From a65306ba5d80891b20ac86fa3a3242f9497751e6 Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom@tomsmeding.com>
Date: Thu, 30 May 2024 11:58:40 +0200
Subject: Refactor Mixed (modules, regular function names)

---
 src/Data/Array/Mixed.hs | 757 +++---------------------------------------------
 1 file changed, 43 insertions(+), 714 deletions(-)

(limited to 'src/Data/Array/Mixed.hs')

diff --git a/src/Data/Array/Mixed.hs b/src/Data/Array/Mixed.hs
index 4ae89a1..0100ec8 100644
--- a/src/Data/Array/Mixed.hs
+++ b/src/Data/Array/Mixed.hs
@@ -30,300 +30,23 @@ module Data.Array.Mixed where
 import Control.DeepSeq (NFData(..))
 import qualified Data.Array.RankedS as S
 import qualified Data.Array.Ranked as ORB
-import Data.Bifunctor (first)
 import Data.Coerce
-import qualified Data.Foldable as Foldable
-import Data.Functor.Const
 import Data.Kind
-import Data.List (sort)
-import Data.Monoid (Sum(..))
 import Data.Proxy
-import Data.Type.Bool
 import Data.Type.Equality
 import Data.Type.Ord
 import qualified Data.Vector.Storable as VS
 import Foreign.Storable (Storable)
 import GHC.Generics (Generic)
-import GHC.IsList (IsList)
-import qualified GHC.IsList as IsList
-import GHC.TypeError
 import GHC.TypeLits
-import qualified GHC.TypeNats as TypeNats
-import Unsafe.Coerce (unsafeCoerce)
 
-import Data.Array.Nested.Internal.Arith
+import Data.Array.Mixed.Internal.Arith
+import Data.Array.Mixed.Lemmas
+import Data.Array.Mixed.Permutation
+import Data.Array.Mixed.Shape
+import Data.Array.Mixed.Types
 
