1 files changed, 38 insertions, 6 deletions

diff --git a/src/Data/Array/XArray.hs b/src/Data/Array/XArray.hs
index ee83654..64bbb0c 100644
--- a/src/Data/Array/XArray.hs
+++ b/src/Data/Array/XArray.hs
@@ -1,9 +1,11 @@
+{-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ImportQualifiedPost #-}
+{-# LANGUAGE MultiWayIf #-}
 {-# LANGUAGE NoStarIsType #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StandaloneKindSignatures #-}
@@ -15,10 +17,11 @@
 module Data.Array.XArray where
 
 import Control.DeepSeq (NFData)
+import Control.Monad (foldM)
+import Control.Monad.ST
 import Data.Array.Internal qualified as OI
 import Data.Array.Internal.RankedG qualified as ORG
 import Data.Array.Internal.RankedS qualified as ORS
-import Data.Array.Ranked qualified as ORB
 import Data.Array.RankedS qualified as S
 import Data.Coerce
 import Data.Foldable (toList)
@@ -27,9 +30,9 @@ import Data.List.NonEmpty (NonEmpty)
 import Data.Proxy
 import Data.Type.Equality
 import Data.Type.Ord
-import Data.Vector qualified as V
 import Data.Vector.Generic.Checked qualified as VGC
 import Data.Vector.Storable qualified as VS
+import Data.Vector.Storable.Mutable qualified as VSM
 import Foreign.Storable (Storable)
 import GHC.Generics (Generic)
 import GHC.TypeLits
@@ -237,6 +240,7 @@ transpose ssh perm (XArray arr)
   = XArray (unsafeCoerce (S.transpose (permToList' perm) arr))
 #endif
 
+
 -- | The list argument gives indices into the original dimension list.
 --
 -- The permutation (the list) must have length <= @n@. If it is longer, this
@@ -303,21 +307,49 @@ sumOuter ssh ssh' arr
 
 -- | If @n@ is an 'SKnown' dimension, the list is streamed. If @n@ is unknown,
 -- the list's spine must be fully materialised to compute its length before
--- constructing the array.
+-- constructing the array. The list can't be empty (not enough information
+-- in the given shape to guess the shape of the empty array, in general).
 fromListOuter :: forall n sh a. Storable a
               => StaticShX (n : sh) -> [XArray sh a] -> XArray (n : sh) a
 fromListOuter ssh l
-  | Dict <- lemKnownNatRankSSX ssh
+  | Dict <- lemKnownNatRankSSX (ssxTail ssh)
   , let l' = coerce @[XArray sh a] @[S.Array (Rank sh) a] l
   = case ssh of
       _ :!% ZKX ->
         fromList1 ssh (map S.unScalar l')
       SKnown m :!% _ ->
         let n = fromSNat' m
-        in XArray (S.ravel (ORB.fromVector [n] (VGC.fromListNChecked n l')))
+        in XArray (ravelOuterN n l')
       _ ->
         let n = length l
-        in XArray (S.ravel (ORB.fromVector [n] (V.fromListN          n l')))
+        in XArray (ravelOuterN n l')
+
+-- This checks that the list has the given length and that each shape
+-- on the list equals the given shape. The list is streamed.
+-- The list can't be empty.
+ravelOuterN :: (KnownNat k, Storable a)
+            => Int -> [S.Array k a] -> S.Array (1 + k) a
+ravelOuterN 0 _ = error "ravelOuterN: N == 0"
+ravelOuterN _ [] = error "ravelOuterN: empty list"
+ravelOuterN k as@(a0 : _) = runST $ do
+  let sh0 = S.shapeL a0
+      len = product sh0
+      vecSize = k * len
+  vec <- VSM.unsafeNew vecSize
+  let f !n a =
+        if | n >= k ->
+             error $ "ravelOuterN: list too long " ++ show (n, k)
+               -- if we do this check just once at the end, we may
+               -- crash instead of producing an accurate error message
+           | S.shapeL a == sh0 -> do
+             VS.unsafeCopy (VSM.slice (n * len) len vec) (S.toVector a)
+             return $! n + 1
+           | otherwise ->
+             error $ "ravelOuterN: unequal shapes " ++ show (S.shapeL a, sh0)
+  nFinal <- foldM f 0 as
+  if nFinal == k
+  then S.fromVector (k : sh0) <$> VS.unsafeFreeze vec
+  else error $ "ravelOuterN: list too short " ++ show (nFinal, k)
 
 toListOuter :: forall a n sh. Storable a => XArray (n : sh) a -> [XArray sh a]
 toListOuter (XArray arr@(ORS.A (ORG.A _ t))) =