newtype如何帮助隐藏任何东西？ [英] How does newtype help hiding anything?

问题描述

真实世界haskell说：

Real world haskell says:

我们将使用newtype
声明隐藏解析器类型的细节

we will hide the details of our parser type using a newtype declaration

我不明白我们如何使用newtype隐藏任何东西。任何人都可以详述吗？

I don't get how we can hide anything using the newtype. Can anyone elaborate? What are we trying to hide and how do we do it.

data ParseState = ParseState {
  string :: L.ByteString
, offset :: Int64           -- imported from Data.Int
} deriving (Show)


newtype Parse a = Parse {
    runParse :: ParseState -> Either String (a, ParseState)
}

推荐答案

这个想法是将modules + newtypes结合起来，以防止人们看到我们如何实现内部事物。

The idea is to combine modules + newtypes to keep people from seeing the internals of how we implement things.

-- module A
module A (A, toA) where -- Notice we limit our exports
newtype A = A {unA :: Int}

toA :: Int -> A
toA = -- Do clever validation

-- module B
import A
foo :: A
foo = toA 1 -- Must use toA and can't see internals of A

这可以防止模式匹配和任意构造 A 。这让我们的 A 模块对 A 做一些假设，并且改变 A 逍遥法外！

This prevents from pattern matching and arbitrarily constructing A. This let's our A module make certain assumptions about A and also change the internals of A with impunity!

这非常好，因为在运行时， newtype 几乎没有像这样做的开销

This is particularly nice because at runtime the newtypes are erased so there's almost no overhead from doing something like this

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