Currently, GHC.Arr.listArray is pretty lazy: listArray (1,3) $ [1,2,3] ++ undefined will work perfectly fine. This is actually lazier than the current array: array (1,3) $ [(1,1), (2,2), (3,3)] ++ undefined = undefined Unfortunately, I don't think it's possible to make listArray fuse with a list producer while preserving quite that level of laziness. If we're willing to be slightly stricter, however, I think everything works. Specifically, I propose that we allow listArray (length xs) (xs ++ _|_) = _|_ The resulting listArray code is below. {-# INLINE mylistArray #-} listArray :: Ix i => (i,i) -> [e] -> Array i e listArray (l,u) es = runST (ST $ \s1# -> case safeRangeSize (l,u) of { n@(I# n#) -> case newArray# n# arrEleBottom s1# of { (# s2#, marr# #) -> let fillFromList y r i# s3# | isTrue# (i# ==# n#) = s3# | otherwise = case writeArray# marr# i# y s3# of s4# -> r (i# +# 1#) s4# in case foldr fillFromList (\_ s# -> s#) es 0# s2# of s5# -> done l u n marr# s5# }})