'instance Flat Day' reports errors.

[highfly22]

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DeriveAnyClass #-}

import Data.Flat
import Data.Time

instance Flat Day

error:
    • No instance for (Data.Flat.Class.GEncode (Rep Day))
        arising from a use of ‘Data.Flat.Class.$dmencode’
    • In the expression: Data.Flat.Class.$dmencode @Day
      In an equation for ‘encode’:
          encode = Data.Flat.Class.$dmencode @Day
      In the instance declaration for ‘Flat Day’

[tittoassini]

To automatically derive a Flat instance for a data type, the data type must be an instance of Generic.

Unfortunately, this is not the case for Day.

[unhammer]

See https://github.com/fpco/store/blob/247f2863bed0f5c1003ab835075b6dcafb9aa773/src/Data/Store/Internal.hs#L669 for an instance of Day for store – I suppose it'd be something like (completely untested)

size = Data.Flat.Encoder.Size.sInteger
encode = Data.Flat.encode (Time.toModifiedJulianDay d)
decode = Time.ModifiedJulianDay <$> Data.Flat.decode

(it might make sense for flat to also include some instances for the popular non-generic types? or would it be better to have that in a flat-orphans package to avoid deps?)

[kk-hainq]

Is there anything I can help with to make flat-orphans possible? Will anyone support the idea?