optional/README.md

# optional

Single header implementation of `std::optional` with functional-style extensions and support for references.

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`std::optional` is the preferred way to represent an object which may or may not have a value. Unfortunately, chaining together many computations which may or may not produce a value can be verbose, as empty-checking code will be mixed in with the actual programming logic. This implementation provides a number of utilities to make coding with `optional` cleaner.

For example, instead of writing this code:

```c++
std::optional<image> get_cute_cat (const image& img) {
    auto cropped = crop_to_cat(img);
    if (!cropped) {
      return std::nullopt;
    }

    auto with_tie = add_bow_tie(*cropped);
    if (!with_tie) {
      return std::nullopt;
    }

    auto with_sparkles = make_eyes_sparkle(*with_tie);
    if (!with_sparkles) {
      return std::nullopt;
    }

    return add_rainbow(make_smaller(*with_sparkles));
}
```

You can do this:

```c++
tl::optional<image> get_cute_cat (const image& img) {
    return crop_to_cat(img)
           .and_then(add_bow_tie)
           .and_then(make_eyes_sparkle)
           .map(make_smaller)
           .map(add_rainbow);
}
```

The interface is the same as `std::optional`, but the following member functions are also defined. Explicit types are for clarity.

- `map`: carries out some operation on the stored object if there is one.
  * `tl::optional<std::size_t> s = opt_string.map(&std::string::size);`
- `and_then`: like `map`, but for operations which return a `tl::optional`.
  * `tl::optional<int> stoi (const std::string& s);`
  * `tl::optional<int> i = opt_string.and_then(stoi);`
- `or_else`: calls some function if there is no value stored.
  * `opt.or_else([] { throw std::runtime_error{"oh no"}; });`
- `map_or`: carries out a `map` if there is a value, otherwise returns a default value.
  * `tl::optional<std::size_t> s = opt_string.map_or(&std::string::size, 0);`
- `map_or_else`: carries out a `map` if there is a value, otherwise returns the result of a given default function.
  * `std::size_t get_default();`
  * `tl::optional<std::size_t> s = opt_string.map_or_else(&std::string::size, get_default);`
- `conjunction`: returns the argument if a value is stored in the optional, otherwise an empty optional.
  * `tl::make_optional(42).conjunction(13); //13`
  * `tl::optional<int>{}.conjunction(13); //empty`
- `disjunction`: returns the argument if the optional is empty, otherwise the current value.
  * `tl::make_optional(42).disjunction(13); //42`
  * `tl::optional<int>{}.disjunction(13); //13`
- `take`: returns the current value, leaving the optional empty.
  * `opt_string.take().map(&std::string::size); //opt_string now empty;`

In addition to those member functions, optional references are also supported:

```c++
int i = 42;
tl::optional<int&> o = i;
*o == 42; //true
i = 12;
*o == 12; //true
&*o == &i; //true
```

Assignment has rebind semantics rather than assign-through semantics:

```c++
int j = 8;
o = j;

&*o == &j; //true
```

### Compiler support

Tested on:


- Linux
  * clang 6.0.1
  * clang 5.0.2
  * clang 4.0.1
  * clang 3.9
  * clang 3.8
  * clang 3.7
  * clang 3.6
  * clang 3.5
  * g++ 8.0.1
  * g++ 7.3
  * g++ 6.4
  * g++ 5.5
  * g++ 4.9
  * g++ 4.8
- Windows
  * MSVC 2015
  * MSVC 2017

### Standards Proposal

This library also serves as an implementation of WG21 standards paper [P0798R0: Monadic operations for std::optional](https://wg21.tartanllama.xyz/monadic-optional). This paper proposes adding `map`, `and_then`, and `or_else` to `std::optional`.

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To the extent possible under law, [Sy Brand](https://twitter.com/TartanLlama) has waived all copyright and related or neighboring rights to the `optional` library. This work is published from: United Kingdom.