 
--- | Evidence for the constraint @c a@.
-data Dict c a where
-  Dict :: c a => Dict c a
-
-fromSNat' :: SNat n -> Int
-fromSNat' = fromIntegral . fromSNat
-
-pattern SZ :: () => (n ~ 0) => SNat n
-pattern SZ <- ((\sn -> testEquality sn (SNat @0)) -> Just Refl)
-  where SZ = SNat
-
-pattern SS :: forall np1. () => forall n. (n + 1 ~ np1) => SNat n -> SNat np1
-pattern SS sn <- (snatPred -> Just (SNatPredResult sn Refl))
-  where SS = snatSucc
-
-{-# COMPLETE SZ, SS #-}
-
-snatSucc :: SNat n -> SNat (n + 1)
-snatSucc SNat = SNat
-
-data SNatPredResult np1 = forall n. SNatPredResult (SNat n) (n + 1 :~: np1)
-snatPred :: forall np1. SNat np1 -> Maybe (SNatPredResult np1)
-snatPred snp1 =
-  withKnownNat snp1 $
-    case cmpNat (Proxy @1) (Proxy @np1) of
-      LTI -> Just (SNatPredResult (SNat @(np1 - 1)) Refl)
-      EQI -> Just (SNatPredResult (SNat @(np1 - 1)) Refl)
-      GTI -> Nothing
-
-
--- | Type-level list append.
-type family l1 ++ l2 where
-  '[] ++ l2 = l2
-  (x : xs) ++ l2 = x : xs ++ l2
-
-lemAppNil :: l ++ '[] :~: l
-lemAppNil = unsafeCoerce Refl
-
-lemAppAssoc :: Proxy a -> Proxy b -> Proxy c -> (a ++ b) ++ c :~: a ++ (b ++ c)
-lemAppAssoc _ _ _ = unsafeCoerce Refl
-
-type family Replicate n a where
-  Replicate 0 a = '[]
-  Replicate n a = a : Replicate (n - 1) a
-
-
-type role ListX nominal representational
-type ListX :: [Maybe Nat] -> (Maybe Nat -> Type) -> Type
-data ListX sh f where
-  ZX :: ListX '[] f
-  (::%) :: f n -> ListX sh f -> ListX (n : sh) f
-deriving instance (forall n. Eq (f n)) => Eq (ListX sh f)
-deriving instance (forall n. Ord (f n)) => Ord (ListX sh f)
-infixr 3 ::%
-
-instance (forall n. Show (f n)) => Show (ListX sh f) where
-  showsPrec _ = showListX shows
-
-instance (forall n. NFData (f n)) => NFData (ListX sh f) where
-  rnf ZX = ()
-  rnf (x ::% l) = rnf x `seq` rnf l
-
-data UnconsListXRes f sh1 =
-  forall n sh. (n : sh ~ sh1) => UnconsListXRes (ListX sh f) (f n)
-unconsListX :: ListX sh1 f -> Maybe (UnconsListXRes f sh1)
-unconsListX (i ::% shl') = Just (UnconsListXRes shl' i)
-unconsListX ZX = Nothing
-
-fmapListX :: (forall n. f n -> g n) -> ListX sh f -> ListX sh g
-fmapListX _ ZX = ZX
-fmapListX f (x ::% xs) = f x ::% fmapListX f xs
-
-foldListX :: Monoid m => (forall n. f n -> m) -> ListX sh f -> m
-foldListX _ ZX = mempty
-foldListX f (x ::% xs) = f x <> foldListX f xs
-
-lengthListX :: ListX sh f -> Int
-lengthListX = getSum . foldListX (\_ -> Sum 1)
-
-snatLengthListX :: ListX sh f -> SNat (Rank sh)
-snatLengthListX ZX = SNat
-snatLengthListX (_ ::% l) | SNat <- snatLengthListX l = SNat
-
-showListX :: forall sh f. (forall n. f n -> ShowS) -> ListX sh f -> ShowS
-showListX f l = showString "[" . go "" l . showString "]"
-  where
-    go :: String -> ListX sh' f -> ShowS
-    go _ ZX = id
-    go prefix (x ::% xs) = showString prefix . f x . go "," xs
-
-listXToList :: ListX sh' (Const i) -> [i]
-listXToList ZX = []
-listXToList (Const i ::% is) = i : listXToList is
-
-
-type role IxX nominal representational
-type IxX :: [Maybe Nat] -> Type -> Type
-newtype IxX sh i = IxX (ListX sh (Const i))
-  deriving (Eq, Ord, Generic)
-
-pattern ZIX :: forall sh i. () => sh ~ '[] => IxX sh i
-pattern ZIX = IxX ZX
-
-pattern (:.%)
-  :: forall {sh1} {i}.
-     forall n sh. (n : sh ~ sh1)
-  => i -> IxX sh i -> IxX sh1 i
-pattern i :.% shl <- IxX (unconsListX -> Just (UnconsListXRes (IxX -> shl) (getConst -> i)))
-  where i :.% IxX shl = IxX (Const i ::% shl)
-infixr 3 :.%
-
-{-# COMPLETE ZIX, (:.%) #-}
-
-type IIxX sh = IxX sh Int
-
-instance Show i => Show (IxX sh i) where
-  showsPrec _ (IxX l) = showListX (\(Const i) -> shows i) l
-
-instance Functor (IxX sh) where
-  fmap f (IxX l) = IxX (fmapListX (Const . f . getConst) l)
-
-instance Foldable (IxX sh) where
-  foldMap f (IxX l) = foldListX (f . getConst) l
-
-instance NFData i => NFData (IxX sh i)
-
-
-data SMayNat i f n where
-  SUnknown :: i -> SMayNat i f Nothing
-  SKnown :: f n -> SMayNat i f (Just n)
-deriving instance (Show i, forall m. Show (f m)) => Show (SMayNat i f n)
-deriving instance (Eq i, forall m. Eq (f m)) => Eq (SMayNat i f n)
-deriving instance (Ord i, forall m. Ord (f m)) => Ord (SMayNat i f n)
-
-instance (NFData i, forall m. NFData (f m)) => NFData (SMayNat i f n) where
-  rnf (SUnknown i) = rnf i
-  rnf (SKnown x) = rnf x
-
-fromSMayNat :: (n ~ Nothing => i -> r) -> (forall m. n ~ Just m => f m -> r) -> SMayNat i f n -> r
-fromSMayNat f _ (SUnknown i) = f i
-fromSMayNat _ g (SKnown s) = g s
-
-fromSMayNat' :: SMayNat Int SNat n -> Int
-fromSMayNat' = fromSMayNat id fromSNat'
-
-type role ShX nominal representational
-type ShX :: [Maybe Nat] -> Type -> Type
