[Haskell-cafe] Data.Time.Calendar.Day does not seem to have an instance for Read

[Erik Hesselink]

On Thu, Mar 17, 2011 at 07:30, C K Kashyap <ckkashyap at gmail.com> wrote:
>> On Thu, Mar 17, 2011 at 12:30 AM, C K Kashyap <ckkashyap at gmail.com> wrote:
>> > Hi,
>> > I was wondering if this is a defect -
>> > Prelude> import Data.Time.Calendar
>> > Prelude Data.Time.Calendar> read "2011-10-10" :: Day
>> > <interactive>:1:1:
>> >     No instance for (Read Day)
>> >       arising from a use of `read'
>> >     Possible fix: add an instance declaration for (Read Day)
>> >     In the expression: read "2011-10-10" :: Day
>> >     In an equation for `it': it = read "2011-10-10" :: Day
>> > Prelude Data.Time.Calendar>
>> >
>>
>> I see the same problem with GHC 7.0.2, time-1.2.0.3.
>>
>> Does it work if you use it in a Haskell source file, instead of GHCi?
>
> Nope, even compiling with ghc causes the error.

So then it's not a bug. The instance is defined in
Data.Time.Format.Parse, and Data.Time.Calendar doesn't import that
module.

This, however is a bug, I think:

Prelude> import Data.Time
Prelude Data.Time> read "2011-10-10" :: Day

... no instance for (Read Day) ...

Prelude> :m +Data.Time
Prelude Data.Time> read "2011-10-10" :: Day
2011-10-10


Erik