1 files changed, 11 insertions, 2 deletions
diff --git a/src/Data/Array/Nested/Mixed.hs b/src/Data/Array/Nested/Mixed.hs
index 54f8fe6..0a2fc17 100644
--- a/src/Data/Array/Nested/Mixed.hs
+++ b/src/Data/Array/Nested/Mixed.hs
@@ -16,6 +16,7 @@
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE UndecidableSuperClasses #-}
 {-# LANGUAGE ViewPatterns #-}
 module Data.Array.Nested.Mixed where
 
@@ -29,7 +30,7 @@ import Data.Bifunctor (bimap)
 import Data.Coerce
 import Data.Foldable (toList)
 import Data.Int
-import Data.Kind (Type)
+import Data.Kind (Type, Constraint)
 import Data.List.NonEmpty (NonEmpty(..))
 import Data.List.NonEmpty qualified as NE
 import Data.Proxy
@@ -290,7 +291,9 @@ matan2Array = mliftNumElt2 (liftO2 . floatEltAtan2)
 -- | Allowable element types in a mixed array, and by extension in a 'Ranked' or
 -- 'Shaped' array. Note the polymorphic instance for 'Elt' of @'Primitive'
 -- a@; see the documentation for 'Primitive' for more details.
-class Elt a where
+class EltC a => Elt a where
+  type EltC a :: Constraint
+
   -- ====== PUBLIC METHODS ====== --
 
   mshape :: Mixed sh a -> IShX sh
@@ -396,6 +399,8 @@ class Elt a => KnownElt a where
 
 -- Arrays of scalars are basically just arrays of scalars.
 instance Storable a => Elt (Primitive a) where
+  type EltC (Primitive a) = Storable a
+
   mshape (M_Primitive sh _) = sh
   mindex (M_Primitive _ a) i = Primitive (X.index a i)
   mindexPartial (M_Primitive sh a) i = M_Primitive (shxDropIx sh i) (X.indexPartial a i)
@@ -500,6 +505,8 @@ deriving via Primitive () instance KnownElt ()
 
 -- Arrays of pairs are pairs of arrays.
 instance (Elt a, Elt b) => Elt (a, b) where
+  type EltC (a, b) = (Elt a, Elt b)
+
   mshape (M_Tup2 a _) = mshape a
   mindex (M_Tup2 a b) i = (mindex a i, mindex b i)
   mindexPartial (M_Tup2 a b) i = M_Tup2 (mindexPartial a i) (mindexPartial b i)
@@ -549,6 +556,8 @@ instance (KnownElt a, KnownElt b) => KnownElt (a, b) where
 
 -- Arrays of arrays are just arrays, but with more dimensions.
 instance Elt a => Elt (Mixed sh' a) where
+  type EltC (Mixed sh' a) = Elt a
+
   -- TODO: this is quadratic in the nesting depth because it repeatedly
   -- truncates the shape vector to one a little shorter. Fix with a
   -- moverlongShape method, a prefix of which is mshape.