无法从“Learn you a Haskell”中编译Writer Monad示例。 [英] Unable to compile Writer Monad example from "Learn you a Haskell"

问题描述

以下代码是逐字从LYAH ，不编译。代码和编译时错误包含在下面。在LYAH页面上，代码约15％，yay emacs浏览器：）

任何想法为什么？我忽略了一些完全明显的东西？

（尽管在后期标题中有相似之处，但我认为我的问题不同于 this one 。）

---

以下是代码（在我命名为 testcopy.hs 的文件中）

  import Control.Monad.Writer 
 
 logNumber :: Int  - > Writer [String] Int 
 logNumber x = Writer（x，[Got number：++ show x]）
 
 multWithLog :: Writer [String] Int 
 multWithLog = 
a<  -  logNumber 3 
b<  -  logNumber 5 
 return（a * b）
  

这里是编译时错误：

  Prelude> ：l testcopy.hs 
 [1的1]编译主（testcopy.hs，解释）
 testcopy.hs：4：15：
不在范围内：数据构造函数`Writer'
也许你的意思是'WriterT'（从Control.Monad.Writer导入）
失败，模块加载：无。 
  

解决方案

LYAH在这个例子中已经过时了。您应该使用 writer 智能构造函数方法而不是（现在不存在的） Writer 数据构造函数。

扩展一下，这些数据类型被更新为与monad变换器更加兼容。因此，在monad变量堆栈中使用一般 WriterT ，并使用 Writer 类型同义词用标识编写 WriterT 。因此，不再有与​​ Writer 类型相关联的数据构造函数（因为 Writer 是一个类型同义词）。幸运的是，尽管存在这种复杂性，解决方案非常简单：用 Writer >作家。

The following code, which is verbatim from LYAH, doesn't compile. Code and compile-time error are included below. On the LYAH page, the code is ~15% down the page, yay emacs browser :)

Any ideas why? Am I overlooking something totally obvious?

(Despite the similarity in post-titles, I think my question is different from this one.)

---

Here's the code (in a file which I named testcopy.hs)

import Control.Monad.Writer

logNumber :: Int -> Writer [String] Int
logNumber x = Writer (x, ["Got number: " ++ show x])

multWithLog :: Writer [String] Int
multWithLog = do
    a <- logNumber 3
    b <- logNumber 5
    return (a*b)

And here's the compile-time error:

Prelude> :l testcopy.hs
[1 of 1] Compiling Main             ( testcopy.hs, interpreted )
testcopy.hs:4:15:
    Not in scope: data constructor `Writer'
    Perhaps you meant `WriterT' (imported from Control.Monad.Writer)
Failed, modules loaded: none.

解决方案

LYAH is outdated in this example. You should use the writer smart constructor method instead of the (now non-existent) Writer data constructor.

To expand a bit, these data types were updated to be more compatible with monad transformers. As a result, there is a general WriterT, for use in a monad transformer stack, and a Writer type synonym that composes WriterT with Identity. Because of this, there is no longer a data constructor associated specifically with the Writer type (since Writer is a type synonym).

Luckily, despite this complication, the solution is pretty simple: replace Writer with writer.

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