Fix genDate generator frequency #83
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src/Data/Date/Gen.purs
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| let maxDays = if isLeapYear year then 365 else 364 | ||
| janFirst = unsafePartial $ fromJust $ exactDate year bottom bottom | ||
| days <- Days <<< toNumber <$> chooseInt 0 maxDays | ||
| pure $ unsafePartial $ fromJust $ adjust days janFirst |
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I think we could consolidate the use of unsafePartial by moving janFirst down:
genDate :: forall m. MonadGen m => m Date
genDate = do
year <- genYear
let maxDays = if isLeapYear year then 365 else 364
days <- Days $ map toNumber $ chooseInt 0 maxDays
pure $ unsafePartial fromJust do
janFirst <- exactDate year bottom bottom
adjust days janFirst
thomashoneyman
approved these changes
Dec 17, 2020
I discovered that the generator was generating random years but the day was always December 31st, January 31st, December 1st or January 1st (in frequency order). This was because of purescript/purescript-gen#22 which is now fixed. However, reading at the current implementation, there is still a slight skew toward first days of months following a month with only 30 days or less. This is because `genYear <*> genMonth <*> genDay` could generate something like 2020-04-31 which then get converted to 2020-05-01 by `canonicalDate`. This means that 2020-05-01 has twice as much chance to get generated as other dates. The fix is to pick a day number between 1 and 365 (or 366 for leap years) instead.
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I discovered that the generator was generating random years but the day
was always December 31st, January 31st, December 1st or January 1st (in
frequency order).
This was because of
purescript/purescript-gen#22 which is now
fixed.
However, reading at the current implementation, there is still a slight
skew toward first days of months following a month with only 30 days or
less. This is because
genYear <*> genMonth <*> genDaycould generatesomething like 2020-04-31 which then get converted to 2020-05-01 by
canonicalDate. This means that 2020-05-01 has twice as much chance toget generated as other dates.
The fix is to pick a day number between 1 and 365 (or 366 for leap
years) instead.