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无法为关联数据族派生Typeable(Cannot derive Typeable for associated data family)

 我正在尝试为相关数据系列派生Typeable ，如下所示： 
 

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE TypeFamilies #-}
module Test where

import Data.Typeable

class Test a where
  data DTest a :: *

instance Test () where
  data DTest () = Foo
    deriving Typeable

 

 但我收到以下错误消息，它让我有点困惑： 
 

[1 of 1] Compiling Test             ( test.hs, test.o )

test.hs:12:14:
    Cannot eta-reduce to an instance of form
      instance (...) => Typeable DTest
    In the data instance declaration for ‘DTest’

 

 有人可以详细说明这里出了什么问题吗？ 

I'm trying to derive Typeable for an associated data family like so:
 

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE TypeFamilies #-}
module Test where

import Data.Typeable

class Test a where
  data DTest a :: *

instance Test () where
  data DTest () = Foo
    deriving Typeable

 

But I'm getting the following error message and it leaves me a little puzzled:
 

[1 of 1] Compiling Test             ( test.hs, test.o )

test.hs:12:14:
    Cannot eta-reduce to an instance of form
      instance (...) => Typeable DTest
    In the data instance declaration for ‘DTest’

 

Could someone please elaborate as to what is going wrong here?

原文：https://stackoverflow.com/questions/29994790
更新时间：2023-11-28 22:11

最满意答案

 一些基于正则表达式的预处理怎么样？ 
 

 示例input.html ： 
 

<html>
  <head>
    <title>My example</title>
  </head>
  <body>
    <h1>Test</h1>
    <div id="foo">&nbsp;</div>
    <script type="text/javascript">
      document.getElementById('foo').innerHTML = '<span style="color:red;">Hello World!</span>';
    </script>
  </body>
</html>

 

 脚本标记删除PHP脚本： 
 

<?php

    // unformatted source output:
    header("Content-Type: text/plain");

    // read the example input file given above into a string:
    $input = file_get_contents('input.html');

    echo "Before:\r\n";
    echo $input;
    echo "\r\n\r\n-----------------------\r\n\r\n";

    // replace script tags including their contents by ""
    $output = preg_replace("~<script[^<>]*>.*</script>~Uis", "", $input);

    echo "After:\r\n";
    echo $output;
    echo "\r\n\r\n-----------------------\r\n\r\n";

?>


How about some regex-based pre-processing?
 

Example input.html:
 

<html>
  <head>
    <title>My example</title>
  </head>
  <body>
    <h1>Test</h1>
    <div id="foo">&nbsp;</div>
    <script type="text/javascript">
      document.getElementById('foo').innerHTML = '<span style="color:red;">Hello World!</span>';
    </script>
  </body>
</html>

 

Script tag removing php script:
 

<?php

    // unformatted source output:
    header("Content-Type: text/plain");

    // read the example input file given above into a string:
    $input = file_get_contents('input.html');

    echo "Before:\r\n";
    echo $input;
    echo "\r\n\r\n-----------------------\r\n\r\n";

    // replace script tags including their contents by ""
    $output = preg_replace("~<script[^<>]*>.*</script>~Uis", "", $input);

    echo "After:\r\n";
    echo $output;
    echo "\r\n\r\n-----------------------\r\n\r\n";

?>

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