Amazing rant but I really wish they gave you a quick tldr on what the fuck the difference is between imperitive and declarative.
There’s a sort of useless sense in which everything is declarative if you are allowed to shift around what the relevant “thing to be done” is, and it seems like many people are complaining that the article does this exact thing.
No flow control? Declarative.
What is flow control? Typically if/then/else statements.
Importantly, setting a variable with a conditional does not make the language necessarily imperative.
I wonder if a language can be Turing-complete without flow control. My guess is no?
> Flow control? Imperative. > No flow control? Declarative.
I think I'd rather say:
Imperative: Tell the system what to do.
Declarative: Tell the system what you would like to be. The system figures out what to do to get there.
You could well have flow control to say things like "if things are like that, I want things to be such", or "here is a loop building up what I would the state of the system to look like", still without telling the system what to do to get to that state.
> I wonder if a language can be Turing-complete without flow control. My guess is no?
Depends on your definition of "flow control". Untyped lambda calculous is effectively entirely just declaration and application of anonymous functions with only one argument, but it's Turing complete.
You can even try it in any language that has lambdas as first class citizens, e.g. here's a factorial function I wrote in Lambda calculous, but implemented in python:
fac = ((lambda p: p(p)(lambda q: lambda n: ((n(lambda e: lambda e:
lambda a: a)(lambda i: lambda l: i))(lambda d: lambda c: c)
(lambda m: (lambda m: lambda z: m(n(z)))(q(m)))((lambda c: lambda x:
n(lambda g: lambda h: h(g(c)))(lambda u: x)(lambda u: u))))))
(lambda s: lambda q: lambda w: q(s(s)(q))(w)))
Would I call Lambda calculous declarative? Not at all!
Yep. That’s why this is a potentially useful rule of thumb for comparing programming languages, especially ones that skew very far in one direction or the other, but it’s not a rigorous definition of the concepts.
Some incredibly simple algorithms are difficult to explain naturally without it sounding like control flow. How would you describe the absolute value function without saying “if” or “when” or specifying one particular element from a set? Obviously any computer system needs to be able to determine what steps to take depending on specified conditions.
I don't think you even want a conditional when describing the absolute value function on the complex numbers.
How is this? absolute value is distance a number is from 0.
I get your point, though.
Just his example of `x = a ? b : c`, I never saw anybody try to claim code like this makes a language imperative. Instead, I've seen many people calling this logic "functional if".
If you go with his definition, only simple variable declaration is declarative.
(By the way, a very popular definition is that imperative languages have flow, not flow control. AKA, it makes a difference if you go and execute the instructions on a different order. That one is reasonable, but then, why single out non-associativity of operations when there are many other things that no language makes non-associative?)
I agree, unless something like pattern-matching does not count as control flow. But then isn't a conditional just a basic pattern? Why does it get a pass?
I can’t imagine why not. I think you have to make the distinction between explicit control flow in the syntax, which declarative languages don’t have (according to this rule of thumb), and the notion of a computer choosing to do one thing among several options depending on some condition. Of course any Turing-complete system can do the latter, but that’s totally unrelated to the syntax of the programming language.
squares = map(square, inputs)
Expressions instead of statements is a key tool to make things declarative; the functional and logic paradigms are both declarative programming paradigms, as opposed to the structured/procedural and OO paradigms, which are imperative.
That they're also present in the functional and logic paradigms doesn't change that. It's a feature common to both, so not a feature that can be used to distinguish the categories from each other.
And regarding expressions, Rust is more expression oriented than most mainstream procedural languages today, and it's not declarative, though it may have more declarative-style (particularly its heavy use of the iterator style) compared to many other imperative languages.
“Declarative” and “imperative” are not really features of languages in the first place. They are features of code (and coding paradigms), and you can write code of either style (and usually most paradigms) in almost any real-world, Turing-complete, higher-level-than-assembly language.
The specific replacement of an imperative loop with a map call you made the non-sequitur response about function calls to upthread however, made the code it was in more declarative, though, and is typical of the ways that the functional paradigm (where map is typically the idiomatic way to do that) is more declarative than the structured/procedural paradigm (where building up a collection in an imperative loop would be.)
> At their core, imperative and declarative differ in one major and fundamental way. Imperative has control flow and declarative does not.
The SQL guy sort of made sense I guess maybe.
When you write an SQL query, you're not describing steps to take, you're describing the outcome you want (what rows and fields from which tables and how those tables relate to each other). The query engine then takes that description and figures out how to actually do the query (what indices to use, in what order, how to actually loop/hash/map/scan the tables and indices, etc). This is what EXPLAIN shows you, the actual imperative plan the query engine came up with given the declarative query you gave it.
Another common example would be CSS: It's all about describing the desired result and spatial relationships between elements, then the CSS engine takes those rules and figures out how to actually get those elements to do what you want. Like something as seemingly simple as "float: left" - for the text to wrap around it, the engine has to deal with the image size, position, font and font size, figuring out when to wrap the text, height of the font, which line of text is greater than the image height, etc. The engine does a lot of work figuring out the steps to take to get the result you've described.
Declarative: N(N+1)/2
Imperative: for(int sum=0, int i=1; i<=N; i++) {sum = sum+i;}