With [a] -> b, when production of the input elements requires I/O and you don't want to read the entire list beforehand, you are forced to use lazy I/O, which i

With [a] -> b, when production of the input elements requires I/O and you don't want to read the entire list beforehand, you are forced to use lazy I/O, which is notoriously flaky and doesn't handle errors well. Meanwhile, with a Collector, you can read the inputs with standard IO actions just fine, and feed them as they are produced.