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How would you log outside of the IO monad?

By collecting status messages in addition to computation results. The Writer monad is one (convenient) way to do this.

Can you clarify? I thought that keeping separate streams of side effects is not possible when one stream influences the other. The logging process depends on what actions happen in the IO sequence though not the other way around. Are you talking about keeping the normal IO actions in one monad, logging actions in another & then composing the two into an outer IO monad?

It depends on what you're logging. Pure code can keep its own log in Writer and then depose the whole thing to the IO logger later. Since there's no guarantee of the timing or synchronicity on non-IO code, this works well.

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