您如何在Haskell中使用TypeApplications? [英] How do you use TypeApplications in Haskell?

问题描述

使用GHC 8.0中的-XTypeApplications，可以使用@前面的函数参数显式指定类型.它确切指定了什么类型，特别是在引入几个@时?

With -XTypeApplications in GHC 8.0, you can specify types explicitly with @ preceding function arguments. What types does it exactly specify, especially when several @ are introduced?

推荐答案

如果您查看函数的类型

elem :: (Foldable t, Eq a) => a -> t a -> Bool

我们看到它有两个多态变量，t和a.这些变量是应用程序指定的@类型.似乎上下文中引入的变量(类型类约束所在的位置)会影响顺序，因此第一个@指定t，第二个a指定.在没有上下文变量的函数中

we see it has two polymorphic variables, t and a. These variables are what the @ type applications specify. It seems that variables introduced in the context — where typeclass constraints go — affect order, and hence the first @ specifies the t, and the second the a. In functions without context variables

const :: a -> b -> a

顺序更明显，a是第一位，b是第二位.正如仙人掌在上面的评论中提到的，您还可以使用显式的forall自己指定订单.

the order is more obvious, the a is first and b is second. As Cactus mentioned in a comment above, you can also use explicit foralls to specify the order yourself.

myConst :: forall b a. a -> b -> a

现在，第一种类型的应用程序将指定b，第二种类型的应用程序将指定a.

Now the first type application will specify the b and the second the a.

您可能会遇到需要指定类型的问题，尤其是在使用重载字符串或列表的情况下

You may run into this problem of needing to specify types particularly if you're using overloaded strings or lists

elem c "abc...xyz" -- What string type is this?
elem c ['a' .. 'z'] -- What list constructor is this?

因此，我们使用显式类型的应用程序

therefore we use explicit type applications

elem @[] @Char c ['a' .. 'z']

在这种情况下，我们仅必须指定@[]，并说这是[]列表类型构造函数"，因为GHC从列表元素中推断出Char，所以@Char可以在这里省略.

in this case we only have to specify the @[] and say "this is a [] list type constructor" because GHC infers Char from the list elements, so @Char can be omitted here.

如果多态参数GHC能够推断碰巧首先出现，则可以利用-XPartialTypeSignatures，它允许您在类型签名(包括类型应用程序签名)中使用_，告诉GHC只是推断出[一部分]类型，使事情变得不太冗长.

If a polymorphic argument GHC is able to infer happens to come first you can leverage -XPartialTypeSignatures which allows you to use _ in type signatures including type application signatures, telling GHC to just infer that [part of the] type, to make things less verbose.

f @_ @[]

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