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不存在量化方法时重叠实例错误的原因(Reason for overlapping instances error when not quantifying method existentially)

 给定以下代码，从go类型中删除forall ar失败，并显示“Typeable（D r）重叠实例”。 我想知道为什么？  
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}
module M where

import Data.Typeable (Proxy, Typeable, cast)

class C r where
    data D r :: *

deriving instance Typeable D

data A = forall r . A (D r)

go :: forall r a . (Typeable a, Typeable (D r)) => a -> Proxy r -> A
go a _ = case cast a of
  Just (b :: D r) -> A b
  Nothing -> error "fail to cast"
 
 该错误还说“选择取决于r的实例化” - 但是不是由提供的Proxy r固定的？ 

Given the following code, removing forall a r from the type of go fails with "Overlapping instances for Typeable (D r)". I wonder why? 
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}
module M where

import Data.Typeable (Proxy, Typeable, cast)

class C r where
    data D r :: *

deriving instance Typeable D

data A = forall r . A (D r)

go :: forall r a . (Typeable a, Typeable (D r)) => a -> Proxy r -> A
go a _ = case cast a of
  Just (b :: D r) -> A b
  Nothing -> error "fail to cast"
 
The error also says "The choice depends on the instantiation of r" - but isn't that pinned by the supplied Proxy r?

原文：https://stackoverflow.com/questions/31114770

更新时间：2022-02-10 15:02

最满意答案

 希望这可以帮助！  
 使用bootstrap beforeShowDay选项启用日期。  
 
 
  
  $(function() {
var datesEnabled = ['2017-9-10', '2017-9-15', '2017-9-25'];
$('#datepicker').datepicker({
format: 'yyyy-mm-dd',
beforeShowDay: function (date) {
  var allDates = date.getFullYear() + '-' + (date.getMonth() + 1) + '-' + date.getDate();
  if(datesEnabled.indexOf(allDates) != -1)
  return true;
  else
  return false;
   }
});
}); 
  td.day.disabled {
opacity: 0.2;
} 
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.0/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.7.1/js/bootstrap-datepicker.min.js"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.0/css/bootstrap.min.css" rel="stylesheet"/>
<input id="datepicker"> 
  
 

Hope this helps! 
use bootstrap beforeShowDay option to enable dates. 
 
 
  
  $(function() {
var datesEnabled = ['2017-9-10', '2017-9-15', '2017-9-25'];
$('#datepicker').datepicker({
format: 'yyyy-mm-dd',
beforeShowDay: function (date) {
  var allDates = date.getFullYear() + '-' + (date.getMonth() + 1) + '-' + date.getDate();
  if(datesEnabled.indexOf(allDates) != -1)
  return true;
  else
  return false;
   }
});
}); 
  td.day.disabled {
opacity: 0.2;
} 
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.0/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-datepicker/1.7.1/js/bootstrap-datepicker.min.js"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.0/css/bootstrap.min.css" rel="stylesheet"/>
<input id="datepicker">

不存在量化方法时重叠实例错误的原因(Reason for overlapping instances error when not quantifying method existentially)

最满意答案

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