conversion from Integer to Double rounds differently depending on the value

I'm not sure if this is intended behavior or a bug.

For values less than 2^53^, the the Integer can be represented exactly as a Double so there is no issue. For values from 2^53^ to 2^63^, fromInteger n rounds to the nearest Double, where ties go toward the Double with even least significant bit. This matches the behavior of read. For values between 2^63^ and 2^1024^, fromInteger n rounds down to the next representable Double.

Steps to reproduce:

These values are all the same except the last one because it falls just below the threshold for the "round to nearest, ties to even" rule. In this example, (2^63^) and (2^63^-2^10^) are the consecutive values of type Double.

> fromInteger (2^63) :: Double           -- exactly 9223372036854775808
9.223372036854776e18
> fromInteger (2^63 - 1) :: Double       -- rounded up
9.223372036854776e18
> fromInteger (2^63 - 2^9) :: Double     -- rounded up
9.223372036854776e18
> fromInteger (2^63 - 2^9 - 1) :: Double -- rounded down
9.223372036854775e18
> fromInteger (2^63 - 2^10) :: Double    -- exactly 9223372036854774784
9.223372036854775e18

Here you can see the behavior of fromInteger matches that of read . show

> (read . show) (2^63) :: Double           -- exactly 9223372036854775808
9.223372036854776e18
> (read . show) (2^63 - 1) :: Double       -- rounded up
9.223372036854776e18
> (read . show) (2^63 - 2^9) :: Double     -- rounded up
9.223372036854776e18
> (read . show) (2^63 - 2^9 - 1) :: Double -- rounded down
9.223372036854775e18
> (read . show) (2^63 - 2^10) :: Double    -- exactly 9223372036854774784
9.223372036854775e18

Here you can see fromInteger is rounding down. In this example, (2^64^) and (2^64^-2^11^) are the consecutive values of type Double.

> fromInteger (2^64) :: Double            -- exactly 18446744073709551616
1.8446744073709552e19
> fromInteger (2^64 - 1) :: Double        -- rounded down
1.844674407370955e19
> fromInteger (2^64 - 2^10) :: Double     -- rounded down
1.844674407370955e19
> fromInteger (2^64 - 2^10 - 1) :: Double -- rounded down
1.844674407370955e19
> fromInteger (2^64 - 2^11) :: Double     -- exactly 18446744073709549568
1.844674407370955e19

Here you can see the behavior of fromInteger does not match the behavior of read . show

> (read . show) (2^64) :: Double            -- exactly 18446744073709551616
1.8446744073709552e19
> (read . show) (2^64 - 1) :: Double        -- rounded up
1.8446744073709552e19
> (read . show) (2^64 - 2^10) :: Double     -- rounded up
1.8446744073709552e19
> (read . show) (2^64 - 2^10 - 1) :: Double -- rounded down
1.844674407370955e19
> (read . show) (2^64 - 2^11) :: Double     -- exactly 18446744073709549568
1.844674407370955e19

Trac metadata

Trac field	Value
Version	8.6.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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