Re: [Haskell-cafe] mapping a concept to a type

I find both heavy and redundant. The first forces me to specify if I want an argument of not (with the constructors MR and NR)

I'm sorry, I don't understand what you mean here.

Do you know of a construction/abstraction that allows having or not an argument (a variable number of arguments, here zero or one)?

Either way you do it (Maybe, or building the variant yourself), you're going to need a constructor. This is a feature, not a bug. Edward