Why isn't Int moving forward?

Question

I would like to have points in two or three dimensions, so that two-dimensional points and three-dimensional points can share the code, but the compiler can tell them apart. Here is the first attempt.

{-# LANGUAGE DataKinds, GADTs, KindSignatures #-}

data Dimension = D2 | D3
data Point :: Dimension -> * where
  Point :: Dimension -> [Int] -> Point d
origin = Point D2 [0, 0]

It still works. Here is a simplified version:

data Point' :: Int -> * where
  Point' :: Int -> [Int] -> Point' d
origin' = Point' 2 [0, 0]

It will not compile: ‘Int’ of kind ‘*’ is not promotable. The data type promotion documentation for GHC 7.10.3 lists various reasons why a type may not be promoted (for example, if it already includes advanced types), but I don’t understand why they exclude Int.

(1) Why does this error occur?

And as a bonus

(2) Is there a reasonable solution or alternative approach? A search shows, for example, fixed-length Vector types in Haskell , but that seems complicated.

Answer 1

Int , (?). , C ; , ( " Int" ), , , , .

- . Peano nats - , . , , , Hackage -.

Answer 2

, Int , GHC.TypeLits.Nat. Point'

import GHC.TypeLits

data Point' :: Nat -> * where
    Point' :: Int -> [Int] -> Point' d

origin, GHC , d in Point d - phantom.