According to the documentation,

-- If the accumulating function is strict, then 'accumArray' is strict in
-- the values, as well as the indices, in the association list.  Thus,
-- unlike ordinary arrays built with 'array', accumulated arrays should
-- not in general be recursive.

I believe that strictness is desirable, because it prevents thunks from accumulating when we use the passed function. But the strictness isn't actually there! We never force the result of the function.

I believe we should fix the code (and documentation) to maintain the invariant that all elements in the result of accumArray are in WHNF.