a.hs

data Foo :: * -> * where -- the * -> * means that Foo expects a type to be concrete, so like Foo Int, Foo Bool, etc.
  FooInt :: Int -> Foo Int
  FooBool :: Bool -> Foo Bool
  
-- so the compiler can enforce that there is no way to build a Foo t where t is not either Int or Bool.

-- then, matching on Foo Int is exhaustive here:
matchIntOnly :: Foo Int -> Int
matchIntOnly (FooInt x) = x

something like that :

data FooType = Int | Bool 

data Foo :: FooType -> Foo FooType

What's the difference ?

It would be

data FooType = FooInt Int | FooBool Bool

And you cannot inject get the « favor » (int or bool) at compile type of a FooType, so your second line doesn’t make sense.