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

On Thu, Mar 17, 2011 at 07:30, C K Kashyap wrote:

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On Thu, Mar 17, 2011 at 12:30 AM, C K Kashyap wrote:

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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