Dear all, Today I spend nearly one day's time to debug this exception, but still haven't got a solution. My code is pasted [here][1]. The program throws an exception when evaluating this line: points = assert (len > 0) $ A.listArray (1::Int, len) $ map (\(u,i,r) -> [u,i]) rts Below is my ghci debug session. I still don't understand why it has a range of (0, -193459561) when the 'len' binding is actually 90570. *Main> :break 125 Breakpoint 4 activated at SVMCF.hs:125:13-86 *Main> :trace main Stopped at SVMCF.hs:125:13-86 _result :: UA.Array Int [User] = _ len :: Int = 90570 rts :: [RTuple] = (1,1,5.0) : (1,2,3.0) : (1,3,4.0) : (1,4,3.0) : (1,5,3.0) : .... [SVMCF.hs:125:13-86] *Main> :lis 124 points :: A.Array Int [Int] 125 points = assert (len > 0) $ A.listArray (1::Int, len) $ map (\(u,i,r) -> [u,i]) rts 126 values :: UA.UArray Int Double [SVMCF.hs:125:13-86] *Main> :ste Stopped at SVMCF.hs:125:13-28 _result :: UA.Array Int [User] -> UA.Array Int [User] = _ len :: Int = 90570 [SVMCF.hs:125:13-28] *Main> :ste Stopped at SVMCF.hs:125:21-27 _result :: Bool = _ len :: Int = 90570 [SVMCF.hs:125:21-27] *Main> :ste Stopped at SVMCF.hs:125:32-86 _result :: UA.Array Int [User] = _ len :: Int = 90570 rts :: [RTuple] = (1,1,5.0) : (1,2,3.0) : (1,3,4.0) : (1,4,3.0) : (1,5,3.0) : .... [SVMCF.hs:125:32-86] *Main> :ste Stopped at SVMCF.hs:125:32-56 _result :: [[User]] -> UA.Array Int [User] = _ len :: Int = 90570 [SVMCF.hs:125:32-56] *Main> :lis 124 points :: A.Array Int [Int] 125 points = assert (len > 0) $ A.listArray (1::Int, len) $ map (\(u,i,r) -> [u,i]) rts 126 values :: UA.UArray Int Double [SVMCF.hs:125:32-56] *Main> len 90570 [SVMCF.hs:125:32-56] *Main> :ste Stopped at SVMCF.hs:125:60-86 _result :: [[User]] = _ rts :: [RTuple] = (1,1,5.0) : (1,2,3.0) : (1,3,4.0) : (1,4,3.0) : (1,5,3.0) : .... [SVMCF.hs:125:60-86] *Main> :ste *** Exception: Ix{Int}.index: Index (1) out of range ((1,-193459561)) [1]: http://hpaste.org/47460/haskell_array_index_out_of_ran