blocks|key|3182662|text|问得好！|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3182663|这里有几个关键的区别。|3182664|Representation|offset|length|style|BOLD|3182665|3182666|+newtype保证您的数据在运行时将具有与您包装的类型完全相同的表示。|unordered-list-item|CODE|3182667|而data在运行时声明了一个全新的数据结构。|3182668|3182669|所以这里的关键点是保证newtype的构造在编译时被擦除。|3182670|示例：|3182671|3182672|data+Book+=+Book+Int+Int|3182673|3182674|​|3182675|📷|atomic|3182676|3182677|3182678|newtype+Book+=+Book+(Int,+Int)|3182679|3182680|3182681|3182682|3182683|请注意，由于Book构造函数已被擦除，因此它的表示与(Int,Int)完全相同。|3182684|3182685|data+Book+=+Book+(Int,+Int)|3182686|3182687|3182688|3182689|3182690|具有newtype中不存在的附加Book构造函数。|3182691|3182692|data+Book+=+Book+{-#+UNPACK+#-}!Int+{-#+UNPACK+#-}!Int|3182693|3182694|3182695|3182696|3182697|没有指针！在Book构造函数中，这两个Int字段是未装箱的单词大小的字段。|3182698|代数数据类型|3182699|由于需要擦除构造函数，因此仅当使用作为单个构造函数包装数据类型时，newtype才有效。没有“代数”新类型的概念。也就是说，你不能写一个新类型的等价物，比如说，|3182700|data+Maybe+a+=+Nothing
+++++++++++++%7C+Just+a|code-block|syntax|javascript|3182701|因为它有多个构造函数。你也不会写|3182702|newtype+Book+=+Book+Int+Int|3182703|严格性|3182704|构造函数被擦除这一事实导致了data和newtype之间在严格性方面的一些非常细微的差异。特别是，data引入了一个“提升”的类型，这意味着，本质上，它有一种额外的方法来计算到最低值。因为newtype在运行时没有额外的构造函数，所以这个属性不成立。|3182705|到(,)的Book构造函数中的额外指针允许我们将底部值放入。|3182706|因此，与explained+in+the+Haskell+wiki+article一样，newtype和data具有略微不同的严格属性。|3182707|拆箱|3182708|拆开newtype的组件是没有意义的，因为没有构造函数。虽然这样写是完全合理的：|3182709|data+T+=+T+{-#+UNPACK+#-}!Int|3182710|生成具有T构造函数和Int#组件的运行时对象。您只需使用newtype获得一个空Int即可。|3182711|3182712|参考文献|3182713|3182714|"Newtype"+on+the+Haskell+wiki|3182715|Norman+Ramsey's+answer关于|3182716|的严格性属性|3182717|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://ask.qcloudimg.com/http-save/yehe-900000/236accc6500f72b6515fc53d11bdeb70.png|imageAlt|1|https://ask.qcloudimg.com/http-save/yehe-900000/07eff29e208a7371d10bc1f27eafd321.png|2|https://ask.qcloudimg.com/http-save/yehe-900000/5e72bbabba6fc9ef341302bd15182e4b.png|3|https://ask.qcloudimg.com/http-save/yehe-900000/3536f2746d1e030cb14ece80d21407e1.png|4|LINK|MUTABLE|url|http://www.haskell.org/haskellwiki/Newtype#The_messy_bits|5|http://www.haskell.org/haskellwiki/Newtype|6|https://stackoverflow.com/questions/2649305/why-is-there-data-and-newtype-in-haskell/2650051#2650051^0|0|0|0|E|0|0|1|7|0|1|4|0|0|B|7|0|0|0|0|O|0|0|0|0|1|0|0|0|0|0|U|0|0|0|0|1|1|0|0|6|4|Q|9|0|0|0|R|0|0|0|0|1|2|0|0|2|7|G|4|0|0|0|1I|0|0|0|0|1|3|0|0|6|4|J|3|0|0|6|0|H|8|X|7|0|0|0|0|0|3|0|E|4|J|7|1D|4|2M|7|0|1|3|5|4|0|18|7|1G|4|4|11|4|0|0|2|0|2|7|0|0|4|1|A|4|S|7|14|3|0|0|0|0|0|T|5|0|0|M|6|0|0^^$0|@$1|2|3|4|5|6|7|3N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|3O|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|3P|8|@$F|3Q|G|3R|H|I]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|3S|8|@]|9|@]|A|$]]|$1|K|3|L|5|M|7|3T|8|@$F|3U|G|3V|H|N]]|9|@]|A|$]]|$1|O|3|P|5|M|7|3W|8|@$F|3X|G|3Y|H|N]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|3Z|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|40|8|@$F|41|G|42|H|N]]|9|@]|A|$]]|$1|T|3|U|5|6|7|43|8|@]|9|@]|A|$]]|$1|V|3|-4|5|6|7|44|8|@]|9|@]|A|$]]|$1|W|3|X|5|M|7|45|8|@$F|46|G|47|H|N]]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|48|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|49|8|@]|9|@]|A|$]]|$1|11|3|12|5|13|7|4A|8|@]|9|@$F|4B|G|4C|1|4D]]|A|$]]|$1|14|3|10|5|6|7|4E|8|@]|9|@]|A|$]]|$1|15|3|-4|5|6|7|4F|8|@]|9|@]|A|$]]|$1|16|3|17|5|M|7|4G|8|@$F|4H|G|4I|H|N]]|9|@]|A|$]]|$1|18|3|-4|5|6|7|4J|8|@]|9|@]|A|$]]|$1|19|3|10|5|6|7|4K|8|@]|9|@]|A|$]]|$1|1A|3|12|5|13|7|4L|8|@]|9|@$F|4M|G|4N|1|4O]]|A|$]]|$1|1B|3|10|5|6|7|4P|8|@]|9|@]|A|$]]|$1|1C|3|1D|5|6|7|4Q|8|@$F|4R|G|4S|H|N]|$F|4T|G|4U|H|N]]|9|@]|A|$]]|$1|1E|3|-4|5|6|7|4V|8|@]|9|@]|A|$]]|$1|1F|3|1G|5|M|7|4W|8|@$F|4X|G|4Y|H|N]]|9|@]|A|$]]|$1|1H|3|-4|5|6|7|4Z|8|@]|9|@]|A|$]]|$1|1I|3|10|5|6|7|50|8|@]|9|@]|A|$]]|$1|1J|3|12|5|13|7|51|8|@]|9|@$F|52|G|53|1|54]]|A|$]]|$1|1K|3|10|5|6|7|55|8|@]|9|@]|A|$]]|$1|1L|3|1M|5|6|7|56|8|@$F|57|G|58|H|N]|$F|59|G|5A|H|N]]|9|@]|A|$]]|$1|1N|3|-4|5|6|7|5B|8|@]|9|@]|A|$]]|$1|1O|3|1P|5|M|7|5C|8|@$F|5D|G|5E|H|N]]|9|@]|A|$]]|$1|1Q|3|-4|5|6|7|5F|8|@]|9|@]|A|$]]|$1|1R|3|10|5|6|7|5G|8|@]|9|@]|A|$]]|$1|1S|3|12|5|13|7|5H|8|@]|9|@$F|5I|G|5J|1|5K]]|A|$]]|$1|1T|3|10|5|6|7|5L|8|@]|9|@]|A|$]]|$1|1U|3|1V|5|6|7|5M|8|@$F|5N|G|5O|H|N]|$F|5P|G|5Q|H|N]]|9|@]|A|$]]|$1|1W|3|1X|5|6|7|5R|8|@$F|5S|G|5T|H|I]]|9|@]|A|$]]|$1|1Y|3|1Z|5|6|7|5U|8|@$F|5V|G|5W|H|I]|$F|5X|G|5Y|H|N]]|9|@]|A|$]]|$1|20|3|21|5|22|7|5Z|8|@]|9|@]|A|$23|24]]|$1|25|3|26|5|6|7|60|8|@]|9|@]|A|$]]|$1|27|3|28|5|22|7|61|8|@]|9|@]|A|$23|24]]|$1|29|3|2A|5|6|7|62|8|@$F|63|G|64|H|I]]|9|@]|A|$]]|$1|2B|3|2C|5|6|7|65|8|@$F|66|G|67|H|N]|$F|68|G|69|H|N]|$F|6A|G|6B|H|N]|$F|6C|G|6D|H|N]]|9|@]|A|$]]|$1|2D|3|2E|5|6|7|6E|8|@$F|6F|G|6G|H|N]|$F|6H|G|6I|H|N]]|9|@]|A|$]]|$1|2F|3|2G|5|6|7|6J|8|@$F|6K|G|6L|H|N]|$F|6M|G|6N|H|N]]|9|@$F|6O|G|6P|1|6Q]]|A|$]]|$1|2H|3|2I|5|6|7|6R|8|@$F|6S|G|6T|H|I]]|9|@]|A|$]]|$1|2J|3|2K|5|6|7|6U|8|@$F|6V|G|6W|H|N]]|9|@]|A|$]]|$1|2L|3|2M|5|22|7|6X|8|@]|9|@]|A|$23|24]]|$1|2N|3|2O|5|6|7|6Y|8|@$F|6Z|G|70|H|N]|$F|71|G|72|H|N]|$F|73|G|74|H|N]|$F|75|G|76|H|N]]|9|@]|A|$]]|$1|2P|3|-4|5|6|7|77|8|@]|9|@]|A|$]]|$1|2Q|3|2R|5|6|7|78|8|@]|9|@]|A|$]]|$1|2S|3|-4|5|6|7|79|8|@]|9|@]|A|$]]|$1|2T|3|2U|5|M|7|7A|8|@]|9|@$F|7B|G|7C|1|7D]]|A|$]]|$1|2V|3|2W|5|M|7|7E|8|@]|9|@$F|7F|G|7G|1|7H]]|A|$]]|$1|2X|3|2Y|5|6|7|7I|8|@]|9|@]|A|$]]|$1|2Z|3|-4|5|6|7|7J|8|@]|9|@]|A|$]]]|30|$31|$5|32|33|34|A|$35|36|37|-4]]|38|$5|32|33|34|A|$35|39|37|-4]]|3A|$5|32|33|34|A|$35|3B|37|-4]]|3C|$5|32|33|34|A|$35|3D|37|-4]]|3E|$5|3F|33|3G|A|$3H|3I]]|3J|$5|3F|33|3G|A|$3H|3K]]|3L|$5|3F|33|3G|A|$3H|3M]]]]

Great question!

There are several key differences.

Representation

<ul>
<li>A <code>newtype</code> guarantees that your data will have exactly the same representation at runtime, as the type that you wrap.</li>
<li>While <code>data</code> declares a brand new data structure at runtime.</li>
</ul>

So the key point here is that the construct for the <code>newtype</code> is guaranteed to be erased at compile time. 

Examples:

<ul>
<li><code>data Book = Book Int Int</code></li>
</ul>

<img src="https://i.stack.imgur.com/PQWvT.png" alt="data">

<ul>
<li><code>newtype Book = Book (Int, Int)</code></li>
</ul>

<img src="https://i.stack.imgur.com/Qca1X.png" alt="newtype">

Note how it has exactly the same representation as a <code>(Int,Int)</code>, since the <code>Book</code> constructor is erased.

<ul>
<li><code>data Book = Book (Int, Int)</code></li>
</ul>

<img src="https://i.stack.imgur.com/FwKcy.png" alt="data tuple">

Has an additional <code>Book</code> constructor not present in the <code>newtype</code>.

<ul>
<li><code>data Book = Book {-# UNPACK #-}!Int {-# UNPACK #-}!Int</code></li>
</ul>

<img src="https://i.stack.imgur.com/btUqH.png" alt="enter image description here">

No pointers! The two <code>Int</code> fields are unboxed word-sized fields in the <code>Book</code> constructor.

Algebraic data types

Because of this need to erase the constructor, a <code>newtype</code> only works when wrapping a data type with a single constructor. There's no notion of "algebraic" newtypes. That is, you can't write a newtype equivalent of, say,

<pre><code>data Maybe a = Nothing
 | Just a
</code></pre>

since it has more than one constructor. Nor can you write

<pre><code>newtype Book = Book Int Int
</code></pre>

Strictness

The fact that the constructor is erased leads to some very subtle differences in strictness between <code>data</code> and <code>newtype</code>. In particular, <code>data</code> introduces a type that is "lifted", meaning, essentially, that it has an additional way to evaluate to a bottom value. Since there's no additional constructor at runtime with <code>newtype</code>, this property doesn't hold. 

That extra pointer in the <code>Book</code> to <code>(,)</code> constructor allows us to put a bottom value in.

As a result, <code>newtype</code> and <code>data</code> have slightly different strictness properties, as <a href="http://www.haskell.org/haskellwiki/Newtype#The_messy_bits" rel="noreferrer">explained in the Haskell wiki article</a>.

Unboxing

It doesn't make sense to unbox the components of a <code>newtype</code>, since there's no constructor. While it is perfectly reasonable to write:

<pre><code>data T = T {-# UNPACK #-}!Int
</code></pre>

yielding a runtime object with a <code>T</code> constructor, and an <code>Int#</code> component. You just get a bare <code>Int</code> with <code>newtype</code>.

<hr>

References:

<ul>
<li><a href="http://www.haskell.org/haskellwiki/Newtype" rel="noreferrer">"Newtype" on the Haskell wiki</a></li>
<li><a href="https://stackoverflow.com/questions/2649305/why-is-there-data-and-newtype-in-haskell/2650051#2650051">Norman Ramsey's answer</a> about the strictness properties</li>
</ul>

What is the difference when I write this?

<pre><code>data Book = Book Int Int
</code></pre>

versus

<pre><code>newtype Book = Book (Int, Int) -- "Book Int Int" is syntactically invalid
</code></pre>

Difference between `data` and `newtype` in Haskell

数据结构

我写这篇文章的时候有什么不同？data Book = Book Int Int对比newtype Book = Book (Int, Int) -- "Book Int Int" is syntactically invalid

问Haskell中`data`和`newtype`的区别
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