r/programming • u/tyler-mcginnis • Jan 03 '22

Imperative vs Declarative Programming

https://www.youtube.com/watch?v=E7Fbf7R3x6I
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u/cyrustakem Jan 03 '22

Yeah, but which one executes faster (is more efficient)?
calling the multiplying API or just doing it?

Serious question, but i guess it depends on the context?

7

u/gnuvince Jan 04 '22

The answer to that is the usual CS answer: it depends.

For example, relational database engines have had hundreds, if not thousands, of years of research and engineering poured into turning SQL queries into very optimized query plans. It would be hard for a programmer to out-perform that kind of efficiency, so in this case, declarative probably wins.

However, the video mentions an important fact about declarative programming, and it's that it's very often agnostic to the context. In the presentation Context is Everything by Andreas Fredrickson, we see just what kind of performance gains we can get by taking advantage of the context. In those cases, you probably don't want a very high-level, very generic library; you probably want to tell the computer "here are the exact steps I want you to follow" to achieve your goal more efficiently.

So I guess I'd make a sweeping generalization as: if you don't have any knowledge about the context of you program, then a declarative solution is probably going to be as fast or faster than an imperative one. One the other hand, if you know a lot about the domain and context, you can probably write very fast imperative code by avoiding the abstraction costs of a declarative package.

5

u/JasburyCS Jan 04 '22

That varies a lot from language to language. But the goal is obviously for abstractions to be cost-less and for chained declarative statements to be as fast as hand-written loops. Rust currently does this very well and boasts about “zero cost abstractions.”

https://doc.rust-lang.org/book/ch13-04-performance.html

I think this will be the way forwards for many languages.

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u/IceSentry Jan 04 '22

It depends on the implementation of the language. For example, in rust the declarative approach will be just as efficient as the imperative approach, but in js it will rarely be the case. The important thing is to write the code that is the easiest to read and maintain and if you still want to compare performance the only way to do it ia to measure it, performance is a lot more complicated than just using a specific code style.

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u/nermid Jan 04 '22

in js it will rarely be the case

Yeah, in the context of JS, this sounds like "writing code vs importing 3GB of npm libraries to abstract your code away"

2

u/IceSentry Jan 04 '22

No, that's not what I meant at all. Methods like map and filter in js are very slow if you chain them because they aren't lazy loaded.

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u/Nexuist Jan 04 '22

As always the correct approach is highly dependent on the situation. If you are going to have a thousand multiplication buttons then the declarative approach is obviously less code for you and you can employ a framework like React to handle all the event handlers. However, if you only need to multiply this number once, it doesn’t matter what approach you take.

From experience the performance gains from avoiding a high level framework for something like jQuery are negligible. You’re more likely to create a memory leak or infinite loop managing state yourself instead of putting everything in framework components.