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| author | Herbert Valerio Riedel <hvr@gnu.org> | 2014-06-24 22:58:41 +0200 |
|---|---|---|
| committer | Herbert Valerio Riedel <hvr@gnu.org> | 2014-06-24 23:30:12 +0200 |
| commit | ec550e8f951e50fb91c89389e2e77a3358079c3a (patch) | |
| tree | 50295797463e2fa4bcb9ecef53ce652925b58344 | |
| parent | d8abf85f8ca176854e9d5d0b12371c4bc402aac3 (diff) | |
| download | haskell-ec550e8f951e50fb91c89389e2e77a3358079c3a.tar.gz | |
Fixup c1035d51e to behave more like in GHC 7.6
The fix in c1035d51e (addressing #9231) was done under the assumption
that `Data.Fixed` is used only with power-of-10 `resolution`. This
follow-up fixup changes the behavior to be more consistent with the
previous behavior in GHC 7.6
For instance, for the following `B7` resolution
> data B7
> instance HasResolution B7 where resolution _ = 128
After this patch, the following behavior is now observable:
> 1.070 :: Fixed B7
1.062
> 1.062 :: Fixed B7
1.054
> read "1.070" :: Fixed B7
1.062
> read "1.062" :: Fixed B7
1.054
This doesn't provide the desirable "read . show == id" property
yet (which didn't hold in GHC 7.6 either), but at least `fromRational`
and `read` are consistent.
Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
| -rw-r--r-- | libraries/base/Data/Fixed.hs | 9 |
1 files changed, 6 insertions, 3 deletions
diff --git a/libraries/base/Data/Fixed.hs b/libraries/base/Data/Fixed.hs index b3af208469..cadbb61ac1 100644 --- a/libraries/base/Data/Fixed.hs +++ b/libraries/base/Data/Fixed.hs @@ -159,9 +159,12 @@ instance (HasResolution a) => Read (Fixed a) where convertFixed :: forall a . HasResolution a => Lexeme -> ReadPrec (Fixed a) convertFixed (Number n) | Just (i, f) <- numberToFixed e n = - return (fromInteger i + (fromInteger f / fromInteger r)) - where r = resolution (undefined :: Fixed a) -- = 10^e - e = round (logBase 10 (fromInteger r) :: Double) + return (fromInteger i + (fromInteger f / (10 ^ e))) + where r = resolution (undefined :: Fixed a) + -- round 'e' up to help make the 'read . show == id' property + -- possible also for cases where 'resolution' is not a + -- power-of-10, such as e.g. when 'resolution = 128' + e = ceiling (logBase 10 (fromInteger r) :: Double) convertFixed _ = pfail data E0 = E0 |