```swift
... earlier we said top-level code isn’t allowed in most of your app’s source files. The exception is a special file named 「main.swift」, which behaves much like a playground file, but is built with your app’s source code. The 「main.swift」 file can contain top-level code, and the order-dependent rules apply as well. In effect, the first line of code to run in 「main.swift」 is implicitly defined as the main entrypoint for the program. This allows the minimal Swift program to be a single line — as long as that line is in 「main.swift」.app
```ui
緣由就是你把print等等表達式放到其餘文件去了。在main.swift這個文件中寫就沒問題code
知道怎麼改就能夠了。若是不知足的話,能夠看看上面這段官方代碼,寫得真好ci