anthony Bullard (Jun 19 2025 at 12:22):

If you are writing a REPL, you would write out your prompt, read the input, append that to whatever state, and then recurse passing that state as an arg

Anthony Bullard (Jun 19 2025 at 12:24):

like this (psuedo-code):

main = |_| run_repl([])

run_repl = |state|
    Stdout.write!("> ")
    line = Stdin.read!()
    # do something
    run_repl(List.append(state, line))

Anthony Bullard (Jun 19 2025 at 12:25):

Obviously you would want to have a condition based on the line that would be your base case, something like if line == ":exit" return 0 else start_repl(List.append(state, line))

Brendan Hansknecht (Jun 19 2025 at 16:25):

Oh, if it is something like the above, no worries at all. Tail calls are optimized to loop without increasing the stack size.

Anton (Jun 20 2025 at 13:08):

Possibly helpful example of a terminal app: https://github.com/roc-lang/basic-cli/blob/main/examples/terminal-app-snake.roc

Anton (Jun 20 2025 at 13:11):

In order to capture and maintain some state