4 ms·

Amazing rant but I really wish they gave you a quick tldr on what the fuck the difference is between imperitive and declarative.

TedDoesntTalk 4y ago
Flow control? Imperative. 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?
- anyfoo 4y ago
  I'm not even sure I agree with the author with that specifically: > 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))) Does it have flow control? Depends. No python flow control is used, but some of those lambdas implement flow control. But they're not any part of the language (Lambda calculous), we just build them. Would I call Lambda calculous declarative? Not at all!
  - tshaddox 4y ago
    > Depends on your definition of "flow control". 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.
    - TedDoesntTalk 4y ago
      > How would you describe the absolute value function without saying “if” or “when How is this? absolute value is distance a number is from 0. I get your point, though.
      - tshaddox 4y ago
        True, the syntax will obviously depend on what primitives the language provides. Of course it could also just provide the abs() function directly.
    - naniwaduni 4y ago
      > How would you describe the absolute value function without saying “if” or “when” or specifying one particular element from a set? I don't think you even want a conditional when describing the absolute value function on the complex numbers.
- mLuby 4y ago
  > I wonder if a language can be Turing-complete without flow control. My guess is no? 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?
- tshaddox 4y ago
  > I wonder if a language can be Turing-complete without flow control. My guess is no? 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.
- jfoutz 4y ago
  Prolog would like to have a word.
- marcosdumay 4y ago
  The author seems as confused as the people he complains about. 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?)
forbiddenlake 4y ago
They do. > At their core, imperative and declarative differ in one major and fundamental way. Imperative has control flow and declarative does not.
- tshaddox 4y ago
  That’s a great rule of thumb to quickly identify whether some new syntax is more likely to be imperative or declarative, but it’s not a great explanation of what the two terms mean for someone new to the terms. A banana doesn’t have control flow but it’s definitely not a declarative programming language.
  - epgui 4y ago
    I would eat a declarative banana.
    - WaxProlix 4y ago
      In fact, one should eat a declarative banana.
      - Izkata 4y ago
        The declarative banana should be in my stomach.
        fjdbeb76 4y ago
        After the 20 or so comments that responded I'm still no closer to understanding. The SQL guy sort of made sense I guess maybe.
        Izkata 4y ago
        The "has control flow" vs "doesn't have control flow" description is a bad one, an attempt to reduce "declarative vs imperative" down to syntax instead of semantics. The parent to my comment is still in imperative style, despite having no control flow: It's specifying what actions to take to reach the desired outcome ("eat"). Declarative is describing the desired outcome, then letting the system figure out how to get there. 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.
eloisius 4y ago
As an example, declarative would be something like squares = map(square, inputs) While imperative would be a for loop, store this here, multiply that with this, push that onto the end of my results array, etc
- Jtsummers 4y ago
  Function calls don't make something declarative, otherwise C is declarative. And once you accept that, then words have no meaning anymore and we can't meaningfully discuss the topic.
  - crb3 4y ago
    Unless they're talking about layers, with the function prototypes and function calls being declarative, and the next layer down being where the code is actually written out. Even that doesn't really work, though; main() usually has code in it... <shrug>
  - dragonwriter 4y ago
    > Function calls don't make something declarative 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.
    - Jtsummers 4y ago
      I don't know what you're trying to get at, you seem to be agreeing with me. Function calls (present in C and other structured, procedural, and OO paradigm languages which are also imperative) do not make a language declarative. 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.
femto113 4y ago
It's much like the difference between "how" and "what". SQL select statements are my favored example of declarative: you simply "declare" what you want ("all rows from this table with these values in those columns") and it's up to an engine to actually figure out how to do that for you. In an imperative language like C you could produce the same output, but you'd be specifying how, not just what: a for loop to iterate over each row in the table, if statements to compare values, etc.
- icambron 4y ago
  But each layer’s what is the layer above its how. My C code is just declaring the program I want to run. How it is run is determined by a compiler, assembler, linker, operating system, and CPU. 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.
  - andreareina 4y ago
    Sure it's a spectrum and the exact edges can be blurry. But no-one's going to argue that there isn't a qualitative difference between a sql query and the same logic written out in c
    - icambron 4y ago
      And I’m not arguing that. The article is dealing with those blurry edges, though, and what layer “matters” in evaluating the distinction is very much at issue. So if we’re going to compare to SQL/C, we need to bring in the relevant analogous nuances
rramadass 4y ago
Example Problem: Calculate the sum of the first N natural numbers. Declarative: N(N+1)/2 Imperative: for(int sum=0, int i=1; i<=N; i++) {sum = sum+i;}

