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46 lines
1.4 KiB
Plaintext
46 lines
1.4 KiB
Plaintext
[section Monadic interface]
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The monadic interface of `optional` allows the application of functions
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to optional values without resorting to the usage of explicit `if`-statements.
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Function `map` takes a function mapping type `T` onto type `U` and maps an `optional<T>`
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onto an `optional<U>` using the provided function.
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int length(const string& s){ return s.size(); };
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optional<string> null{}, thin{""}, word{"word"};
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assert (null.map(length) == none);
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assert (thin.map(length) == 0);
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assert (word.map(length) == 4);
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Function `flat_map` is similar, but it requires the function to return an
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`optional<V>` for some type `V`. This `optional<V>` becomes the return type of
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`flat_map`.
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optional<char> first_char(const string& s) {
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if (s.empty()) return none;
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else return s[0];
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};
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optional<string> null{}, thin{""}, word{"word"};
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assert (null.flat_map(first_char) == none);
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assert (thin.flat_map(first_char) == none);
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assert (word.flat_map(first_char) == 'w');
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These functions can be combined in one expression reflecting a chain of computations:
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auto get_contents(path p) -> optional<string>;
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auto trim(string) -> string;
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auto length(string) -> int;
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auto trimmed_size_of(optional<path> p) -> int
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{
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return p.flat_map(get_contents)
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.map(trim)
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.map(length)
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.value_or(0);
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}
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[endsect]
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