-newtype ShX sh i = ShX (ListX sh (SMayNat i SNat))
-  deriving (Eq, Ord, Generic)
-
-pattern ZSX :: forall sh i. () => sh ~ '[] => ShX sh i
-pattern ZSX = ShX ZX
-
-pattern (:$%)
-  :: forall {sh1} {i}.
-     forall n sh. (n : sh ~ sh1)
-  => SMayNat i SNat n -> ShX sh i -> ShX sh1 i
-pattern i :$% shl <- ShX (unconsListX -> Just (UnconsListXRes (ShX -> shl) i))
-  where i :$% ShX shl = ShX (i ::% shl)
-infixr 3 :$%
-
-{-# COMPLETE ZSX, (:$%) #-}
-
-type IShX sh = ShX sh Int
-
-instance Show i => Show (ShX sh i) where
-  showsPrec _ (ShX l) = showListX (fromSMayNat shows (shows . fromSNat)) l
-
-instance Functor (ShX sh) where
-  fmap f (ShX l) = ShX (fmapListX (fromSMayNat (SUnknown . f) SKnown) l)
-
-instance NFData i => NFData (ShX sh i) where
-  rnf (ShX ZX) = ()
-  rnf (ShX (SUnknown i ::% l)) = rnf i `seq` rnf (ShX l)
-  rnf (ShX (SKnown SNat ::% l)) = rnf (ShX l)
-
-lengthShX :: ShX sh i -> Int
-lengthShX (ShX l) = lengthListX l
-
-shXToList :: IShX sh -> [Int]
-shXToList ZSX = []
-shXToList (smn :$% sh) = fromSMayNat' smn : shXToList sh
-
-
--- | The part of a shape that is statically known.
-type StaticShX :: [Maybe Nat] -> Type
-newtype StaticShX sh = StaticShX (ListX sh (SMayNat () SNat))
-  deriving (Eq, Ord)
-
-pattern ZKX :: forall sh. () => sh ~ '[] => StaticShX sh
-pattern ZKX = StaticShX ZX
-
-pattern (:!%)
-  :: forall {sh1}.
-     forall n sh. (n : sh ~ sh1)
-  => SMayNat () SNat n -> StaticShX sh -> StaticShX sh1
-pattern i :!% shl <- StaticShX (unconsListX -> Just (UnconsListXRes (StaticShX -> shl) i))
-  where i :!% StaticShX shl = StaticShX (i ::% shl)
-infixr 3 :!%
-
-{-# COMPLETE ZKX, (:!%) #-}
-
-instance Show (StaticShX sh) where
-  showsPrec _ (StaticShX l) = showListX (fromSMayNat shows (shows . fromSNat)) l
-
-lengthStaticShX :: StaticShX sh -> Int
-lengthStaticShX (StaticShX l) = lengthListX l
-
-geqStaticShX :: StaticShX sh -> StaticShX sh' -> Maybe (sh :~: sh')
-geqStaticShX ZKX ZKX = Just Refl
-geqStaticShX (SKnown n@SNat :!% sh) (SKnown m@SNat :!% sh')
-  | Just Refl <- sameNat n m
-  , Just Refl <- geqStaticShX sh sh'
-  = Just Refl
-geqStaticShX (SUnknown () :!% sh) (SUnknown () :!% sh')
-  | Just Refl <- geqStaticShX sh sh'
-  = Just Refl
-geqStaticShX _ _ = Nothing
-
-
--- | Evidence for the static part of a shape. This pops up only when you are
--- polymorphic in the element type of an array.
-type KnownShX :: [Maybe Nat] -> Constraint
-class KnownShX sh where knownShX :: StaticShX sh
-instance KnownShX '[] where knownShX = ZKX
-instance (KnownNat n, KnownShX sh) => KnownShX (Just n : sh) where knownShX = SKnown natSing :!% knownShX
-instance KnownShX sh => KnownShX (Nothing : sh) where knownShX = SUnknown () :!% knownShX
-
-
--- | Very untyped: only length is checked (at runtime).
-instance KnownShX sh => IsList (ListX sh (Const i)) where
-  type Item (ListX sh (Const i)) = i
-  fromList topl = go (knownShX @sh) topl
-    where
-      go :: StaticShX sh' -> [i] -> ListX sh' (Const i)
-      go ZKX [] = ZX
-      go (_ :!% sh) (i : is) = Const i ::% go sh is
-      go _ _ = error $ "IsList(ListX): Mismatched list length (type says "
-                         ++ show (lengthStaticShX (knownShX @sh)) ++ ", list has length "
-                         ++ show (length topl) ++ ")"
-  toList = listXToList
-
--- | Very untyped: only length is checked (at runtime), index bounds are __not checked__.
-instance KnownShX sh => IsList (IxX sh i) where
-  type Item (IxX sh i) = i
-  fromList = IxX . IsList.fromList
-  toList = Foldable.toList
-
--- | Untyped: length and known dimensions are checked (at runtime).
-instance KnownShX sh => IsList (ShX sh Int) where
-  type Item (ShX sh Int) = Int
-  fromList topl = ShX (go (knownShX @sh) topl)
-    where
-      go :: StaticShX sh' -> [Int] -> ListX sh' (SMayNat Int SNat)
-      go ZKX [] = ZX
-      go (SKnown sn :!% sh) (i : is)
-        | i == fromSNat' sn = SKnown sn ::% go sh is
-        | otherwise = error $ "IsList(ShX): Value does not match typing (type says "
-                                ++ show (fromSNat' sn) ++ ", list contains " ++ show i ++ ")"
-      go (SUnknown () :!% sh) (i : is) = SUnknown i ::% go sh is
-      go _ _ = error $ "IsList(ShX): Mismatched list length (type says "
-                         ++ show (lengthStaticShX (knownShX @sh)) ++ ", list has length "
-                         ++ show (length topl) ++ ")"
-  toList = shXToList
-
-
-type family Rank sh where
-  Rank '[] = 0
-  Rank (_ : sh) = Rank sh + 1
-
 type XArray :: [Maybe Nat] -> Type -> Type
 newtype XArray sh a = XArray (S.Array (Rank sh) a)
   deriving (Show, Eq, Generic)
@@ -333,180 +56,6 @@ deriving instance (Ord a, Storable a) => Ord (XArray '[] a)
 
