Question

I have started this a new question as it became a follow-on from my previous question.

If I have two data types which are composed of similar constructors:

data A = X | Y | Z
data B = X | Y

is there no way I can somehow represent this as:

data A = C | Z
data B = C

data C = X | Y

if you can see what I am doing- I am trying to group the X | Y into one data type, which can then be used by multiple other data types. I cannot seem to get the compiler to allow this, or if it does, I cannot pattern-match against the X or Y, only the C??

I get the error message that C has been declared multiple times.

I thought I could maybe use types, but they do not allow multiple typings.

EDIT

Even if I declare the long way (like below), it still won’t compile and says X and Y have multiple declarations:

data A = X | Y | Z
data B = X | Y

Answer 1

Not only can’t you do this, you also can’t do your first option – i.e. you can’t have two types in the same module that both have constructors named X and Y.

If you could do this, what should the type of X be – C, A or B? The most obvious answer would be C, but then you wouldn’t be able to use it in a context where an A or a B are required (note that Haskell has no subtyping), so that would defeat the purpose of the whole construct.

The best you can do is to wrap C in a constructor of A and B, i.e.:

data A = AC C | Z
data B = BC C
data C = X | Y

Then you could wrap a C with either the AC or the BC constructor to create a value of type A or B respectively.