There are Haskell types that have an associated monad structure, but cannot be made instances of the
Monad class. The reason is typically that the return or the bind operation of such a type
m has a constraint on the type parameter of
m. As a result, all the nice library support for monads is unusable for such types. This problem is called the constrained-monad problem.
In my article The
Constraint kind, I described a solution to this problem, which involved changing the
Monad class. In this article, I present a solution that works with the standard
Monad class. This solution has been developed by Neil Sculthorpe, Jan Bracker, George Giorgidze, and Andy Gill. It is described in their paper The Constrained-Monad Problem and implemented in the constrained-normal package. Continue reading