haskell - Haskell 中未导入记录的类型类实例

Question

我正在尝试使用 time-recurrence 中找到的 UTCTime 实例 CalendarTimeConvertible图书馆。它在文档中列出，但是当我将库导入 ghci 并评估 i: CalendarTimeConvertible 时，该实例不会出现。

class CalendarTimeConvertible t where
  toCalendarTime :: t -> CalendarTime
  fromCalendarTime :: CalendarTime -> Maybe t
    -- Defined in ‘time-recurrence-0.9.3:Data.Time.CalendarTime.CalendarTime’
instance CalendarTimeConvertible CalendarTime
  -- Defined in ‘time-recurrence-0.9.3:Data.Time.CalendarTime.CalendarTime’

它也不会出现在 i: UTCTime 的输出中。

data UTCTime = UTCTime {utctDay :: Day, utctDayTime :: DiffTime}
    -- Defined in ‘time-1.9.1:Data.Time.Clock.Internal.UTCTime’
instance Eq UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Clock.Internal.UTCTime’
instance Ord UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Clock.Internal.UTCTime’
instance Read UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Format.Parse’
instance Show UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.LocalTime.Internal.ZonedTime’
instance ParseTime UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Format.Parse.Instances’
instance FormatTime UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Format.Format.Instances’

我想不通。我缺少导入步骤吗？我搜索了显式导入类型类实例的方法，但一无所获。

编辑:这是我的 session :

λ> import Data.Time.Clock
λ> :i UTCTime
data UTCTime
  = UTCTime {utctDay :: time-1.9.1:Data.Time.Calendar.Days.Day,
             utctDayTime :: DiffTime}
    -- Defined in ‘time-1.9.1:Data.Time.Clock.Internal.UTCTime’
instance Eq UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Clock.Internal.UTCTime’
instance Ord UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Clock.Internal.UTCTime’
instance Read UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Format.Parse’
instance Show UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.LocalTime.Internal.ZonedTime’
λ> import Data.Time.CalendarTime
λ> :i CalendarTimeConvertible
class CalendarTimeConvertible t where
  toCalendarTime :: t -> CalendarTime
  fromCalendarTime :: CalendarTime -> Maybe t
    -- Defined in ‘time-recurrence-0.9.3:Data.Time.CalendarTime.CalendarTime’
instance CalendarTimeConvertible CalendarTime
  -- Defined in ‘time-recurrence-0.9.3:Data.Time.CalendarTime.CalendarTime’
λ> :i UTCTime
data UTCTime
  = UTCTime {utctDay :: time-1.9.1:Data.Time.Calendar.Days.Day,
             utctDayTime :: DiffTime}
    -- Defined in ‘time-1.9.1:Data.Time.Clock.Internal.UTCTime’
instance Eq UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Clock.Internal.UTCTime’
instance Ord UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Clock.Internal.UTCTime’
instance Read UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.Format.Parse’
instance Show UTCTime
  -- Defined in ‘time-1.9.1:Data.Time.LocalTime.Internal.ZonedTime’
λ> 

Answer 1

通常这意味着您在范围内有相同类型的两个不同版本。例如。你有UTCTime来自time-1.9.1 ，但是time-recurrence定义 UTCTime 的实例来自time-1.5.0.1 .

要检查理论，请尝试运行 ghc-pkg list time 。如果我是对的，它将列出已安装的两个不同版本。

另外，请注意 time-recurrence不支持time-1.9.1 。它对time有以下约束: :

time >= 1.4 && < 1.6

它证实了我的理论。

您可以使用 ghci -hide-package time-1.5.0.1 修复它