#15081: Finite list becomes infinite after maping fractional function for high numbers -------------------------------------+------------------------------------- Reporter: Onsed | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: Incorrect result | (amd64) at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by sighingnow): Look at the enumerate method for fractional types: {{{#!hs numericEnumFrom :: (Fractional a) => a -> [a] numericEnumFrom n = n `seq` (n : numericEnumFrom (n + 1)) numericEnumFromTo :: (Ord a, Fractional a) => a -> a -> [a] numericEnumFromTo n m = takeWhile (<= m + 1/2) (numericEnumFrom n) }}} A possible improvement should be recording the accumulate increment and add it to the base number, rather than `+1` every time. For `Double` (64-bit floating point), the number `9007199254740991 + 1/2` has binary representation: {{{ 0x4340000000000000 }}} However, the number * `9007199254740990` is: `0x433ffffffffffffe` * `9007199254740990 + 1` is `0x433fffffffffffff` * `(9007199254740990 + 1) + 1` is `0x4340000000000000` * `((9007199254740990 + 1) + 1) + 1` is `0x4340000000000000` Note that `(9007199254740990 + 1) + 1` has the same binary representation with `((9007199254740990 + 1) + 1) + 1`, so the condition `(<= m + 1/2)` will always hold. Then we get the infinite list. If we change the upper bound, for example, {{{ map (/2) [9007199254740989..9007199254740990] }}} It works fine and prints `[4.5035996273704945e15,4.503599627370495e15]`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15081#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler