Use fixed-width integers to represent days

[charles-cooper]

Currently https://hackage.haskell.org/package/time-1.8.0.1/docs/Data-Time-Calendar.html#t:Day is represented in memory as an Integer, and many similar functions like fromGregorian which interface with Integers. While this is admirably future-proof (we can deal with years larger than the year 2,147,481,790 AD!) I think we will be using intuitionistic type theory sooner than we will need those dates :)

Using something like Int32 would give us nearly the same flexibility, better Storable instances, interfacing with C and serialization/deserialization. I'm not sure how much this will break downstream code, but I have a feeling most people are using fromIntegral to interface with the API anyways, and this change will actually lessen the amount of marshalling people need to do.

[charles-cooper]

Something like charles-cooper@404f094.

To get a sense of practicality I was able to get ghc to build (https://github.com/charles-cooper/ghc/tree/day_int32) with only one change to libraries/Cabal:

diff --git a/Cabal/Distribution/Simple/SrcDist.hs b/Cabal/Distribution/Simple/SrcDist.hs
index 90a6323..a0b5e0e 100644
--- a/Cabal/Distribution/Simple/SrcDist.hs
+++ b/Cabal/Distribution/Simple/SrcDist.hs
@@ -391,10 +391,10 @@ snapshotVersion date = alterVersion (++ [dateToSnapshotNumber date])
 -- For example given a date @18/03/2008@ produce the number @20080318@.
 --
 dateToSnapshotNumber :: UTCTime -> Int
-dateToSnapshotNumber date = case toGregorian (utctDay date) of
+dateToSnapshotNumber date = fromIntegral $ case toGregorian (utctDay date) of
                             (year, month, day) ->
-                                fromIntegral year * 10000
-                              + month             * 100
+                                fromIntegral{-backwards compat time-}year  * 10000
+                              + month * 100
                               + day

Which needs an extra fromIntegral for one of the bootstrapping stages.

[AshleyYakeley]

It's about 6 million years each way, which is probably reasonable.

Prelude Data.Int Data.Time> ModifiedJulianDay $ toInteger (maxBound :: Int32)
5881469-05-27
Prelude Data.Int Data.Time> ModifiedJulianDay $ toInteger (minBound :: Int32)
-5877752-05-08

[AshleyYakeley]

#80 is the larger issue.