# fold

## definition

```scala
def fold[A1 >: A](z: A1)(op: (A1, A1) ⇒ A1): A1
```

## demo

```scala
val prices: Seq[Double] = Seq(1.5, 2.0, 2.5)

// sum all the donut prices using fold function
val sum = prices.fold(0.0)(_ + _)
```

## demo2

```scala
val donuts: Seq[String] = Seq("Plain", "Strawberry", "Glazed")

// create a String of all donuts using fold function
val singleString = donuts.fold("")((acc,s) => acc + s + " Donut ")
```

输出的内容是

```scala
scala> val singleString = donuts.fold("")((acc,s) => acc + s + " Donut ")
singleString: String = "Plain Donut Strawberry Donut Glazed Donut "
```

## demo3

```scala
val donuts: Seq[String] = Seq("Plain", "Strawberry", "Glazed")
// declare a value function to create the donut string

val concatDonuts:(String,String) => String = (acc,s) =>  acc + s + " Donut "

val singleString = donuts.fold("")(concatDonuts)
```

输出的结果与 demo2 等价，只不过将函数显式地定义在外面，比较好理解。

```scala
scala> val concatDonuts:(String,String) => String = (acc,s) =>  acc + s + " Donut "
concatDonuts: (String, String) => String = $$Lambda$1470/1463209038@32eea4f7

scala> val singleString = donuts.fold("")(concatDonuts)
singleString: String = "Plain Donut Strawberry Donut Glazed Donut "
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://www.selinux.tech/scala/function-operation/f/fold.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.