[Haskell-cafe] RebindableSyntax Strangeness

[Li-yao Xia]

Hi Henry,

RebindableSyntax implies NoImplicitPrelude, which puts Eq, Ord, Show, 
and Read out of scope.

Cheers,
Li-yao

On 8/31/2020 10:12 AM, Henry Laxen wrote:
> Hi Cafe,
> 
> I have encountered the following puzzling behaviour from a very simple
> program:
> 
> Consider
> 
> -- {-# LANGUAGE RebindableSyntax   #-}
> module A where
> import Data.Time
> 
> running this with ghci gives:
> 
> A> :i UTCTime
> type UTCTime :: *
> data UTCTime = UTCTime {utctDay :: Day, utctDayTime :: DiffTime}
>    	-- Defined in ‘time-1.9.3:Data.Time.Clock.Internal.UTCTime’
> instance [safe] Eq UTCTime
>    -- Defined in ‘time-1.9.3:Data.Time.Clock.Internal.UTCTime’
> instance [safe] Ord UTCTime
>    -- Defined in ‘time-1.9.3:Data.Time.Clock.Internal.UTCTime’
> instance [safe] Show UTCTime
>    -- Defined in ‘time-1.9.3:Data.Time.LocalTime.Internal.ZonedTime’
> instance [safe] FormatTime UTCTime
>    -- Defined in ‘time-1.9.3:Data.Time.Format.Format.Instances’
> instance [safe] Read UTCTime
>    -- Defined in ‘time-1.9.3:Data.Time.Format.Parse’
> instance [safe] ParseTime UTCTime
>    -- Defined in ‘time-1.9.3:Data.Time.Format.Parse.Instances’
> 
> Now uncomment the first line (RebindableSyntax) and rerun it:
> 
> A> :i UTCTime
> type UTCTime :: *
> data UTCTime = UTCTime {utctDay :: Day, utctDayTime :: DiffTime}
>    	-- Defined in ‘time-1.9.3:Data.Time.Clock.Internal.UTCTime’
> instance [safe] FormatTime UTCTime
>    -- Defined in ‘time-1.9.3:Data.Time.Format.Format.Instances’
> instance [safe] ParseTime UTCTime
>    -- Defined in ‘time-1.9.3:Data.Time.Format.Parse.Instances’
> 
> And all of the nice instances for UTCTime have disappeared.  Why?
> 
> Best wishes,
> Henry Laxen
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