Encapsulating data definitions in Haskell

问题

I am trying to define a data type using other data types like this:

data A = Something String | SomethingElse Int

data B = Another B | YetAnother A

data C = A | B

x :: [ C ]
x = [ YetAnother (SomethingElse 0), Something "Hello World" ]

But this is giving me an error saying that I cannot have a type A when expecting a type B. Why is this?

回答1:

You're missing the data constructors for C.

data A = Something String
       | SomethingElse Int

data B = Another    B
       | YetAnother A

data C = C0 A
       | C1 B

x :: [ C ]
x = [ C1 (YetAnother (SomethingElse 0))
    , C0 (Something "Hello World")
     ]

回答2:

The A and B in data C = A | B are declarations of new data constructors, not references to your existing types A and B. (Constructors are not optional in data declarations.)

回答3:

Haskell does not have true "union" types, unions in Haskell must be tagged with constructors. (see Wikipedia > Tagged union).

Either is the general-purpose tagged union type in Haskell, where data is tagged as Left or Right.

data Either a b = Left a | Right b

来源：https://stackoverflow.com/questions/10553803/encapsulating-data-definitions-in-haskell