typical usage of result.unwrap and result.or

result.unwrap and result.or are both useful functions in Gleam for working with Result types, but they serve different purposes.

result.unwrap

result.unwrap is used to extract the value from a Result, providing a default value if the Result is an Error. It’s typically used when you want to proceed with a default value rather than propagating an error.

Typical usage:

import gleam/result

pub fn get_user_name(user_id: Int) -> Result(String, Nil) {
  // Simulated user lookup
  case user_id {
    1 -> Ok("Alice")
    2 -> Ok("Bob")
    _ -> Error(Nil)
  }
}

pub fn greet_user(user_id: Int) -> String {
  let name = get_user_name(user_id)
    |> result.unwrap("Guest")

  "Hello, " <> name
}

// Usage:
pub fn main() {
  io.println(greet_user(1))  // Prints: "Hello, Alice"
  io.println(greet_user(3))  // Prints: "Hello, Guest"
}

In this example, result.unwrap allows us to use a default value (“Guest”) when the user lookup fails, ensuring that we always have a name to greet.

result.or

result.or is used to provide an alternative Result when the first Result is an Error. It’s typically used when you have a fallback operation or value that you want to try if the primary operation fails.

Typical usage:

import gleam/result

pub fn get_config_from_file() -> Result(Config, String) {
  // Simulated file read
  Error("File not found")
}

pub fn get_default_config() -> Result(Config, String) {
  // Return a default configuration
  Ok(Config(..))
}

pub fn get_config() -> Result(Config, String) {
  get_config_from_file()
  |> result.or(get_default_config())
}

// Usage:
pub fn main() {
  case get_config() {
    Ok(config) -> io.println("Config loaded")
    Error(err) -> io.println("Failed to load config: " <> err)
  }
}

In this example, result.or allows us to try loading the configuration from a file first, and if that fails, fall back to using a default configuration. The get_config function will only return an Error if both operations fail.

Key differences and when to use each:

• Use result.unwrap when you want to extract a value from a Result and have a sensible default to use if it’s an Error. This effectively “throws away” the error information.
• Use result.or when you want to try an alternative operation if the first one fails, while still preserving the Result type. This allows you to chain multiple fallback options.
• result.unwrap returns the unwrapped value directly, while result.or returns another Result.
• result.unwrap is often used at the “edges” of your program where you need to interface with code that doesn’t use Results, while result.or is more commonly used within the “core” logic where you’re still working with Results.

Both functions are valuable tools for error handling in Gleam, and understanding when to use each can lead to more robust and expressive code.

Rust

Same principle can be applied to Rust as the design is very alike.