 instance NFData a => NFData (XArray sh a)
 
-zeroIxX :: StaticShX sh -> IIxX sh
-zeroIxX ZKX = ZIX
-zeroIxX (_ :!% ssh) = 0 :.% zeroIxX ssh
-
-zeroIxX' :: IShX sh -> IIxX sh
-zeroIxX' ZSX = ZIX
-zeroIxX' (_ :$% sh) = 0 :.% zeroIxX' sh
-
--- This is a weird operation, so it has a long name
-completeShXzeros :: StaticShX sh -> IShX sh
-completeShXzeros ZKX = ZSX
-completeShXzeros (SUnknown () :!% ssh) = SUnknown 0 :$% completeShXzeros ssh
-completeShXzeros (SKnown n :!% ssh) = SKnown n :$% completeShXzeros ssh
-
-listxAppend :: ListX sh f -> ListX sh' f -> ListX (sh ++ sh') f
-listxAppend ZX idx' = idx'
-listxAppend (i ::% idx) idx' = i ::% listxAppend idx idx'
-
-ixAppend :: forall sh sh' i. IxX sh i -> IxX sh' i -> IxX (sh ++ sh') i
-ixAppend = coerce (listxAppend @_ @(Const i))
-
-shAppend :: forall sh sh' i. ShX sh i -> ShX sh' i -> ShX (sh ++ sh') i
-shAppend = coerce (listxAppend @_ @(SMayNat i SNat))
-
-listxDrop :: forall f g sh sh'. ListX (sh ++ sh') f -> ListX sh g -> ListX sh' f
-listxDrop long ZX = long
-listxDrop long (_ ::% short) = case long of _ ::% long' -> listxDrop long' short
-
-ixDrop :: forall sh sh' i. IxX (sh ++ sh') i -> IxX sh i -> IxX sh' i
-ixDrop = coerce (listxDrop @(Const i) @(Const i))
-
-shDropSSX :: forall sh sh' i. ShX (sh ++ sh') i -> StaticShX sh -> ShX sh' i
-shDropSSX = coerce (listxDrop @(SMayNat i SNat) @(SMayNat () SNat))
-
-shDropIx :: forall sh sh' i j. ShX (sh ++ sh') i -> IxX sh j -> ShX sh' i
-shDropIx = coerce (listxDrop @(SMayNat i SNat) @(Const j))
-
-shDropSh :: forall sh sh' i. ShX (sh ++ sh') i -> ShX sh i -> ShX sh' i
-shDropSh = coerce (listxDrop @(SMayNat i SNat) @(SMayNat i SNat))
-
-shTakeSSX :: forall sh sh' i. Proxy sh' -> ShX (sh ++ sh') i -> StaticShX sh -> ShX sh i
-shTakeSSX _ = flip go
-  where
-    go :: StaticShX sh1 -> ShX (sh1 ++ sh') i -> ShX sh1 i
-    go ZKX _ = ZSX
-    go (_ :!% ssh1) (n :$% sh) = n :$% go ssh1 sh
-
-ssxDropIx :: forall sh sh' i. StaticShX (sh ++ sh') -> IxX sh i -> StaticShX sh'
-ssxDropIx = coerce (listxDrop @(SMayNat () SNat) @(Const i))
-
--- TODO: generalise all these things to arbitrary @i@
-shTail :: IShX (n : sh) -> IShX sh
-shTail (_ :$% sh) = sh
-
-ssxTail :: StaticShX (n : sh) -> StaticShX sh
-ssxTail (_ :!% ssh) = ssh
-
-shAppSplit :: Proxy sh' -> StaticShX sh -> IShX (sh ++ sh') -> (IShX sh, IShX sh')
-shAppSplit _ ZKX idx = (ZSX, idx)
-shAppSplit p (_ :!% ssh) (i :$% idx) = first (i :$%) (shAppSplit p ssh idx)
-
-ssxAppend :: StaticShX sh -> StaticShX sh' -> StaticShX (sh ++ sh')
-ssxAppend ZKX sh' = sh'
-ssxAppend (n :!% sh) sh' = n :!% ssxAppend sh sh'
-
-shapeSize :: IShX sh -> Int
-shapeSize ZSX = 1
-shapeSize (n :$% sh) = fromSMayNat' n * shapeSize sh
-
--- | This may fail if @sh@ has @Nothing@s in it.
-ssxToShape' :: StaticShX sh -> Maybe (IShX sh)
-ssxToShape' ZKX = Just ZSX
-ssxToShape' (SKnown n :!% sh) = (SKnown n :$%) <$> ssxToShape' sh
-ssxToShape' (SUnknown _ :!% _) = Nothing
-
-lemReplicateSucc :: (a : Replicate n a) :~: Replicate (n + 1) a
-lemReplicateSucc = unsafeCoerce Refl
-
-ssxReplicate :: SNat n -> StaticShX (Replicate n Nothing)
-ssxReplicate SZ = ZKX
-ssxReplicate (SS (n :: SNat n'))
-  | Refl <- lemReplicateSucc @(Nothing @Nat) @n'
-  = SUnknown () :!% ssxReplicate n
-
-fromLinearIdx :: IShX sh -> Int -> IIxX sh
-fromLinearIdx = \sh i -> case go sh i of
-  (idx, 0) -> idx
-  _ -> error $ "fromLinearIdx: out of range (" ++ show i ++
-               " in array of shape " ++ show sh ++ ")"
-  where
-    -- returns (index in subarray, remaining index in enclosing array)
-    go :: IShX sh -> Int -> (IIxX sh, Int)
-    go ZSX i = (ZIX, i)
-    go (n :$% sh) i =
-      let (idx, i') = go sh i
-          (upi, locali) = i' `quotRem` fromSMayNat' n
-      in (locali :.% idx, upi)
-
-toLinearIdx :: IShX sh -> IIxX sh -> Int
-toLinearIdx = \sh i -> fst (go sh i)
-  where
-    -- returns (index in subarray, size of subarray)
-    go :: IShX sh -> IIxX sh -> (Int, Int)
-    go ZSX ZIX = (0, 1)
-    go (n :$% sh) (i :.% ix) =
-      let (lidx, sz) = go sh ix
-      in (sz * i + lidx, fromSMayNat' n * sz)
-
-enumShape :: IShX sh -> [IIxX sh]
-enumShape = \sh -> go sh id []
-  where
-    go :: IShX sh -> (IIxX sh -> a) -> [a] -> [a]
-    go ZSX f = (f ZIX :)
-    go (n :$% sh) f = foldr (.) id [go sh (f . (i :.%)) | i <- [0 .. fromSMayNat' n - 1]]
-
-shapeLshape :: IShX sh -> S.ShapeL
-shapeLshape ZSX = []
-shapeLshape (n :$% sh) = fromSMayNat' n : shapeLshape sh
-
-ssxLength :: StaticShX sh -> Int
-ssxLength ZKX = 0
-ssxLength (_ :!% ssh) = 1 + ssxLength ssh
-
-ssxIotaFrom :: Int -> StaticShX sh -> [Int]
-ssxIotaFrom _ ZKX = []
-ssxIotaFrom i (_ :!% ssh) = i : ssxIotaFrom (i+1) ssh
-
-type Flatten sh = Flatten' 1 sh
-
-type family Flatten' acc sh where
-  Flatten' acc '[] = Just acc
-  Flatten' acc (Nothing : sh) = Nothing
-  Flatten' acc (Just n : sh) = Flatten' (acc * n) sh
-
-flattenSSX :: StaticShX sh -> SMayNat () SNat (Flatten sh)
-flattenSSX = go (SNat @1)
-  where
-    go :: SNat acc -> StaticShX sh -> SMayNat () SNat (Flatten' acc sh)
-    go acc ZKX = SKnown acc
-    go _ (SUnknown () :!% _) = SUnknown ()
-    go acc (SKnown sn :!% sh) = go (mulSNat acc sn) sh
-
-flattenSh :: IShX sh -> SMayNat Int SNat (Flatten sh)
-flattenSh = go (SNat @1)
-  where
-    go :: SNat acc -> IShX sh -> SMayNat Int SNat (Flatten' acc sh)
-    go acc ZSX = SKnown acc
-    go acc (SUnknown n :$% sh) = SUnknown (goUnknown (fromSNat' acc * n) sh)
-    go acc (SKnown sn :$% sh) = go (mulSNat acc sn) sh
-
-    goUnknown :: Int -> IShX sh -> Int
-    goUnknown acc ZSX = acc
-    goUnknown acc (SUnknown n :$% sh) = goUnknown (acc * n) sh
-    goUnknown acc (SKnown sn :$% sh) = goUnknown (acc * fromSNat' sn) sh
-
-staticShapeFrom :: IShX sh -> StaticShX sh
-staticShapeFrom ZSX = ZKX
-staticShapeFrom (n :$% sh) = fromSMayNat (\_ -> SUnknown ()) SKnown n :!% staticShapeFrom sh
-
-lemRankApp :: StaticShX sh1 -> StaticShX sh2
-           -> Rank (sh1 ++ sh2) :~: Rank sh1 + Rank sh2
-lemRankApp _ _ = unsafeCoerce Refl  -- TODO improve this
-
-lemRankAppComm :: StaticShX sh1 -> StaticShX sh2
-               -> Rank (sh1 ++ sh2) :~: Rank (sh2 ++ sh1)
-lemRankAppComm _ _ = unsafeCoerce Refl  -- TODO improve this
-
-lemKnownNatRank :: IShX sh -> Dict KnownNat (Rank sh)
-lemKnownNatRank ZSX = Dict
-lemKnownNatRank (_ :$% sh) | Dict <- lemKnownNatRank sh = Dict
-
-lemKnownNatRankSSX :: StaticShX sh -> Dict KnownNat (Rank sh)
-lemKnownNatRankSSX ZKX = Dict
-lemKnownNatRankSSX (_ :!% ssh) | Dict <- lemKnownNatRankSSX ssh = Dict
 
 shape :: forall sh a. StaticShX sh -> XArray sh a -> IShX sh
 shape = \ssh (XArray arr) -> go ssh (S.shapeL arr)
@@ -519,7 +68,7 @@ shape = \ssh (XArray arr) -> go ssh (S.shapeL arr)
 fromVector :: forall sh a. Storable a => IShX sh -> VS.Vector a -> XArray sh a
 fromVector sh v
   | Dict <- lemKnownNatRank sh
-  = XArray (S.fromVector (shapeLshape sh) v)
+  = XArray (S.fromVector (shxToList sh) v)
 
 toVector :: Storable a => XArray sh a -> VS.Vector a
 toVector (XArray arr) = S.toVector arr
@@ -527,23 +76,18 @@ toVector (XArray arr) = S.toVector arr
 scalar :: Storable a => a -> XArray '[] a
 scalar = XArray . S.scalar
 
