模板Haskell类型引用问题 [英] Template Haskell type quoting problems

问题描述

TemplateHaskell引用两个引号（''）作为获取类型名称的方式：

The TemplateHaskell quoting documents two quotes ('') as the way to get the Name of a type:

> ''String
GHC.Base.String

这适用于此类型（名称） 。但是，我无法找到一种使其适用于 Maybe String ：

This works fine for this type (name). However, I can't find a way to make it work nice for e.g. Maybe String:

> ''Maybe String -- interprets String as a data constructor
> ''Maybe ''String -- wants to apply ''String to the Name type

我知道我可以通过使用 [t |可能是String |] ，但这是在Q monad中，并且需要更改类型，并且我认为不是在相应时刻进行类型检查，仅当拼接时。

I know I can workaround via using [t| Maybe String |], but this is then in the Q monad, and requires type changes, and I think is not type-checked at the respective moment, only when spliced in.

我也可以通过首先定义一个类型别名， type MaybeString = Maybe String ，然后使用' 'MaybeString ，但这也很麻烦。

I can also work around by first defining a type alias, type MaybeString = Maybe String, and then using ''MaybeString, but this is also cumbersome.

通过''直接获取我想要的任何方法 quotation？

Any way to directly get what I want simply via the '' quotation?

推荐答案

用于引用名称，而不是类型。 也许是一个名字，也许字符串不是。因此，在引用该名称之前，必须通过定义类型别名来赋予类型名称并不令人感到意外。

'' is used to quote names, not types. Maybe is a name, Maybe String is not. It is therefore not too surprising that you have to give your type a name by defining a type alias, before you can quote that name.

[ T | |] ，引号类型。注意这里的区别。

[t| |] on the other hand, quotes types. Note the difference here.

Prelude> :t ''String
''String :: Language.Haskell.TH.Syntax.Name
Prelude> :t [t| String |]
[t| String |]
  :: Language.Haskell.TH.Syntax.Q Language.Haskell.TH.Syntax.Type

所以我担心你不能用'来做你想做的事情。

So I'm afraid you cannot use '' for what you're trying to do.

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