| I think, I've found the problem. GHC seems to not implement Data.Array.array | correctly. The implementation doesn't adhere to the following sentence from | the Data.Array.IArray.array documentation: | If any two associations in the list have the same index, the value at that | index is undefined. | Could this be fixed? It's a documented incompatibility with H98 http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs-and-infelic ities.html#HASKELL98-DIVERGENCE We don't intend to fix this because fixing it will have a significant overhead on every array construction. Actually I rather think H98 should have left the behaviour of double indices undefined, rather than defining it to be bottom. But it's too late now. Simon