CODEWITHSUNDEEP
haskelltypessyntax
user400055
user400055

Reputation:

Encapsulating data definitions in Haskell

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?

Upvotes: 1

Views: 483

Answers (3)

Dan Burton
Dan Burton

Reputation: 53705

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

Upvotes: 0

geekosaur
geekosaur

Reputation: 61439

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.)

Upvotes: 4

Don Stewart
Don Stewart

Reputation: 137987

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")
     ]

Upvotes: 7

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