Monadic `optional` utilities

[vittorioromeo]

scelta now provides various monadic operations that work on any supported optional type. Here's an example inspired by Simon Brand's "Functional exceptionless error-handling with optional and expected" article:

optional<image_view> crop_to_cat(image_view);
optional<image_view> add_bow_tie(image_view);
optional<image_view> make_eyes_sparkle(image_view);
image_view make_smaller(image_view);
image_view add_rainbow(image_view);

optional<image_view> get_cute_cat(image_view img)
{
    using namespace scelta::infix;
    return crop_to_cat(img)
         | and_then(add_bow_tie)
         | and_then(make_eyes_sparkle)
         | map(make_smaller)
         | map(add_rainbow);
}

The provided monadic optional operations can be used in two different ways:

optional<int> o{/* ... */};

// Free function syntax:
scelta::map(o, [](int x){ return x + 1; });

// Infix syntax:
o | scelta::infix::map([](int x){ return x + 1; });

These are the available operations:

• map_or_else(o, f_def, f)

  • Returns f(*o) if o is set, f_def() otherwise.

• map_or(o, def, f)

  • Returns f(*o) if o is set, def otherwise.

• map(o, f)

  • Returns optional{f(*o)} if o is set, an empty optional otherwise.

• and_then(o, f)

  • Returns f(*o) if o is set, an empty optional otherwise.

• and_(o, ob)

  • Returns ob if o is set, an empty ob otherwise.

• or_else(o, f)

  • Returns o if o is set, f() otherwise.

• or_(o, def)

  • Returns o if o is set, def otherwise.

The example file example/optional_cat.cpp shows usage of map and and_then using scelta::infix syntax.