Why this exception occurs ?
Hi. In GHCi ver 6.6, why this happens ? Prelude Data.ByteString> Data.ByteString.pack $! Prelude.map (`rem` 256) $ [0..511] "*** Exception: divide by zero -------------- L.Guo 2007-05-25
leaveye.guo:
Hi.
In GHCi ver 6.6, why this happens ?
Prelude Data.ByteString> Data.ByteString.pack $! Prelude.map (`rem` 256) $ [0..511] "*** Exception: divide by zero
It's the use of `rem` on Word8, by the way: Prelude> (0 `rem` 256) :: Data.Word.Word8 *** Exception: divide by zero -- Don
dons:
leaveye.guo:
Hi.
In GHCi ver 6.6, why this happens ?
Prelude Data.ByteString> Data.ByteString.pack $! Prelude.map (`rem` 256) $ [0..511] "*** Exception: divide by zero
It's the use of `rem` on Word8, by the way:
Prelude> (0 `rem` 256) :: Data.Word.Word8 *** Exception: divide by zero
Sorry, I should clarify, think about how to represent: 256 :: Word8 ;-) -- Don
When I was tring manually truncate data to Word8 to fill into ByteString, I got the exception. Thanks. Now I understand the reason for that exception. And I know it is no need to manually truncate data. ------------------ L.Guo 2007-05-25 ------------------------------------------------------------- From: Donald Bruce Stewart At: 2007-05-25 15:33:46 Subject: Re: [Haskell-cafe] Why this exception occurs ? dons:
leaveye.guo:
Hi.
In GHCi ver 6.6, why this happens ?
Prelude Data.ByteString> Data.ByteString.pack $! Prelude.map (`rem` 256) $ [0..511] "*** Exception: divide by zero
It's the use of `rem` on Word8, by the way:
Prelude> (0 `rem` 256) :: Data.Word.Word8 *** Exception: divide by zero
Sorry, I should clarify, think about how to represent: 256 :: Word8 ;-) -- Don
I think it likes a trap. See this. Data.ByteString.unpack . Data.ByteString.pack $! ([0,17..255] ++ [1,18..256]) Without thinking about for Word8, [1,18..256] is equal to [1,18..0]. Though I try to use "$!" to let GHC generate the list as Integer. It would not do so. :-L ------------------ L.Guo 2007-05-25
L.Guo wrote:
Without thinking about for Word8, [1,18..256] is equal to [1,18..0]. Though I try to use "$!" to let GHC generate the list as Integer. It would not do so. $! merely forces evaluation; it doesn't change the types.
What you want is more like: ws: [Word8] ws = map fromIntegral [1,18..256 :: Int] There is no need for multi-precision Integers here: Int will do just fine. Paul.
On Fri, 2007-05-25 at 17:33 +1000, Donald Bruce Stewart wrote:
Sorry, I should clarify, think about how to represent:
256 :: Word8
So the error isn't really divide by zero, but overflow. I've been bitten by this, too, and L.Guo should count him/herself lucky to get an error, and not just incorrect results. IMO, this is a rather nasty hole in the safety nets that Haskell is so abundantly endowed with. Does any Haskell implementation support overflow detection? -k
ketil:
On Fri, 2007-05-25 at 17:33 +1000, Donald Bruce Stewart wrote:
Sorry, I should clarify, think about how to represent:
256 :: Word8
So the error isn't really divide by zero, but overflow. I've been bitten by this, too, and L.Guo should count him/herself lucky to get an error, and not just incorrect results.
I've always thought that the obfuscation opportunities for Num literal overloading, combined with Num *overflowing* were underappreciated. instance Num String anyone? Mwhaha -- Don
Donald Bruce Stewart wrote:
instance Num String anyone? Mwhaha
addString xs ys = add 0 xs ys where m = fromEnum (maxBound :: Char) + 1 alu c x y = let s = c + fromEnum x + fromEnum y in if s >= m then (1, s-m) else (0, s) add c (x:xs) (y:ys) = case alu c x y of (c', s') -> toEnum s' : add c' xs ys add c [] (y:ys) = case alu c 0 y of (0, s') -> toEnum s' : ys (1, s') -> toEnum s' : add 1 [] ys add c xs@(_:_) [] = add c [] xs add c [] [] = if c==0 then [] else [toEnum c]
participants (6)
-
Albert Y. C. Lai -
Clifford Beshers -
dons＠cse.unsw.edu.au -
Ketil Malde -
L.Guo -
Paul Johnson