From Jason Turner

[optional.monadic]

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tmp/tmpnu1c7wic/{from.md → to.md} RENAMED
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+ #### Monadic operations <a id="optional.monadic">[[optional.monadic]]</a>
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+
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+ ``` cpp
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+ template<class F> constexpr auto and_then(F&& f) &;
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+ template<class F> constexpr auto and_then(F&& f) const &;
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+ ```
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+
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+ Let `U` be `invoke_result_t<F, decltype(value())>`.
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+
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+ *Mandates:* `remove_cvref_t<U>` is a specialization of `optional`.
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+
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+ *Effects:* Equivalent to:
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+
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+ ``` cpp
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+ if (*this) {
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+ return invoke(std::forward<F>(f), value());
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+ } else {
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+ return remove_cvref_t<U>();
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+ }
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+ ```
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+
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+ ``` cpp
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+ template<class F> constexpr auto and_then(F&& f) &&;
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+ template<class F> constexpr auto and_then(F&& f) const &&;
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+ ```
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+
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+ Let `U` be `invoke_result_t<F, decltype(std::move(value()))>`.
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+
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+ *Mandates:* `remove_cvref_t<U>` is a specialization of `optional`.
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+
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+ *Effects:* Equivalent to:
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+
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+ ``` cpp
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+ if (*this) {
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+ return invoke(std::forward<F>(f), std::move(value()));
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+ } else {
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+ return remove_cvref_t<U>();
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+ }
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+ ```
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+
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+ ``` cpp
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+ template<class F> constexpr auto transform(F&& f) &;
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+ template<class F> constexpr auto transform(F&& f) const &;
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+ ```
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+
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+ Let `U` be `remove_cv_t<invoke_result_t<F, decltype(value())>>`.
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+
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+ *Mandates:* `U` is a non-array object type other than `in_place_t` or
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+ `nullopt_t`. The declaration
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+
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+ ``` cpp
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+ U u(invoke(std::forward<F>(f), value()));
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+ ```
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+
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+ is well-formed for some invented variable `u`.
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+
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+ [*Note 1*: There is no requirement that `U` is
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+ movable [[dcl.init.general]]. — *end note*]
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+
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+ *Returns:* If `*this` contains a value, an `optional<U>` object whose
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+ contained value is direct-non-list-initialized with
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+ `invoke(std::forward<F>(f), value())`; otherwise, `optional<U>()`.
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+
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+ ``` cpp
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+ template<class F> constexpr auto transform(F&& f) &&;
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+ template<class F> constexpr auto transform(F&& f) const &&;
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+ ```
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+
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+ Let `U` be
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+ `remove_cv_t<invoke_result_t<F, decltype(std::move(value()))>>`.
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+
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+ *Mandates:* `U` is a non-array object type other than `in_place_t` or
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+ `nullopt_t`. The declaration
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+
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+ ``` cpp
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+ U u(invoke(std::forward<F>(f), std::move(value())));
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+ ```
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+
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+ is well-formed for some invented variable `u`.
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+
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+ [*Note 2*: There is no requirement that `U` is
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+ movable [[dcl.init.general]]. — *end note*]
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+
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+ *Returns:* If `*this` contains a value, an `optional<U>` object whose
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+ contained value is direct-non-list-initialized with
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+ `invoke(std::forward<F>(f), std::move(value()))`; otherwise,
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+ `optional<U>()`.
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+
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+ ``` cpp
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+ template<class F> constexpr optional or_else(F&& f) const &;
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+ ```
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+
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+ *Constraints:* `F` models `invocable<>` and `T` models
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+ `copy_constructible`.
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+
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+ *Mandates:* `is_same_v<remove_cvref_t<invoke_result_t<F>>, optional>` is
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+ `true`.
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+
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+ *Effects:* Equivalent to:
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+
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+ ``` cpp
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+ if (*this) {
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+ return *this;
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+ } else {
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+ return std::forward<F>(f)();
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+ }
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+ ```
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+
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+ ``` cpp
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+ template<class F> constexpr optional or_else(F&& f) &&;
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+ ```
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+
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+ *Constraints:* `F` models `invocable<>` and `T` models
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+ `move_constructible`.
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+
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+ *Mandates:* `is_same_v<remove_cvref_t<invoke_result_t<F>>, optional>` is
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+ `true`.
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+
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+ *Effects:* Equivalent to:
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+
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+ ``` cpp
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+ if (*this) {
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+ return std::move(*this);
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+ } else {
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+ return std::forward<F>(f)();
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+ }
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+ ```
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+