混淆纯粹的应用函子

Question

I have following example with pure function: 我有以下具有pure函数的示例：

Prelude> :t pure ((+1) 1)
pure ((+1) 1) :: (Num a, Applicative f) => f a

What is above the concrete type of f wrapped a ? f包裹a的具体类型之上是什么？

For example, the f (structure) is here Maybe: 例如， f （结构）可能在这里：

Prelude> pure ((+1) 1) :: Maybe Int
Just 2

and what is the structure of: 以及什么是结构：

pure ((+1) 1)

? ？

The second example: 第二个例子：

Prelude> :t pure ((+1) 1) :: [Int]
pure ((+1) 1) :: [Int] :: [Int]

Why does the GHCi show the type twice, namely :: [Int] :: [Int] not only :: [Int] ? 为什么GHCi会两次显示类型，即:: [Int] :: [Int]不仅:: [Int] ？

Answer 1

f and a are both type variables. f和a都是类型变量。 There is no concrete type. 没有具体类型。 It will use whatever type is required by the surrounding context. 它将使用周围上下文所需的任何类型。
When you type :t EXPR , ghci prints the type as EXPR :: TYPE . 当键入:t EXPR ，ghci将类型打印为EXPR :: TYPE 。 The first :: [Int] is part of the expression you typed; 第一个:: [Int]是您键入的表达式的一部分； the second :: [Int] is the type computed by ghci. 第二个:: [Int]是ghci计算的类型。