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Comment by mrkeen | Hacker News Reader

The article's good, but misses my most mentally-fatiguing issue when reading code: mutability.

It is such a gift to be able to "lock in" a variable's meaning exactly once while reading a given method, and to hold it constant while reasoning about the rest of the method.

Your understanding of the method should monotonically increase from 0% to 100%, without needing to mentally "restart" the method because you messed up what the loop body did to an accumulator on a particular iteration.

This is the real reason why GOTOs are harmful: I don't have a hard time moving my mind's instruction-pointer around a method; I have a hard time knowing the state of mutable variables when GOTOs are in play.

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Disagree. There's an abstract "information space" that the code is modeling, and you have to move around your mind's instruction pointer in that space. This can be helped or hindered by both mutable and immutable vars--it depends on how cleanly the code itself maps into that space. This can be a problem w/ both mutable and immutable vars. There's a slight tactical advantage to immutable vars b/c you don't have to worry about the value changing or it changing in a way that's misleading, but IME it's small and not worth adopting a "always use immutability" rule-of-thumb. Sometimes mutability makes it way easier to map into that "information space" cleanly.

> This is the real reason why GOTOs are harmful: I don't have a hard time moving my mind's instruction-pointer around a method; I have a hard time knowing the state of mutable variables when GOTOs are in play.

Well, total complexity is not only about moving the instruction pointer given a known starting point. Look at it from the callee’s pov instead of the call site. If someone can jump to a line, you can’t backtrack and see what happened before, because it could have come from anywhere. Ie you needed global program analysis, instead of local.

If mutability were the true source of goto complexity then if-statements and for loops have the same issue. While I agree mutability and state directly causes complexity, I think goto was in a completely different (and harmful) category.

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