A small discrete implementation of a Maybe Monad in C++. Other monads such as Promises to come!

Copy monads.h to somewhere in your local path or library search path. Note that it currently doesn't namespace its types (Maybe and MaybeMonad) so you'll need to use a forwarding header if you need them to be namespaced.

Example:

#include <string>

#include 'monads.h'

std::string getAnswer(const std::string &input) {
  int len;
  Maybe<std::string> maybe_val =
    MaybeMonad<std::string>(input)
      .bind([&len](const std::string &str) { len = str.length(); return str; })
      .predicate([](const std::string &str) { return str.length() > 12; })
      .bind([](const std::string &str) { return str + "\nYour Answer is: "; })
      .bind([](const std::string &str) { return str + "42"; })
      .extract();
  return maybe_val.value_or("There is No Answer");
}

See test.cpp for more usage examples.

Lambdas as arguments are hard to disambiguate by the compiler, even when return types are explicit. std::function is generous with up-casting given non-explicit copy-constructor conversion, so separate methods were required for the interface (bind and predicate) rather than relying simply on the return type. Given that booleans could be used as a contained type, having separate methods was likely desirable anyhow.

MIT