[Haskell-beginners] Maybe and Just

[Larry Evans]

On 03/27/2015 02:01 PM, Joel Neely wrote:
> Shishir,
> 
> Would I be right to guess that you're coming from an OO background? If
> so, I'd suggest thinking about the difference between declaring a
> constructor and invoking it.
> 
> I suggest that you can read the Haskell declaration
> 
>     data Maybe tp = Just tp | Nothing
> 
> 
> as declaring ​a data type and specifying two constructors that can
> return an instance (in the OO sense of the word) of that data type. But
> when the expression
> 
>     Just "FOO"
> 
> 
> is invoking one of those constructors on a value of type String, so the
> resulting value has type Maybe String.
> 
[snip]
Or, Maybe can be thought of as an abstract class with concrete
subclasses, Just and Nothing.

Or, Maybe can be thought of like a tagged union.  In c++ syntax:

  enum MaybeTag{ JustTag, NothingTag};
  template<typename ValType>
  struct JustType
  {
    ValType my_val;
    JustType(ValType a_val)
    : my_val(a_val)
    {}
  };
  struct NothingType
  {};
  template<typename ValType>
  struct Maybe
  {
  private:
      MaybeTag
    my_tag;
      union possible_values
      { JustType<ValType> just_val;
        NothingType nothing_val;
        possible_values(ValType a_val)
          : just_val(a_val)
          {}
        possible_values()
          : nothing_val()
          {}
      }
    my_val;
    Maybe(ValType a_val)
      : my_tag(JustTag)
      , my_val(a_val)
      {
      }
    Maybe()
      : my_tag(NothingTag)
      , my_val()
      {}
  public:
      MaybeTag
    tag()const
      { return my_tag;
      }
      static Maybe<ValType>
    Just(ValType a_val)
      { return Maybe<ValType>(a_val);
      }
      static Maybe<ValType>
    Nothing()
      { return Maybe<ValType>();
      }
  };

#include <iostream>

int main()
{
  auto mj=Maybe<int>::Just(5);
  std::cout<<"mj.tag="<<mj.tag()<<"\n";
  auto mn=Maybe<int>::Nothing();
  std::cout<<"mn.tag="<<mn.tag()<<"\n";
  return 0;
}

The following is the compile and run of above code:

g++ -std=c++11 tagged_union.cpp -o tagged_union.exe
./tagged_union.exe
mj.tag=0
mn.tag=1