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Generic instance based on ISO 8601 representation #19

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Generic instance based on ISO 8601 representation #19

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3 changes: 2 additions & 1 deletion bower.json
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Expand Up @@ -26,6 +26,7 @@
"dependencies": {
"purescript-enums": "^0.7.0",
"purescript-functions": "^0.1.0",
"purescript-globals": "^0.2.0"
"purescript-globals": "^0.2.0",
"purescript-generics": "~0.6.2"
}
}
12 changes: 10 additions & 2 deletions src/Data/Date.purs
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Expand Up @@ -24,6 +24,7 @@ import Control.Monad.Eff
import Data.Enum (Enum, Cardinality(..), fromEnum, defaultSucc, defaultPred)
import Data.Function (on, Fn2(), runFn2, Fn3(), runFn3)
import Data.Maybe (Maybe(..))
import Data.Generic
import Data.Time

-- | A native JavaScript `Date` object.
Expand All @@ -36,11 +37,18 @@ foreign import data JSDate :: *
-- | modules.
newtype Date = DateTime JSDate

instance genericDate :: Generic Date where
toSpine d = SString (runFn2 jsDateMethod "toISOString" (toJSDate d))
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This instance doesn't represent what a Date actually is at runtime, so it's an invalid instance. Foreign data types can't have generic instances for this reason.

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So valid instances of Generic have the true representation and not just an isomorphism? That makes sense. I can close the PR if it's nonsensical.

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Yes exactly. If we allowed this, then things like everywhere would apply functions of Strings to Dates which would be bad. This is why it's recommended to derive generic instances, because they're so hard to get right by hand :)

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Ooh, I see! Thanks for the clarification :)

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Thanks for looking at this though. A newtype around String with a smart constructor might be a good way to handle this if a Generic instance is needed for some reason.

toSignature _ = SigString
fromSpine (SString s) = fromStringStrict s
fromSpine _ = Nothing


instance eqDate :: Eq Date where
eq = eq `on` toEpochMilliseconds
eq = gEq

instance ordDate :: Ord Date where
compare = compare `on` toEpochMilliseconds
compare = gCompare

instance showDate :: Show Date where
show d = "(fromEpochMilliseconds " ++ show (toEpochMilliseconds d) ++ ")"
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