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Wed, 17 Nov 2010

The Real World Strikes Back - or why you shouldn't forbid stuff just because you think it's wrong


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tl;dr;; version: arbitrary API limitations do more harm than good, even if meant well in the first place.

Most advanced libraries that help you with date calculations have separate data types for a point in time, and a time span. That's because those two concepts actually have different semantics. It doesn't make sense to add two points in time, but it does make sense to add two durations, or add a duration to a point in time.

In Perl 6, those two types are called Instant and Duration. And obviously it makes sense to multiply a Duration with a number, but it doesn't make sense to multiply two Durations, or take a power of a Duration. Right?

That's the opinion that both the Perl 6 specification and the implementation in Rakudo reflected. Until recently, when somebody started to actually use it.

That's when the real world struck back. Carl Mäsak did some timings, and then calculated averages and standard deviations. And for calculating standard deviations, you actually have to square those durations, add them up, and then calculate the square root.

So this perfectly legitimate use case shows that multiplication (and also exponentiation) are perfectly fine operations on Durations. Likewise the current specification disallows division of two Durations. Why? It's perfectly fine to ask for the ratio of two time spans. How much longer (or shorter) are my meetings with my current boss, compared to those with my previous boss? That's the question that Duration / Duration answers.

So, the real world taught me that putting restrictions on the allowed operations is a bad idea. It was meant well, it was supposed to catch operations that don't made sense to the designer, and presumably would catch some error that a confused beginner might make. But in the end it did more harm than good.

Currently the Duration class stores a number, and re-dispatches all operations to that number, forbidding some of them. Having learned my lesson, I suggest we get rid of it, and have Instant - Instant return that number straight away. If some day we want to add functionality to Duration, we can still create that class as a subclass of the number.

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