TedDoesntTalk 4y ago

Flow control? Imperative. 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?

anyfoo 4y ago
I'm not even sure I agree with the author with that specifically: > 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))) Does it have flow control? Depends. No python flow control is used, but some of those lambdas implement flow control. But they're not any part of the language (Lambda calculous), we just build them. Would I call Lambda calculous declarative? Not at all!
- tshaddox 4y ago
  > Depends on your definition of "flow control". 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.
  - TedDoesntTalk 4y ago
    > How would you describe the absolute value function without saying “if” or “when How is this? absolute value is distance a number is from 0. I get your point, though.
    - tshaddox 4y ago
      True, the syntax will obviously depend on what primitives the language provides. Of course it could also just provide the abs() function directly.
  - naniwaduni 4y ago
    > How would you describe the absolute value function without saying “if” or “when” or specifying one particular element from a set? I don't think you even want a conditional when describing the absolute value function on the complex numbers.
mLuby 4y ago
> I wonder if a language can be Turing-complete without flow control. My guess is no? 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?
tshaddox 4y ago
> I wonder if a language can be Turing-complete without flow control. My guess is no? 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.
jfoutz 4y ago
Prolog would like to have a word.
marcosdumay 4y ago
The author seems as confused as the people he complains about. 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?)

forbiddenlake 4y ago

They do. > At their core, imperative and declarative differ in one major and fundamental way. Imperative has control flow and declarative does not.

tshaddox 4y ago
That’s a great rule of thumb to quickly identify whether some new syntax is more likely to be imperative or declarative, but it’s not a great explanation of what the two terms mean for someone new to the terms. A banana doesn’t have control flow but it’s definitely not a declarative programming language.
- epgui 4y ago
  I would eat a declarative banana.
  - WaxProlix 4y ago
    In fact, one should eat a declarative banana.
    - Izkata 4y ago
      The declarative banana should be in my stomach.
      - fjdbeb76 4y ago
        After the 20 or so comments that responded I'm still no closer to understanding. The SQL guy sort of made sense I guess maybe.
        Izkata 4y ago
        The "has control flow" vs "doesn't have control flow" description is a bad one, an attempt to reduce "declarative vs imperative" down to syntax instead of semantics. The parent to my comment is still in imperative style, despite having no control flow: It's specifying what actions to take to reach the desired outcome ("eat"). Declarative is describing the desired outcome, then letting the system figure out how to get there. 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.

eloisius 4y ago

As an example, declarative would be something like squares = map(square, inputs) While imperative would be a for loop, store this here, multiply that with this, push that onto the end of my results array, etc

Jtsummers 4y ago
Function calls don't make something declarative, otherwise C is declarative. And once you accept that, then words have no meaning anymore and we can't meaningfully discuss the topic.
- crb3 4y ago
  Unless they're talking about layers, with the function prototypes and function calls being declarative, and the next layer down being where the code is actually written out. Even that doesn't really work, though; main() usually has code in it... <shrug>
- dragonwriter 4y ago
  > Function calls don't make something declarative 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.
  - Jtsummers 4y ago
    I don't know what you're trying to get at, you seem to be agreeing with me. Function calls (present in C and other structured, procedural, and OO paradigm languages which are also imperative) do not make a language declarative. 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.

femto113 4y ago

It's much like the difference between "how" and "what". SQL select statements are my favored example of declarative: you simply "declare" what you want ("all rows from this table with these values in those columns") and it's up to an engine to actually figure out how to do that for you. In an imperative language like C you could produce the same output, but you'd be specifying how, not just what: a for loop to iterate over each row in the table, if statements to compare values, etc.

icambron 4y ago
But each layer’s what is the layer above its how. My C code is just declaring the program I want to run. How it is run is determined by a compiler, assembler, linker, operating system, and CPU. 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.
- andreareina 4y ago
  Sure it's a spectrum and the exact edges can be blurry. But no-one's going to argue that there isn't a qualitative difference between a sql query and the same logic written out in c
  - icambron 4y ago
    And I’m not arguing that. The article is dealing with those blurry edges, though, and what layer “matters” in evaluating the distinction is very much at issue. So if we’re going to compare to SQL/C, we need to bring in the relevant analogous nuances

rramadass 4y ago

Example Problem: Calculate the sum of the first N natural numbers. Declarative: N(N+1)/2 Imperative: for(int sum=0, int i=1; i<=N; i++) {sum = sum+i;}