-eqShX :: IShX sh1 -> IShX sh2 -> Bool
-eqShX ZSX ZSX = True
-eqShX (n :$% sh1) (m :$% sh2) = fromSMayNat' n == fromSMayNat' m && eqShX sh1 sh2
-eqShX _ _ = False
-
 -- | Will throw if the array does not have the casted-to shape.
 cast :: forall sh1 sh2 sh' a. Rank sh1 ~ Rank sh2
      => StaticShX sh1 -> IShX sh2 -> StaticShX sh'
      -> XArray (sh1 ++ sh') a -> XArray (sh2 ++ sh') a
 cast ssh1 sh2 ssh' (XArray arr)
   | Refl <- lemRankApp ssh1 ssh'
-  , Refl <- lemRankApp (staticShapeFrom sh2) ssh'
+  , Refl <- lemRankApp (ssxFromShape sh2) ssh'
   = let arrsh :: IShX sh1
-        (arrsh, _) = shAppSplit (Proxy @sh') ssh1 (shape (ssxAppend ssh1 ssh') (XArray arr))
-    in if eqShX arrsh sh2
-         then XArray arr
-         else error $ "Data.Array.Mixed.cast: Cannot cast (" ++ show arrsh ++ ") to (" ++ show sh2 ++ ")"
+        (arrsh, _) = shxSplitApp (Proxy @sh') ssh1 (shape (ssxAppend ssh1 ssh') (XArray arr))
+    in case shxEqual arrsh sh2 of
+         Just _ -> XArray arr
+         Nothing -> error $ "Data.Array.Mixed.cast: Cannot cast (" ++ show arrsh ++ ") to (" ++ show sh2 ++ ")"
 
 unScalar :: Storable a => XArray '[] a -> a
 unScalar (XArray a) = S.unScalar a
@@ -551,24 +95,24 @@ unScalar (XArray a) = S.unScalar a
 replicate :: forall sh sh' a. Storable a => IShX sh -> StaticShX sh' -> XArray sh' a -> XArray (sh ++ sh') a
 replicate sh ssh' (XArray arr)
   | Dict <- lemKnownNatRankSSX ssh'
-  , Dict <- lemKnownNatRankSSX (ssxAppend (staticShapeFrom sh) ssh')
-  , Refl <- lemRankApp (staticShapeFrom sh) ssh'
-  = XArray (S.stretch (shapeLshape sh ++ S.shapeL arr) $
-            S.reshape (map (const 1) (shapeLshape sh) ++ S.shapeL arr) $
+  , Dict <- lemKnownNatRankSSX (ssxAppend (ssxFromShape sh) ssh')
+  , Refl <- lemRankApp (ssxFromShape sh) ssh'
+  = XArray (S.stretch (shxToList sh ++ S.shapeL arr) $
+            S.reshape (map (const 1) (shxToList sh) ++ S.shapeL arr) $
               arr)
 
 replicateScal :: forall sh a. Storable a => IShX sh -> a -> XArray sh a
 replicateScal sh x
   | Dict <- lemKnownNatRank sh
-  = XArray (S.constant (shapeLshape sh) x)
+  = XArray (S.constant (shxToList sh) x)
 
 generate :: Storable a => IShX sh -> (IIxX sh -> a) -> XArray sh a
-generate sh f = fromVector sh $ VS.generate (shapeSize sh) (f . fromLinearIdx sh)
+generate sh f = fromVector sh $ VS.generate (shxSize sh) (f . ixxFromLinear sh)
 
 -- generateM :: (Monad m, Storable a) => IShX sh -> (IIxX sh -> m a) -> m (XArray sh a)
 -- generateM sh f | Dict <- lemKnownNatRank sh =
---   XArray . S.fromVector (shapeLshape sh)
---     <$> VS.generateM (shapeSize sh) (f . fromLinearIdx sh)
+--   XArray . S.fromVector (shxShapeL sh)
+--     <$> VS.generateM (shxSize sh) (f . ixxFromLinear sh)
 
 indexPartial :: Storable a => XArray (sh ++ sh') a -> IIxX sh -> XArray sh' a
 indexPartial (XArray arr) ZIX = XArray arr
@@ -580,24 +124,6 @@ index xarr i
   = let XArray arr' = indexPartial xarr i :: XArray '[] a
     in S.unScalar arr'
 
-type family AddMaybe n m where
-  AddMaybe Nothing _ = Nothing
-  AddMaybe (Just _) Nothing = Nothing
-  AddMaybe (Just n) (Just m) = Just (n + m)
-
--- This should be a function in base
-plusSNat :: SNat n -> SNat m -> SNat (n + m)
-plusSNat n m = TypeNats.withSomeSNat (TypeNats.fromSNat n + TypeNats.fromSNat m) unsafeCoerce
-
--- This should be a function in base
-mulSNat :: SNat n -> SNat m -> SNat (n * m)
-mulSNat n m = TypeNats.withSomeSNat (TypeNats.fromSNat n * TypeNats.fromSNat m) unsafeCoerce
-
-smnAddMaybe :: SMayNat Int SNat n -> SMayNat Int SNat m -> SMayNat Int SNat (AddMaybe n m)
-smnAddMaybe (SUnknown n) m = SUnknown (n + fromSMayNat' m)
-smnAddMaybe (SKnown n) (SUnknown m) = SUnknown (fromSNat' n + m)
-smnAddMaybe (SKnown n) (SKnown m) = SKnown (plusSNat n m)
-
 append :: forall n m sh a. Storable a
        => StaticShX sh -> XArray (n : sh) a -> XArray (m : sh) a -> XArray (AddMaybe n m : sh) a
 append ssh (XArray a) (XArray b)
@@ -639,9 +165,9 @@ rerank :: forall sh sh1 sh2 a b.
        -> XArray (sh ++ sh1) a -> XArray (sh ++ sh2) b
 rerank ssh ssh1 ssh2 f xarr@(XArray arr)
   | Dict <- lemKnownNatRankSSX (ssxAppend ssh ssh2)
-  = let (sh, _) = shAppSplit (Proxy @sh1) ssh (shape (ssxAppend ssh ssh1) xarr)
-    in if any (== 0) (shapeLshape sh)
-         then XArray (S.fromList (shapeLshape (shAppend sh (completeShXzeros ssh2))) [])
+  = let (sh, _) = shxSplitApp (Proxy @sh1) ssh (shape (ssxAppend ssh ssh1) xarr)
+    in if any (== 0) (shxToList sh)
+         then XArray (S.fromList (shxToList (shxAppend sh (shxCompleteZeros ssh2))) [])
          else case () of
            () | Dict <- lemKnownNatRankSSX ssh
               , Dict <- lemKnownNatRankSSX ssh2
@@ -666,9 +192,9 @@ rerank2 :: forall sh sh1 sh2 a b c.
         -> XArray (sh ++ sh1) a -> XArray (sh ++ sh1) b -> XArray (sh ++ sh2) c
 rerank2 ssh ssh1 ssh2 f xarr1@(XArray arr1) (XArray arr2)
   | Dict <- lemKnownNatRankSSX (ssxAppend ssh ssh2)
-  = let (sh, _) = shAppSplit (Proxy @sh1) ssh (shape (ssxAppend ssh ssh1) xarr1)
-    in if any (== 0) (shapeLshape sh)
-         then XArray (S.fromList (shapeLshape (shAppend sh (completeShXzeros ssh2))) [])
+  = let (sh, _) = shxSplitApp (Proxy @sh1) ssh (shape (ssxAppend ssh ssh1) xarr1)
+    in if any (== 0) (shxToList sh)
+         then XArray (S.fromList (shxToList (shxAppend sh (shxCompleteZeros ssh2))) [])
          else case () of
            () | Dict <- lemKnownNatRankSSX ssh
               , Dict <- lemKnownNatRankSSX ssh2
@@ -678,211 +204,14 @@ rerank2 ssh ssh1 ssh2 f xarr1@(XArray arr1) (XArray arr2)
                           (\a b -> let XArray r = f (XArray a) (XArray b) in r)
                           arr1 arr2)
 
