Question

在解释Persistent章节中的模板扩展时，Yesod书看起来像

<div id="register">
  <div class="box">
    <p>IP</p>
  </div>
  <div id="IP" class="box">
    <p></p>
  </div>

  <div class="box">
    <p>IS</p>
  </div>
  <div id="IS" class="box">
    <p></p>
  </div>

  <div class="box">
    <p>R0</p>
  </div>
  <div id="R0" class="box">
    <p></p>
  </div>

  <div class="box">
    <p>R1</p>
  </div>
  <div id="R1" class="box">
    <p></p>
  </div>
</div>
<div id="case">
  <div id="0" class="box">
    <input></input>
    <p class="id">0</p>
  </div>
  <div id="1" class="box">
    <input></input>
    <p class="id">1</p>
  </div>
  <div id="2" class="box">
    <input></input>
    <p class="id">2</p>
  </div>
  <div id="3" class="box">
    <input></input>
    <p class="id">3</p>
  </div>
  <div id="4" class="box">
    <input></input>
    <p class="id">4</p>
  </div>
  <div id="5" class="box">
    <input></input>
    <p class="id">5</p>
  </div>
  <div id="6" class="box">
    <input></input>
    <p class="id">6</p>
  </div>
  <div id="7" class="box">
    <input></input>
    <p class="id">7</p>
  </div>
  <div id="8" class="box">
    <input></input>
    <p class="id">8</p>
  </div>
  <div id="9" class="box">
    <input></input>
    <p class="id">9</p>
  </div>
  <div id="10" class="box">
    <input></input>
    <p class="id">10</p>
  </div>
  <div id="11" class="box">
    <input></input>
    <p class="id">11</p>
  </div>
  <div id="12" class="box">
    <input></input>
    <p class="id">12</p>
  </div>
  <div id="13" class="box">
    <input></input>
    <p class="id">13</p>
  </div>
  <div id="14" class="box">
    <input></input>
    <p class="id">14</p>
  </div>
  <div id="15" class="box">
    <p class="id">15</p>
    <input></input>
  </div>
</div>
<div id="instructions">
  <p class="header">Instructions:</p>
  <p class="info header">1-Byte Instructions</p>
  <p class="info">0 = Halt</p>
  <p class="info">1 = Add (R0 = R0 + R1)</p>
  <p class="info">2 = Subtract (R0 = R0 - R1)</p>
  <p class="info">3 = Increment R0 (R0 = R0 + 1)</p>
  <p class="info">4 = Increment R1 (R1 = R1 + 1)</p>
  <p class="info">5 = Decrement R0 (R0 = R0 - 1)</p>
  <p class="info">6 = Decrement R1 (R1 = R1 - 1)</p>
  <p class="info">7 = Ring Bell/Beep</p>
  <p class="info header">2-Byte Instructions, value of the second Byte is called &ltdata&gt</p>
  <p class="info">8 = Print &ltdata&gt</p>
  <p class="info">9 = Load value at address &ltdata&gt into R0</p>
  <p class="info">10 = Load value at address &ltdata&gt into R1</p>
  <p class="info">11 = Store R0 into address &ltdata&gt</p>
  <p class="info">12 = Store R1 into address &ltdata&gt</p>
  <p class="info">13 = Jump to address &ltdata&gt</p>
  <p class="info">14 = Jump to address &ltdata&gt if R0 == 0</p>
  <p class="info">15 = Jump to address &ltdata&gt if R0 != 0</p>
</div>
<div id="output">
  <p>Output:</p>
</div>

我不太确定如何阅读这个新类型声明：是 newtype Key Person = PersonKey (BackendKey SqlBackend)已在其他地方定义的内容（不能找到它的定义虽然）或PersonKey a只是一个普通的类型或值构造函数，如PersonKey / Just / Nothing / etc。？

Answer 1

好的，首先简化一下，让我们假装说：

Left

在这种情况下，newtype KeyPerson = PersonKey (BackendKey SqlBackend)将是“类型构造函数”，KeyPerson将是“数据构造函数”。我们将在这里创建它们。 PersonKey数据构造函数将保存类型为PersonKey的值，该值需要存在于其他位置并从此处引用。

在这种情况下，我们通常是关联类型（也称为类型系列），这使得略微更复杂：BackendKey SqlBackend意味着“我们正在定义关联的{{1}对于newtype Key Person类型。但我们仍然像以前一样创建Key数据构造函数。

在Persistent库中键入系列

1 个答案: