你在 Haskell 中发现了更高级别的类型有什么用途? [英] What uses have you found for higher-rank types in Haskell?

问题描述

更高等级的类型看起来很有趣.这个例子来自 Haskell wikibook:

Higher rank types look like great fun. From the Haskell wikibook comes this example:

foo :: (forall a. a -> a) -> (Char,Bool)
foo f = (f 'c', f True)

现在我们可以评估 foo id 而不会导致编译器爆炸.这个例子在本书后面很快出现了我在其他几个地方看到的真实例子:ST monad 和 runST.这很酷.

Now we can evaluate foo id without the compiler exploding. This example is quickly followed in the book by the real-world example I have seen in a few other places: the ST monad and runST. That's pretty cool.

但是我还没有遇到过这样的情况，即通过编写自己的函数并使用更高级别的参数来解决问题.你?你有哪些野外 rank-2 或 rank-n 多态性的例子?

But I have yet to come across a situation where I solve a problem by writing my own function with an argument of higher-rank. Have you? What examples do you have of rank-2 or rank-n polymorphism in the wild?

推荐答案

看看 Darcs源.

Take a look at functions like withRepoLock in the Darcs source.

Darcs 支持多种存储库格式，这种支持通过类型类表示.因此，您可以编写比存储库格式通用的代码.在实际读取磁盘存储库时，您希望通过一些通用代码分派到该代码，这些代码确定存储库的格式并选择正确的类型类实例化.

Darcs has support for multiple repository formats, and that support is expressed via a typeclass. So you can write code that is generic over repository formats. When actually reading an on-disk repository you want to dispatch to that code through some common code that figures out what format the repository is in and chooses the right typeclass instantiation.

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