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

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