gistfile1.txt

Prelude> data Foo = Foo
Prelude> data Bar = Bar
Prelude> data Baz = Baz
Prelude> data DoesntHave
Prelude> data Has
Prelude> data MyXml:: * -> * -> * -> * where { DFoo :: Foo -> MyXml Has a b; DBar :: Bar -> MyXml a Has b; DBaz :: Baz -> MyXml a b Has }
Prelude> :t [DFoo Foo, DBaz Baz]
[DFoo Foo, DBaz Baz] :: [MyXml Has a Has]
Prelude> let a = DBar Bar ::  MyXml Has DoesntHave Has

<interactive>:28:9:
    Couldn't match expected type `DoesntHave' with actual type `Has'
    Expected type: MyXml Has DoesntHave Has
      Actual type: MyXml Has Has b0
    In the return type of a call of `DBar'
    In the expression: DBar Bar :: MyXml Has DoesntHave Has
Prelude> let a = DBaz Baz ::  MyXml Has DoesntHave Has
Prelude>