promptor

Simple, functional, monadic command-line prompts.

Heavily inspired by readMaybe from Haskell.

A simple module for handling user input and user input validation from input streams.

Features

• Functional: Each of the methods that return an Option has a translation from Haskell which you can view in the documentation.
• Transparent: Uses only traits in the standard library and defines only a trait alias.
• (Mostly) Safe: All of the methods except for prompt() and rprompt() return Option or Result, and use the chaining methods on Option and Result.
• Usable beyond stdio: You can define a Promptor object for any combination of objects that implement BufRead and Write.
• Use only what you need: don't need validation? Just use input(). Need only a string? Just use get_line().

Usage

Say you'd like to get a number from the user via stdin that is in the closed interval [1, 100]. To do this with Promptor, you first define the Promptor object and then call the prompt() method on it, like so:

use promptor::Promptor;

let stdio = std::io::stdin();
let input = stdio.lock();
let output = std::io::stdout();

let mut promptor = Promptor {
    reader: input,
    writer: output
};

let res: u32 = promptor.prompt("Please enter a number between 1 and 100: ", |x| 1 <= x && x <= 100);

If you only need a string, you can use get_line() instead:

use promptor::Promptor;
let stdio = std::io::stdin();
let input = stdio.lock();
let output = std::io::stdout();

let mut promptor = Promptor {
    reader: input,
    writer: output
};

let name = promptor.get_line("What is your name? ").unwrap();

Motivation

This is the culmination of a few months of researching error handling in several different languages. After learning about monadic error handling in Haskell and applying that to several other languages, I found that Rust, in combination with the std::str::FromStr trait, came closest to the Haskell implementation.

I also found that other prompt libraries like this one did not quite fit my needs: they were either too complicated, too opaque, or did not account for validation internally. So, I made this library as a simple, straightforward prompt library, complete with a prompt() method that allows for a validator. Everything I've done in this library is close to the Rust standard library.

Furthermore, thanks to dependency injection, you can even use this library for more than just stdin/stdout. You could, for instance, pipe input from a file.

Meanwhile, I have tried to keep the error handling sensible: you can ignore internal errors with the methods that return an Option, or you can use the Result versions to decide how to handle errors.

My library does not use any special traits; as you can see, SafeParsable merely uses std::str::FromStr, Sized, Copy, and Default. This way, the types play nicely with the validator and have a sensible result if you call .unwrap_or_default().

You can also use only what you need: if, for instance, you only need to get a string from the user, you can use just get_line(). You don't need to use prompt() at all.