副作用其实就是说谎，你的函数本来要做一件事，但是却偷偷做了其他的事。

Side effects are lies. Your function promises to do one thing, but it also does other hidden things.

youdao

这就是为什么要使纯函数，即无副作用的函数，只能够调用其他纯函数想法的由来。

This is where the idea of making pure functions, functions without side-effects, able to call only other pure functions.

youdao

XQuery是一种高端的、强类型的函数语言(没有副作用)，非常适合表达从XML文档或者大型XML存储库(repository)中获取数据的查询。

XQuery is a high-level, strongly-typed, functional language (free of side-effects) that is ideal to express a query to obtain data both from an XML document and a large XML document repository.

youdao

And again, a related problem that people run into is side-effects. You call a function and it returns a value, but maybe it, on purpose or on accident, modifies one of the actual parameters.

另外，一个人们常碰到的相关问题,就是副作用，你调用一个函数它返回一个值,但是故意或者无意的,它可能会修改一个实参的值。

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