Functions concept (#324)

* add about

* add introduction

* add links

* update config.json

* fix lint issues

concepts/functions/.meta/config.json

@@ -1,7 +1,5 @@
 {
-    "blurb": "<todo>",
-    "authors": [
-        "<your_gh_username>"
-    ],
+    "blurb": "Functions in Cairo let you organize and reuse code by defining reusable blocks with a name, parameters, and an optional return value.",
+    "authors": ["0xNeshi"],
     "contributors": []
 }

concepts/functions/about.md

@@ -1 +1,117 @@
 # Functions
+
+Functions are fundamental in Cairo.
+
+By convention, Cairo uses **snake case** for function and variable names, where words are lowercase and separated by underscores.
+
+Here's an example:
+
+```rust
+fn another_function() {
+    println!("Another function.");
+}
+
+fn main() {
+    println!("Hello, world!");
+    another_function();
+}
+```
+
+Functions are defined with `fn`, followed by a name, parentheses for parameters (if any), and a body enclosed in curly brackets.
+
+You can call a function by its name followed by parentheses.
+
+Function order doesn't matter as long as they're in scope.
+
+## Parameters
+
+Functions can take **parameters**, which are variables defined in the function signature.
+
+When calling the function, you provide **arguments**, the values passed to these parameters.
+
+```rust
+fn main() {
+    another_function(5);
+}
+
+fn another_function(x: felt252) {
+    println!("The value of x is: {}", x);
+}
+```
+
+Here, `x` is a parameter of type `felt252`.
+
+When called with `5`, the function prints "The value of x is: 5".
+
+Multiple parameters are separated by commas:
+
+```rust
+fn main() {
+    print_labeled_measurement(5, "h");
+}
+
+fn print_labeled_measurement(value: u128, unit_label: ByteArray) {
+    println!("The measurement is: {value}{unit_label}");
+}
+```
+
+This prints: `The measurement is: 5h`.
+
+## Statements and Expressions
+
+Cairo functions are made up of **statements** and **expressions**:
+
+- **Statements** perform actions but do not return values.
+For example, `let x = 3;` is a statement.
+- **Expressions** evaluate to a value.
+For example, `5 + 6` evaluates to `11`.
+
+Blocks enclosed in curly brackets are also expressions, and their final value is returned:
+
+```rust
+fn main() {
+    let y = {
+        let x = 3;
+        x + 1
+    };
+    println!("The value of y is: {}", y);
+}
+```
+
+Here, `y` is assigned the value `4`.
+
+## Return Values
+
+Functions can return values using the `->` syntax to specify the return type.
+
+By default, a function returns the value of its final expression:
+
+```rust
+fn five() -> u32 {
+    5
+}
+
+fn main() {
+    let x = five();
+    println!("The value of x is: {}", x);
+}
+```
+
+This outputs: `The value of x is: 5`.
+
+For more complex cases:
+
+```rust
+fn plus_one(x: u32) -> u32 {
+    x + 1
+}
+
+fn main() {
+    let x = plus_one(5);
+    println!("The value of x is: {}", x);
+}
+```
+
+This prints: `The value of x is: 6`.
+
+Be mindful not to add a semicolon at the end of the final expression, as it would turn the expression into a statement, causing the function to return `()` (unit type) instead of the expected value.

concepts/functions/introduction.md

@@ -1 +1,5 @@
 # Introduction
+
+Functions in Cairo let you organize and reuse code by defining reusable blocks with a name, parameters, and an optional return value.
+They consist of statements and expressions, where the latter can produce a value.
+Learning how to define, call, and work with parameters and return values is key to writing clean and efficient Cairo programs.

concepts/functions/links.json

@@ -1 +1,6 @@
-[]
+[
+    {
+        "url": "https://book.cairo-lang.org/ch02-03-functions.html",
+        "description": "Functions in The Cairo book"
+    }
+]