Building Monads in C++

This is a guide/example of implementing various Haskell programming techniques in c++.

data types

Example: Bool

Haskell:

data Bool = False | True

C++:

class Bool {
  const bool b;
  Bool (const bool & b_) : b(b_) {}
  friend const Bool True(const bool & b_);
  friend const Bool False(const bool & b_);
};

const Bool True() { return Bool(true); }
const Bool False() { return Bool(false); }

Rules:

• Data type names should be CamelCase.
• All data members must be const.
• Make a public constructor inside the class to enable default constructors.
• Make a public function with {classname}_ to act as haskell style constructor.

const {classname} classname_(args)

• Make external functions which act as constructors / factories.
  • They must be friends to the class so that they can call the private constructor.

Haskell:

x = True;
y = False;

C++:

auto x = True();
auto y = False();

Operations:

Haskell:

(&&) :: Bool -> Bool -> Bool
(||) :: Bool -> Bool -> Bool
not :: Bool -> Bool

C++

class Bool {
  const bool b;
  ...
  // Operations
  friend Bool operator && (const Bool & x, const Bool & y);
  friend Bool operator || (const Bool & x, const Bool & y);
  friend Bool operator ! (const Bool & x);
  ...
};

Bool operator && (const Bool & x, const Bool & y) { return Bool(x.b && y.b); }
Bool operator || (const Bool & x, const Bool & y) { return Bool(x.b || y.b); }
Bool operator ! (const Bool & x) { return Bool(!x.b); }
const Bool otherwise = True();

Further Reading

Monads in C++ Minimal C++ implementation of Functor, Monad and Maybe