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Pure callbacks aren't universally useless. For instance, the qsort() API can use pure synchronous callbacks effectively: http://linux.die.net/man/3/qsort

The article doesn't advocate always requiring pure callbacks. It's just offered as one possible way to make it easy to reason about the correctness of using an callback API.

Of course, the callback doesn't need to be pure. It just need to be side-effects free with respect to the caller.

Or presumably this is the same as a 'trivial solution' in physics (normally zero)? Makes the equations work, but generally represents nothing happening.