-type family Elem x l where
-  Elem x '[] = 'False
-  Elem x (x : _) = 'True
-  Elem x (_ : ys) = Elem x ys
-
-type family AllElem' as bs where
-  AllElem' '[] bs = 'True
-  AllElem' (a : as) bs = Elem a bs && AllElem' as bs
-
-type AllElem as bs = Assert (AllElem' as bs)
-  (TypeError (Text "The elements of " :<>: ShowType as :<>: Text " are not all in " :<>: ShowType bs))
-
-type family Count i n where
-  Count n n = '[]
-  Count i n = i : Count (i + 1) n
-
-type Permutation as = (AllElem as (Count 0 (Rank as)), AllElem (Count 0 (Rank as)) as)
-
-type family Index i sh where
-  Index 0 (n : sh) = n
-  Index i (_ : sh) = Index (i - 1) sh
-
-type family Permute is sh where
-  Permute '[] sh = '[]
-  Permute (i : is) sh = Index i sh : Permute is sh
-
-type PermutePrefix is sh = Permute is (TakeLen is sh) ++ DropLen is sh
-
-data HList f list where
-  HNil :: HList f '[]
-  HCons :: f a -> HList f l -> HList f (a : l)
-infixr 5 `HCons`
-deriving instance (forall a. Show (f a)) => Show (HList f list)
-deriving instance (forall a. Eq (f a)) => Eq (HList f list)
-
-foldHList :: Monoid m => (forall a. f a -> m) -> HList f list -> m
-foldHList _ HNil = mempty
-foldHList f (x `HCons` l) = f x <> foldHList f l
-
-snatLengthHList :: HList f list -> SNat (Rank list)
-snatLengthHList HNil = SNat
-snatLengthHList (_ `HCons` l) | SNat <- snatLengthHList l = SNat
-
-permFromList :: [Int] -> (forall list. HList SNat list -> r) -> r
-permFromList [] k = k HNil
-permFromList (x : xs) k = withSomeSNat (fromIntegral x) $ \case
-  Just sn -> permFromList xs $ \list -> k (sn `HCons` list)
-  Nothing -> error $ "Data.Array.Mixed.permFromList: negative number in list: " ++ show x
-
-permToList :: HList SNat list -> [Int]
-permToList = foldHList (pure . fromSNat')
-
-type family TakeLen ref l where
-  TakeLen '[] l = '[]
-  TakeLen (_ : ref) (x : xs) = x : TakeLen ref xs
-
-type family DropLen ref l where
-  DropLen '[] l = l
-  DropLen (_ : ref) (_ : xs) = DropLen ref xs
-
-lemRankPermute :: Proxy sh -> HList SNat is -> Rank (Permute is sh) :~: Rank is
-lemRankPermute _ HNil = Refl
-lemRankPermute p (_ `HCons` is) | Refl <- lemRankPermute p is = Refl
-
-lemRankDropLen :: forall is sh. (Rank is <= Rank sh)
-               => StaticShX sh -> HList SNat is -> Rank (DropLen is sh) :~: Rank sh - Rank is
-lemRankDropLen ZKX HNil = Refl
-lemRankDropLen (_ :!% sh) (_ `HCons` is) | Refl <- lemRankDropLen sh is = Refl
-lemRankDropLen (_ :!% _) HNil = Refl
-lemRankDropLen ZKX (_ `HCons` _) = error "1 <= 0"
-
-lemIndexSucc :: Proxy i -> Proxy a -> Proxy l -> Index (i + 1) (a : l) :~: Index i l
-lemIndexSucc _ _ _ = unsafeCoerce Refl
-
-listxTakeLen :: forall f is sh. HList SNat is -> ListX sh f -> ListX (TakeLen is sh) f
-listxTakeLen HNil _ = ZX
-listxTakeLen (_ `HCons` is) (n ::% sh) = n ::% listxTakeLen is sh
-listxTakeLen (_ `HCons` _) ZX = error "Permutation longer than shape"
-
-listxDropLen :: forall f is sh. HList SNat is -> ListX sh f -> ListX (DropLen is sh) f
-listxDropLen HNil sh = sh
-listxDropLen (_ `HCons` is) (_ ::% sh) = listxDropLen is sh
-listxDropLen (_ `HCons` _) ZX = error "Permutation longer than shape"
-
-listxPermute :: forall f is sh. HList SNat is -> ListX sh f -> ListX (Permute is sh) f
-listxPermute HNil _ = ZX
-listxPermute (i `HCons` (is :: HList SNat is')) (sh :: ListX sh f) = listxIndex (Proxy @is') (Proxy @sh) i sh (listxPermute is sh)
-
-listxIndex :: forall f is shT i sh. Proxy is -> Proxy shT -> SNat i -> ListX sh f -> ListX (Permute is shT) f -> ListX (Index i sh : Permute is shT) f
-listxIndex _ _ SZ (n ::% _) rest = n ::% rest
-listxIndex p pT (SS (i :: SNat i')) ((_ :: f n) ::% (sh :: ListX sh' f)) rest
-  | Refl <- lemIndexSucc (Proxy @i') (Proxy @n) (Proxy @sh')
-  = listxIndex p pT i sh rest
-listxIndex _ _ _ ZX _ = error "Index into empty shape"
-
-listxPermutePrefix :: forall f is sh. HList SNat is -> ListX sh f -> ListX (PermutePrefix is sh) f
-listxPermutePrefix perm sh = listxAppend (listxPermute perm (listxTakeLen perm sh)) (listxDropLen perm sh)
-
-ssxTakeLen :: forall is sh. HList SNat is -> StaticShX sh -> StaticShX (TakeLen is sh)
-ssxTakeLen = coerce (listxTakeLen @(SMayNat () SNat))
-
-ssxDropLen :: HList SNat is -> StaticShX sh -> StaticShX (DropLen is sh)
-ssxDropLen = coerce (listxDropLen @(SMayNat () SNat))
-
-ssxPermute :: HList SNat is -> StaticShX sh -> StaticShX (Permute is sh)
-ssxPermute = coerce (listxPermute @(SMayNat () SNat))
-
-ssxIndex :: Proxy is -> Proxy shT -> SNat i -> StaticShX sh -> StaticShX (Permute is shT) -> StaticShX (Index i sh : Permute is shT)
-ssxIndex p1 p2 = coerce (listxIndex @(SMayNat () SNat) p1 p2)
-
