Haskell 私有(private)记录，如 OCaml

Question

在 OCaml 模块签名中，我可以写:

type monotype = private {
  monotype_free_variables: StringSet.t Lazy.t;
  monotype_description: monotype_description;
}

(注意 private 的使用。)

这允许导入我的模块的代码与 monotype_description 等字段进行模式匹配但不允许代码导入我的模块来构造记录。

我可以在 Haskell 中编写具有类似访问权限的代码吗？我想对 monotypeDescription 进行模式匹配字段不允许记录构造。

我知道 PatternSynonyms语言扩展存在，但我不知道如何使用它。如果可以的话，我也更喜欢更简单的解决方案。

Answer 1

如果不至少允许记录更新(您要么导入字段)，就无法匹配字段。我认为限制更多，因为没有明显的语法用于仅导入访问器......

正确的解决方案是真正使用 PatternSynonyms .您需要一个单向模式:

{-# LANGUAGE PatternSynonyms #-}
module MyModule ( Monotype(Monotype) ) where

import Data.Set

data Monotype = MT (Set String) String

pattern Monotype :: Set String -> String -> Monotype
pattern Monotype { monotype_free_variables, monotype_description }
  <- MT monotype_free_variables monotype_description

现在，您可以匹配 Monotype { monotype_free_variables, monotype_description }但你不能构造它:

λ> Monotype { monotype_free_variables = f1, monotype_description = f2 } = m1
λ> Monotype f1 f2 = m1
λ> m3 = Monotype { monotype_free_variables = mempty, monotype_description = "" }
<interactive>:3:6: error:
    • non-bidirectional pattern synonym ‘Monotype’ used in an expression
    • In the expression:
        Monotype
          {monotype_free_variables = mempty, monotype_description = ""}
      In an equation for ‘m3’:
          m3
            = Monotype
                {monotype_free_variables = mempty, monotype_description = ""}
λ> m3 = Monotype mempty ""

<interactive>:4:6: error:
    • non-bidirectional pattern synonym ‘Monotype’ used in an expression
    • In the expression: Monotype mempty ""
      In an equation for ‘m3’: m3 = Monotype mempty ""