[GHC] #15081: Finite list becomes infinite after maping fractional function for high numbers

[GHC]

#15081: Finite list becomes infinite after maping fractional function for high
numbers
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        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:                    |
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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>
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