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CS253 Programming Paradigms

Overview

• Programming paradigms:
  • Imperative programming
  • Declarative programming
    • Functional programming
  • Event-driven programming

Imperative programming

• Languages have an Imperative mood, used for giving commands.
• Statements that cause an immediate action.
• Like a commander in military combat, or a foreman at a construction site.
• No reasoning need be given. Just do this!
• In programming languages, we usually call things like if and while statements, not commands, but that’s just nomenclature.
• Examples: C++, Java, Bash, Perl, Python, Javascript.

Declarative programming

• Not commands as much as description. Not a sergeant, a general.
• More like an architect designing a building.
  • The architect doesn’t tell the electrician where to run the wires.
  • The architect describes the goal, not how to obtain the goal.
• Mathematicians think declaratively. x = y+2, to them, doesn’t mean “fetch y, add 2, store in x”. It means to define x as always being 2 larger than y.
• Examples:
  • spreadsheets (e.g., Microsoft Excel)
  • a regular expression/filename pattern (the pattern, not the pattern-matching code)
  • functional programming
  • Makefile, a mix: dependencies are declarative, commands are imperative

Functional programming example

Parallel JavaScript implementations, taken from Wikipedia:

Traditional imperative look:

const numList = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
let result = 0;
for (let i = 0; i < numList.length; i++) {
  if (numList[i] % 2 === 0) {
    result += numList[i] * 10;
  }
}

Functional style:

const result = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
               .filter(n => n % 2 === 0)
               .map(a => a * 10)
               .reduce((a, b) => a + b);

Sure, down at the machine instruction level, it’s all imperative. However, the programmer doesn’t have to think that way.

C++ declarative programming

Here are two ways to add some numbers in C++:

vector<int> v = {11,22,33,44,55};
int sum = 0;
for (size_t i=0; i<v.size(); i++)  // imperative loop
    sum += v[i];
cout << sum;

165

vector<int> v = {11,22,33,44,55};
int sum = 0;
for (int n : v)  // declarative loop
    sum += n;
cout << sum;

165

It might even be the same instructions both times—it doesn’t matter. Certainly, there are still imperative aspects. In the latter, I don’t have to think about the mechanics of “start at element 0, increment by 1 until I get to the last element of the array”. I just think “do this for each element”.

Event-driven programming

• In event-driven programming, your code is not in charge. Often, you don’t write main().
• Instead, your code spends most of the time waiting to be invoked.
• Examples:
  • web browser: your code gets called on a key press, mouse click, or window motion.
  • clock program: your code gets called each second to update the clock.
  • interrupt service routine: your code gets called to deal with the interrupt.
  • Linux signals: signal handler function gets called to deal with the signal.

Clock example

An imperative clock program might work like this pseudocode:

int main() {
    while (true) {
        sleepms(1000);
        display_time();
    }
}

Or, it might be event-driven, where the event is a clock triggering:

int main() {
    schedule_periodic_callback(1000, &display_time);
    do_nothing_but_service_interrupts();
}

The event-driven code could be easily modified to handle other event-driven tasks: keyboard input, mouse clicks, etc.