[Haskell-cafe] No instance for (Read POSIXTime)

[Yitzchak Gale]

dokondr wrote:
> When I try to read POSIXTime...
> No instance for (Read POSIXTime)...
> What should I do to provide Read instance for POSIXTime?

Short answer: if you are thinking about this as a moment
in time that could be parsed from the usual kind of
string representation for that, you probably want to
use UTCTime in your data type, not POSIXTime.

If you really, really want to represent it internally
as POSIXTime, then you should read those strings
as UTCTime and then convert them to POSIXTime
to store in your data type. I.e., in that case don't
make your data type an instance of Read.

POSIXTime is just a type alias for NominalDiffTime,
i.e., a quantity of time between two moments.

This is what the Show instance looks like:

Prelude Data.Time> realToFrac 1000000 :: NominalDiffTime
1000000s

By convention, the Read instance would expect a
string in that format. Generally people aren't interested
in that, so there is no Read instance.
Even if you did want to parse that, you would just
parse it as a number and then use realToFrac, as I
did above.

The rule of thumb is: always represent moments in
time as a UTCTime.

Regards,
Yitz