p-ranav/result

Result<T, E> for Modern C++

Highlights

• Result is a type that represents either success (Ok) or failure (Err)
• Requires C++17
• MIT License

Quick Start

git clone https://github.com/p-ranav/result
cd result
mkdir build && cd build
cmake -DRESULT_BUILD_TESTS=ON ..
make && ./test/test

Simply include result/result.hpp and you're good to go. Result<T, E> has two variants:

• Ok(value) which indicates that the operation succeeded, and wraps the value returned by the operation. (value has type T)
• Err(why), which indicates that the operation failed, and wraps why, which (hopefully) explains the cause of the failure. (why has type E)

#include <iostream>
#include <string>
#include "result.hpp"
using namespace result;

auto Div(double x, double y) -> Result<double, std::string> {
  if (y == 0.0)
    return Err(std::string{"Division by zero"});
  else
    return Ok(x / y);
}

int main() {
  auto good_result = Div(100.0, 25.0);
  if (good_result.is_ok()) {
    // Continue with result
    std::cout << "Good Result: " << good_result.unwrap() << std::endl;
  }

  auto bad_result = Div(100.0, 0.0);
  if (bad_result.is_err()) {
    auto reason = bad_result.unwrap_err(); // "Division by zero"
    std::cout << "Bad Result: " << reason << std::endl;
  }
}

Aliases for Result

At a module level, creating aliases can be particularly helpful. Errors found in a specific module often have the same Err type, so a single alias can succinctly define all associated Results. Using Result.and_then, one can chain operations and terminate early as needed:

#include <iostream>
#include <cmath>
#include <result/result.hpp>
using namespace result;

enum class MathError {
  DivisionByZero,
  NonPositiveLogarithm,
  NegativeSquareRoot,
};

using MathResult = Result<double, MathError>;

auto Div(double x, double y) -> MathResult {
  if (y == 0.0)
    return Err(MathError::DivisionByZero);
  else
    return Ok(x / y);
}

auto Sqrt(double x) -> MathResult {
  if (x < 0.0) 
    return Err(MathError::NegativeSquareRoot);
  else
    return Ok(sqrt(x));
}

auto Log(double x) -> MathResult {
  if (x <= 0.0)
    return Err(MathError::NonPositiveLogarithm);
  else
    return Ok(log(x));
}

auto Op(double x, double y) -> double {
  return Div(x, y)
    .and_then(Sqrt)
    .and_then(Log)
    .unwrap();
}

int main() {
  std::cout << Op(100.0, 25.0) << std::endl; // 0.693147
}

map for Result

map maps a Result<T, E> to Result<U, E> by applying a function to a contained Ok value, leaving an Err value untouched. The following example implements a string -> int parser function and then uses it's results to multiply two numbers. The multiplication will proceed if both numbers are parsed correctly from their string representation. Else, the resultant error is propagated. Note the combined usage of and_then and map in this example.

#include <iostream>
#include <string>
#include <result/result.hpp>
using namespace result;

Result<int, std::string> 
parse(const std::string &message) {
  try {
    return Ok(std::stoi(message));
  } catch (std::invalid_argument& err) {
    return Err(std::string(err.what()));
  }
}

Result<int, std::string> 
multiply(const std::string &first_string, const std::string &second_string) {
  return parse(first_string).and_then([&](auto first) {
    return parse(second_string).map([&](auto second) { return first * second; });
  });
}

int main() {
  // This still presents a reasonable answer
  auto twenty = multiply("10", "2");
  std::cout << twenty << std::endl;   // prints "20"

  auto tt = multiply("t", "2");
  std::cout << tt << std::endl;       // prints "stoi: no conversion"
}

Different Return Types

map and map_err functions are not constrained to return the same type inside their variants. The map functions can be given Ok(T) and return Ok(U). The map_err function can be given Err(E) and return Err(F).

#include <iostream>
#include <cmath>
#include <result/result.hpp>
using namespace result;

enum class FooError {
  Bad,
};

enum class BarError {
  Horrible,
};

int main() {
  Result<Result<int, FooError>, BarError> res = Ok(Result<int, FooError>{Err(FooError::Bad)});

  auto value = res
    // `map` will only call the closure for `Ok(Result<int, FooError>)`
    .map([&](Result<int, FooError> res) {
      // transform `Ok(Result<int, FooError>)` into `Ok(Result<size_t, std::string>)`
      return res
        // transform int to size_t
        .map([](int n) { return static_cast<size_t>(n); })

        // transform `FooError` into std::string
        .map_err([](FooError e) { return std::string{"bad FooError"}; });
    })

    // `map_err` will only call the closure for `Err(BadError)
    .map_err([](BarError e) {
      // transform `BarError` into std::string
      return std::string{"horrible BarError"};
    });

  // value now has type Result<Result<size_t, std::string>, std::string>
  
  std::cout << value.unwrap() << std::endl; // "bad FooError"
}