-ssxPermutePrefix :: HList SNat is -> StaticShX sh -> StaticShX (PermutePrefix is sh)
-ssxPermutePrefix = coerce (listxPermutePrefix @(SMayNat () SNat))
-
-shPermutePrefix :: HList SNat is -> IShX sh -> IShX (PermutePrefix is sh)
-shPermutePrefix = coerce (listxPermutePrefix @(SMayNat Int SNat))
-
--- TODO: test this thing more properly
-invertPermutation :: HList SNat is
-                  -> (forall is'.
-                           Permutation is'
-                        => HList SNat is'
-                        -> (forall sh. Rank sh ~ Rank is => StaticShX sh -> Permute is' (Permute is sh) :~: sh)
-                        -> r)
-                  -> r
-invertPermutation = \perm k ->
-  genPerm perm $ \(invperm :: HList SNat is') ->
-    let sn = snatLengthHList invperm
-    in case (provePerm1 (Proxy @is') sn invperm, provePerm2 (SNat @0) sn invperm) of
-         (Just Refl, Just Refl) ->
-           k invperm
-             (\ssh -> case provePermInverse perm invperm ssh of
-                        Just eq -> eq
-                        Nothing -> error $ "invertPermutation: did not generate inverse? perm = " ++ show perm
-                                           ++ " ; invperm = " ++ show invperm)
-         _ -> error $ "invertPermutation: did not generate permutation? perm = " ++ show perm
-                      ++ " ; invperm = " ++ show invperm
-  where
-    genPerm :: HList SNat is -> (forall is'. HList SNat is' -> r) -> r
-    genPerm perm =
-      let permList = foldHList (pure . fromSNat) perm
-      in toHList $ map snd (sort (zip permList [0..]))
-      where
-        toHList :: [Natural] -> (forall is'. HList SNat is' -> r) -> r
-        toHList [] k = k HNil
-        toHList (n : ns) k = toHList ns $ \l -> TypeNats.withSomeSNat n $ \sn -> k (HCons sn l)
-
-    lemElemCount :: (0 <= n, Compare n m ~ LT) => proxy n -> proxy m -> Elem n (Count 0 m) :~: True
-    lemElemCount _ _ = unsafeCoerce Refl
-
-    lemCount :: (OrdCond (Compare i n) True False True ~ True) => proxy i -> proxy n -> Count i n :~: i : Count (i + 1) n
-    lemCount _ _ = unsafeCoerce Refl
-
-    lemElem :: Elem x ys ~ True => proxy x -> proxy' (y : ys) -> Elem x (y : ys) :~: True
-    lemElem _ _ = unsafeCoerce Refl
-
-    provePerm1 :: Proxy isTop -> SNat (Rank isTop) -> HList SNat is'
-               -> Maybe (AllElem' is' (Count 0 (Rank isTop)) :~: True)
-    provePerm1 _ _ HNil = Just (Refl)
-    provePerm1 p rtop@SNat (HCons sn@SNat perm)
-      | Just Refl <- provePerm1 p rtop perm
-      = case (cmpNat (SNat @0) sn, cmpNat sn rtop) of
-          (LTI, LTI) | Refl <- lemElemCount sn rtop -> Just Refl
-          (EQI, LTI) | Refl <- lemElemCount sn rtop -> Just Refl
-          _ -> Nothing
-      | otherwise
-      = Nothing
-
-    provePerm2 :: SNat i -> SNat n -> HList SNat is' -> Maybe (AllElem' (Count i n) is' :~: True)
-    provePerm2 = \i@(SNat :: SNat i) n@SNat perm ->
-      case cmpNat i n of
-        EQI -> Just Refl
-        LTI | Refl <- lemCount i n
-            , Just Refl <- provePerm2 (SNat @(i + 1)) n perm
-            -> checkElem i perm
-            | otherwise -> Nothing
-        GTI -> error "unreachable"
-      where
-        checkElem :: SNat i -> HList SNat is' -> Maybe (Elem i is' :~: True)
-        checkElem _ HNil = Nothing
-        checkElem i@SNat (HCons k@SNat perm :: HList SNat is') =
-          case sameNat i k of
-            Just Refl -> Just Refl
-            Nothing | Just Refl <- checkElem i perm, Refl <- lemElem i (Proxy @is') -> Just Refl
-                    | otherwise -> Nothing
-
-    provePermInverse :: HList SNat is -> HList SNat is' -> StaticShX sh -> Maybe (Permute is' (Permute is sh) :~: sh)
-    provePermInverse perm perminv ssh = geqStaticShX (ssxPermute perminv (ssxPermute perm ssh)) ssh
-
-applyPermX :: forall f is sh. HList SNat is -> ListX sh f -> ListX (PermutePrefix is sh) f
-applyPermX perm sh = listxAppend (listxPermute perm (listxTakeLen perm sh)) (listxDropLen perm sh)
-
-applyPermIxX :: forall i is sh. HList SNat is -> IxX sh i -> IxX (PermutePrefix is sh) i
-applyPermIxX = coerce (applyPermX @(Const i))
-
-applyPermShX :: forall i is sh. HList SNat is -> ShX sh i -> ShX (PermutePrefix is sh) i
-applyPermShX = coerce (applyPermX @(SMayNat i SNat))
-
-class KnownNatList l where makeNatList :: HList SNat l
-instance KnownNatList '[] where makeNatList = HNil
-instance (KnownNat n, KnownNatList l) => KnownNatList (n : l) where makeNatList = natSing `HCons` makeNatList
+class KnownNatList l where makeNatList :: Perm l
+instance KnownNatList '[] where makeNatList = PNil
+instance (KnownNat n, KnownNatList l) => KnownNatList (n : l) where makeNatList = natSing `PCons` makeNatList
 
