IMHO this boils down to purity vs side effects. Given a black box function, it's a lot easier to reason about what it did if it behaves in a pure manner returni

IMHO this boils down to purity vs side effects. Given a black box function, it's a lot easier to reason about what it did if it behaves in a pure manner returning some data structure that represents some form of state, than passing some mutable complex real world thing into it and then trying to inspect what the thing did after the fact. But the matter of purity vs side effects is completely orthogonal to the default HTTP handler design. The handler provides the inputs and the output sinks that represent the "real world". But it's entirely up to you how you manage the data therein. You can continue to procedurally consume these input/output entities turtles all the way down, or you can work in a functional style until you are actually ready to flush the result of the computation down the writer.