# Possible errata for Chapter 1: Multi-paradigm: Functional, early-exit code block

**URL:** <https://forums.kodeco.com/t/possible-errata-for-chapter-1-multi-paradigm-functional-early-exit-code-block/141217>\
**Category:** Expert Swift\
**Created:** [May 31, 2021, 4:17am UTC](https://forums.kodeco.com/t/possible-errata-for-chapter-1-multi-paradigm-functional-early-exit-code-block/141217 "2021-05-31T04:17:36Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![atfelix](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/atfelix/32/102075_2.png) [@atfelix](https://forums.kodeco.com/u/atfelix)\
**Post date:** [May 31, 2021, 4:17am UTC](https://forums.kodeco.com/t/possible-errata-for-chapter-1-multi-paradigm-functional-early-exit-code-block/141217/1 "2021-05-31T04:17:36Z")

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In Chapter 1, section **Multi-paradigm** , the function style early-exit code block states

> It works, although the compiler needs to work hard to figure out that an early exit is possible.

I couldn’t get that code block (provided below) to early exit (using either debug or release mode). Am I missing something in my build settings or somewhere else? If not, I think the paragraph after the code block is an errata.

```swift
example("functional, early-exit") {
	let total = numbers.reduce(
		(accumulating: true, total: 0)
	) { state, value in
		if state.accumulating && value >= 0 {
			return (accumulating: true, state.total + value)
		}
		else {
			return (accumulating: false, state.total)
		}
	}
	.total
	
	print(total)
}

```

Here are two alternative functional styles that do early exit

### Approach 1: `reduce`’s throwing API

> ****
>
> If I use `reduce`'s throwing API, then we early exit on negative integers:
> 
> ```swift
> try example("functional, early-exit") {
> struct SumError: Error {
> let sum: Int
> }
> 	
> let total: Result<Int, Never> = Result {
> try numbers.reduce(0) { (accumulator, value) throws -> Int in
> if value >= 0 {
> return accumulator + value
> }
> else {
> throw SumError(sum: accumulator)
> }
> }
> }
> .flatMapError { error in
> guard let error = error as? SumError else { fatalError() }
> return .success(error.sum)
> }
> 	
> try print(total.get())
> }
> 
> ```

### Approach 2: `prefix(while:)`

> ****
>
> Although, I think the above is a harder to read than the following approach:
> 
> ```swift
> example("functional, early-exit") {
> let total = numbers
> .lazy
> .prefix { $0 >= 0 }
> .reduce(0, +)
> 	
> print(total)
> }
> 
> ```

---

<div class="post-metadata">

**Author:** ![rayfix](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/rayfix/32/108995_2.png) [@rayfix](https://forums.kodeco.com/u/rayfix)\
**Post date:** [May 31, 2021, 4:57pm UTC](https://forums.kodeco.com/t/possible-errata-for-chapter-1-multi-paradigm-functional-early-exit-code-block/141217/2 "2021-05-31T16:57:46Z")

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Very cool examples, @atfelix. When I wrote this section, I was imprecise. I didn’t actually check the compiler output and just theorized of a compiler that could, at least someday, deduce an early exit. But given that I actually do look at specific compiler output later in the chapter, it makes sense to check it. Let me mull over the best way to fix it. Thanks for this! (Approach 2 is lovely, BTW.)