 -- | The list argument gives indices into the original dimension list.
-transpose :: forall is sh a. (Permutation is, Rank is <= Rank sh)
+transpose :: forall is sh a. (IsPermutation is, Rank is <= Rank sh)
           => StaticShX sh
-          -> HList SNat is
+          -> Perm is
           -> XArray sh a
           -> XArray (PermutePrefix is sh) a
 transpose ssh perm (XArray arr)
@@ -890,7 +219,7 @@ transpose ssh perm (XArray arr)
   , Refl <- lemRankApp (ssxPermute perm (ssxTakeLen perm ssh)) (ssxDropLen perm ssh)
   , Refl <- lemRankPermute (Proxy @(TakeLen is sh)) perm
   , Refl <- lemRankDropLen ssh perm
-  = XArray (S.transpose (permToList perm) arr)
+  = XArray (S.transpose (permToList' perm) arr)
 
 -- | The list argument gives indices into the original dimension list.
 --
@@ -929,14 +258,14 @@ sumInner :: forall sh sh' a. (Storable a, NumElt a)
          => StaticShX sh -> StaticShX sh' -> XArray (sh ++ sh') a -> XArray sh a
 sumInner ssh ssh' arr
   | Refl <- lemAppNil @sh
-  = let (_, sh') = shAppSplit (Proxy @sh') ssh (shape (ssxAppend ssh ssh') arr)
-        sh'F = flattenSh sh' :$% ZSX
-        ssh'F = staticShapeFrom sh'F
+  = let (_, sh') = shxSplitApp (Proxy @sh') ssh (shape (ssxAppend ssh ssh') arr)
+        sh'F = shxFlatten sh' :$% ZSX
+        ssh'F = ssxFromShape sh'F
 
         go :: XArray (sh ++ '[Flatten sh']) a -> XArray sh a
         go (XArray arr')
           | Refl <- lemRankApp ssh ssh'F
-          , let sn = snatLengthListX (let StaticShX l = ssh in l)
+          , let sn = listxLengthSNat (let StaticShX l = ssh in l)
           = XArray (numEltSum1Inner sn arr')
 
     in go $
@@ -949,10 +278,10 @@ sumOuter :: forall sh sh' a. (Storable a, NumElt a)
          => StaticShX sh -> StaticShX sh' -> XArray (sh ++ sh') a -> XArray sh' a
 sumOuter ssh ssh' arr
   | Refl <- lemAppNil @sh
-  = let (sh, _) = shAppSplit (Proxy @sh') ssh (shape (ssxAppend ssh ssh') arr)
-        shF = flattenSh sh :$% ZSX
-    in sumInner ssh' (staticShapeFrom shF) $
-       transpose2 (staticShapeFrom shF) ssh' $
+  = let (sh, _) = shxSplitApp (Proxy @sh') ssh (shape (ssxAppend ssh ssh') arr)
+        shF = shxFlatten sh :$% ZSX
+    in sumInner ssh' (ssxFromShape shF) $
+       transpose2 (ssxFromShape shF) ssh' $
        reshapePartial ssh ssh' shF $
          arr
 
@@ -988,7 +317,7 @@ toList1 (XArray arr) = S.toList arr
 empty :: forall sh a. Storable a => IShX sh -> XArray sh a
 empty sh
   | Dict <- lemKnownNatRank sh
-  = XArray (S.constant (shapeLshape sh)
+  = XArray (S.constant (shxToList sh)
                        (error "Data.Array.Mixed.empty: shape was not empty"))
 
 slice :: SNat i -> SNat n -> XArray (Just (i + n + k) : sh) a -> XArray (Just n : sh) a
@@ -1005,14 +334,14 @@ reshape :: forall sh1 sh2 a. Storable a => StaticShX sh1 -> IShX sh2 -> XArray s
 reshape ssh1 sh2 (XArray arr)
   | Dict <- lemKnownNatRankSSX ssh1
   , Dict <- lemKnownNatRank sh2
-  = XArray (S.reshape (shapeLshape sh2) arr)
+  = XArray (S.reshape (shxToList sh2) arr)
 
 -- | Throws if the given array and the target shape do not have the same number of elements.
 reshapePartial :: forall sh1 sh2 sh' a. Storable a => StaticShX sh1 -> StaticShX sh' -> IShX sh2 -> XArray (sh1 ++ sh') a -> XArray (sh2 ++ sh') a
 reshapePartial ssh1 ssh' sh2 (XArray arr)
   | Dict <- lemKnownNatRankSSX (ssxAppend ssh1 ssh')
-  , Dict <- lemKnownNatRankSSX (ssxAppend (staticShapeFrom sh2) ssh')
-  = XArray (S.reshape (shapeLshape sh2 ++ drop (lengthStaticShX ssh1) (S.shapeL arr)) arr)
+  , Dict <- lemKnownNatRankSSX (ssxAppend (ssxFromShape sh2) ssh')
+  = XArray (S.reshape (shxToList sh2 ++ drop (ssxLength ssh1) (S.shapeL arr)) arr)
 
 -- this was benchmarked to be (slightly) faster than S.iota, S.generate and S.fromVector(VS.enumFromTo).
 iota :: (Enum a, Storable a) => SNat n -> XArray '[Just n] a
-- 
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