隐藏构造函数但不导入导入类型

时间:2017-04-25 15:08:29

标签: haskell module

我有一个内部模块我想为

提供外部API
module Positive.Internal where

newtype Positive a = Positive { getPositive :: a }
  deriving (Eq, Ord)

-- smart constructor
toPositive :: (Num a, Ord a) => a -> Maybe (Positive a)
toPositive a | a <= 0    = Nothing
             | otherwise = Just $ Positive a
-- ...

我想隐藏哑构造函数,并用单向替换它 模式使用户仍然可以模式匹配值,他们只需使用智能构造函数来使用新值。

由于我希望模式和哑构造函数使用相同的名称,我需要隐藏哑构造函数以防止命名空间冲突。

但是,由于哑构造函数和类型共享名称,导入所有内容但是愚蠢的构造函数有点棘手。

目前我正在这样做,这可行:

{-# LANGUAGE PatternSynonyms #-}
module Positive
  ( module Positive.Internal, pattern Positive
  ) where

import Positive.Internal (Positive())
import Positive.Internal hiding (Positive)
import qualified Positive.Internal as Internal

pattern Positive :: a -> Positive a
pattern Positive a <- Internal.Positive a

我可以通过使用合格的导入来简化我的导入,但我很好奇。

有没有办法在单个import语句中导入除哑构造函数之外的所有Positive.Internal

我尝试了hiding (Positive(Positive)),但是它隐藏了类型和哑构造函数。我对the wiki进行了探讨,但我还没有注意到在hiding列表中区分构造函数和类型的方法。

1 个答案:

答案 0 :(得分:3)

如果我错了,请纠正我,但我几乎可以肯定这就是你要找的东西:

{-# LANGUAGE PatternSynonyms #-}
module Positive
  ( module Positive.Internal, pattern Positive, foo
  ) where

import Positive.Internal hiding (pattern Positive)
import qualified Positive.Internal as Internal (pattern Positive)

pattern Positive :: a -> Positive a
pattern Positive a <- Internal.Positive a

foo :: Positive Int
foo = Internal.Positive 5

Internal模块保持与目前定义的方式相同。并且为了举例:

module Negative where

import Positive

bar :: Maybe Int
bar = getPositive <$> toPositive 6

让我们仔细检查GHCi:

Prelude> :load Negative
[1 of 3] Compiling Positive.Internal ( Positive/Internal.hs, interpreted )
[2 of 3] Compiling Positive         ( Positive.hs, interpreted )
[3 of 3] Compiling Negative         ( Negative.hs, interpreted )
Ok, modules loaded: Negative, Positive, Positive.Internal.
*Negative> bar
Just 6
*Negative> getPositive foo
5
*Negative> :i Positive
newtype Positive a = Positive.Internal.Positive {getPositive :: a}
    -- Defined at Positive/Internal.hs:3:1
instance [safe] Ord a => Ord (Positive a)
  -- Defined at Positive/Internal.hs:4:17
instance [safe] Eq a => Eq (Positive a)
  -- Defined at Positive/Internal.hs:4:13
*Negative> :t Positive

<interactive>:1:1: error:
    • non-bidirectional pattern synonym ‘Positive’ used in an expression
    • In the expression: Positive