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| author | Brian McKenna <brian@brianmckenna.org> | 2014-12-02 17:53:38 +0100 |
|---|---|---|
| committer | Herbert Valerio Riedel <hvr@gnu.org> | 2014-12-02 17:54:50 +0100 |
| commit | 7c38e985aa211ca44039c6d1db9fa13690749c59 (patch) | |
| tree | 1c97a63cd1e023644bc97c8144ed348dab6dffb9 | |
| parent | 6b063ef2a1f68290b51778a38e9b89b6fec5e170 (diff) | |
| download | haskell-7c38e985aa211ca44039c6d1db9fa13690749c59.tar.gz | |
Make `read . show = id` for Data.Fixed (fix #9240)
The QuickCheck property now succeeds:
prop :: Fixed B7 -> Bool
prop a = read (show a) == a
This changes the Show instance for Fixed to round up, rather than down
when calculating a digit. This needs to happen because Read also
rounds down:
data B7
instance HasResolution B7 where
resolution _ = 128
1 / 128 = 0.0078125
read "0.007" = (0.000 :: Fixed B7)
Here is an example of the change to Show:
showFixed False (0.009 :: Fixed B7)
-- Broken: "0.007"
-- Fixed: "0.008"
And now Read can continue to round down:
read "0.008" = (0.0078125 :: Fixed B7)
Reviewed By: hvr, ekmett
Differential Revision: https://phabricator.haskell.org/D547
| -rw-r--r-- | libraries/base/Data/Fixed.hs | 4 | ||||
| -rw-r--r-- | libraries/base/changelog.md | 2 | ||||
| -rw-r--r-- | libraries/base/tests/data-fixed-show-read.hs | 7 | ||||
| -rw-r--r-- | libraries/base/tests/data-fixed-show-read.stdout | 2 |
4 files changed, 14 insertions, 1 deletions
diff --git a/libraries/base/Data/Fixed.hs b/libraries/base/Data/Fixed.hs index 068eec5f12..f12a0e496d 100644 --- a/libraries/base/Data/Fixed.hs +++ b/libraries/base/Data/Fixed.hs @@ -143,7 +143,9 @@ showFixed chopTrailingZeros fa@(MkFixed a) = (show i) ++ (withDot (showIntegerZe -- enough digits to be unambiguous digits = ceiling (logBase 10 (fromInteger res) :: Double) maxnum = 10 ^ digits - fracNum = div (d * maxnum) res + -- read floors, so show must ceil for `read . show = id` to hold. See #9240 + fracNum = divCeil (d * maxnum) res + divCeil x y = (x + y - 1) `div` y instance (HasResolution a) => Show (Fixed a) where show = showFixed False diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md index 07c91a34aa..ef3e9ae95c 100644 --- a/libraries/base/changelog.md +++ b/libraries/base/changelog.md @@ -128,6 +128,8 @@ together with a new exception `AllocationLimitExceeded`. + * Make `read . show = id` for `Data.Fixed` (#9240) + ## 4.7.0.2 *Dec 2014* * Bundled with GHC 7.8.4 diff --git a/libraries/base/tests/data-fixed-show-read.hs b/libraries/base/tests/data-fixed-show-read.hs index 349f639f2c..7e947f466e 100644 --- a/libraries/base/tests/data-fixed-show-read.hs +++ b/libraries/base/tests/data-fixed-show-read.hs @@ -3,6 +3,11 @@ module Main (main) where import Data.Fixed +data B7 + +instance HasResolution B7 where + resolution _ = 128 + main :: IO () main = do doit 38.001 doit 38.009 @@ -14,6 +19,8 @@ main = do doit 38.001 doit (-38.01) doit (-38.09) print (read "-38" :: Centi) + print (read "0.008" :: Fixed B7) + print (read "-0.008" :: Fixed B7) doit :: Centi -> IO () doit c = do let s = show c diff --git a/libraries/base/tests/data-fixed-show-read.stdout b/libraries/base/tests/data-fixed-show-read.stdout index 0e5d7caef5..4abb2d9676 100644 --- a/libraries/base/tests/data-fixed-show-read.stdout +++ b/libraries/base/tests/data-fixed-show-read.stdout @@ -16,3 +16,5 @@ -38.09 -38.09 -38.00 +0.008 +-0.008 |