如果让它自行更改，编译将会失败；这三个组合子知道如何获取String，但不知道如何根据解析的内容生成Expr对象。
If you make that change on its own, it will fail to compile; the three combinators know how to take in Strings but not how to produce Expr objects from what's parsed.
现在，如果与此订单的详细信息有关的数据在server3上不可用，那么它就只好自行获取这些数据。
Now, if the data related to order detail is not available to server3, then all it has to do is fetch that data.
这给人深刻印象的做法排除了因果变量，斯坦福大学的诺阿-古德曼说：“他们实际上自行设计了一个实验以获取他们想要的信息。”
That showed an impressive determination to isolate the causal variables, says Stanford’s Noah Goodman: “They actually designed an experiment to get the information they wanted.”
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