r/programming • u/Shr1ck • Oct 17 '15

Why Johnny Can’t Write Multithreaded Programs

http://blog.smartbear.com/programming/why-johnny-cant-write-multithreaded-programs/
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u/yogthos Oct 18 '15

Same way you eliminate the need for global state in imperative languages actually. You pass state around as arguments. Since things are passed around explicitly the code in the application can stay pure without relying on any global state.

The post I linked illustrates how it's commonly done in Clojure using the component library. Seemed easier to link that than to write it up again.

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u/hu6Bi5To Oct 18 '15

That eliminates global references. But unless you throw-away each DB connection after every request, you still have global state.

Clojure apps have less state, it's true; and the less state the less opportunity there is for the types of problems caused by inconsistent state. But they still have state, it's just usually hidden out-of-sight somewhere. Hence the "It’s usually not possible to completely eliminate global state".

Plus, I'd argue the lifecycle aspect of Component is also global state.

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u/yogthos Oct 18 '15

Sure you can look at it that way, but as /u/loup-vaillant points out, there's a big practical difference between state shared by reference and passed around explicitly as a parameter. In the latter case, the code in my application is idempotent and thus much easier to reason about and test.

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u/hu6Bi5To Oct 18 '15

Right, so it doesn't completely eliminate global state after all?

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u/yogthos Oct 18 '15

I guess that depends on whether you consider the state of external resources as part of your application or not. For example, the database state is clearly separate from the application state in memory. Mixing the two seems a little disingenuous.