在ghci下执行``（读取[Red]"）:: [Color]`时会发生什么？

问题描述

我正在阅读的小节关于

在两个

1. 您为实例定义的 readsPrec Read Color


2. Read [a]

我们在＃2 <$中调用 readsPrec c $ c> readsPrecList 。

评估时

 阅读[Red]:: [Color] 
  

调用code> readsPrecList 。该函数会捕获前导方形paren，并用输入字符串Red调用 readsPrec 。您的函数将删除前三个字符，并使用值 Red 返回< readsPrecList ，并将输入字符串设置为] 。该函数检查下一个字符是一个方括号，并返回列表中的值 - 即 [Red] 。

为什么评估首先调用 readPrecList ？因为您要求阅读来创建一个列表。

为什么 readsPrecList 为类型颜色调用 readsPrec ？因为您要求阅读来创建颜色值的列表。

这是一个类型定向调度的例子 - readsPrec 调用的实例由所请求的值的类型决定。

I am reading the subsection of Real World Haskell Chapter 6 (typeclasses) about aninstance of Read for Color, which implements readsPrec function. I don't know what happens when I type (read "[Red]") :: [Color] in GHCi, getting a [Red] result.

For simplicity, I changed this implementation of the function a little, as shown below:

instance Read Color where
     readsPrec _ value = [(Red, drop (length "Red") value)]

Now, my confusion is: in GHCi, we can use the above as follows:

*Main> let readsPrec 2 "Red]" = [(Red, drop (length "Red") "Red]")]
*Main> readsPrec 2 "Red]"
       [(Red,"]")]

Why readsPrec _ value = [(Red, drop (length "Red") value)] return [Red] when executing (read "[Red]") :: [Color]?

解决方案

There is an interaction going on between two instances of readsPrec:

1. the readsPrec you defined for instance Read Color
2. the readsPrec defined by the Prelude for instance Read [a]

Let's call the readsPrec in #2 readsPrecList.

When evaluating

read "[Red]" :: [Color]

what first happens is the readsPrecList is called. That function gobbles up the leading square paren and calls your readsPrec with the input string "Red]". Your function drops the first three characters and returns back to readsPrecList with the value Red and input string set to "]". That function checks that the next character is a closing square bracket and returns back your value in a list - i.e. [Red].

Why does evaluation begin with a call to readPrecList? Because you are asking read to create a list of something.

Why does readsPrecList call the readsPrec for the type Color? Because you asked read to create a list of Color values.

This is an example of type-directed dispatch - the instance of readsPrec called is determined by the type of the value being requested.

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