Better error message in IsList(ListR)

1 files changed, 4 insertions, 2 deletions

diff --git a/src/Data/Array/Nested/Internal/Shape.hs b/src/Data/Array/Nested/Internal/Shape.hs
index a7797a7..b7bbf4c 100644
--- a/src/Data/Array/Nested/Internal/Shape.hs
+++ b/src/Data/Array/Nested/Internal/Shape.hs
@@ -273,12 +273,14 @@ shrPermutePrefix = coerce (listrPermutePrefix @i)
 -- | Untyped: length is checked at runtime.
 instance KnownNat n => IsList (ListR n i) where
   type Item (ListR n i) = i
-  fromList = go (SNat @n)
+  fromList topl = go (SNat @n) topl
     where
       go :: SNat n' -> [i] -> ListR n' i
       go SZ [] = ZR
       go (SS n) (i : is) = i ::: go n is
-      go _ _ = error "IsList(ListR): Mismatched list length"
+      go _ _ = error $ "IsList(ListR): Mismatched list length (type says "
+                         ++ show (fromSNat (SNat @n)) ++ ", list has length "
+                         ++ show (length topl) ++ ")"
   toList = Foldable.toList
 
 -- | Untyped: length is checked at runtime.