elm/lessons/03-functions.md

# Lesson 3: Functions

## Learning Goals

By the end of this lesson, you will:
- Write pure functions
- Understand currying and partial application
- Use higher-order functions (map, filter, reduce)
- Use the pipe operator for readable code
- Understand function composition

## Defining Functions

### Basic Syntax

```elm
-- JavaScript
// const greet = (name) => "Hello, " + name;

-- Elm
greet : String -> String
greet name =
    "Hello, " ++ name
```

Key differences:
- No parentheses around parameters
- No `return` keyword
- No curly braces
- Last expression is automatically returned

### Multiple Parameters

```elm
-- JavaScript
// const add = (a, b) => a + b;

-- Elm
add : Int -> Int -> Int
add a b =
    a + b

-- Using it
add 5 3  -- 8 (no parentheses or commas!)
```

### If-Then-Else

```elm
-- JavaScript
// const abs = x => x < 0 ? -x : x;

-- Elm
abs : Int -> Int
abs x =
    if x < 0 then
        -x
    else
        x
```

**Important:** In Elm, `if` is an **expression** that returns a value, not a statement. Both branches must return the same type.

```elm
-- This won't compile:
invalid x =
    if x > 0 then
        "positive"
    -- Missing else! What would it return?
```

## Pure Functions

In Elm, ALL functions are **pure**:

1. **Same input → Same output** (always)
2. **No side effects** (no I/O, no mutation, no randomness)

```javascript
// JavaScript - Impure function
let counter = 0;
function increment() {
    counter += 1;  // Side effect: modifies external state
    return counter;
}
increment();  // 1
increment();  // 2 - Different result!
```

```elm
-- Elm - Pure function
increment : Int -> Int
increment counter =
    counter + 1

increment 0  -- 1
increment 0  -- 1 (always!)
```

### Benefits of Pure Functions

1. **Testable** - No mocks needed, just input/output
2. **Cacheable** - Same input = same output, so cache results
3. **Predictable** - No hidden state changes
4. **Parallelizable** - Safe to run simultaneously

## Currying and Partial Application

### What is Currying?

In Elm, every function takes exactly one argument. A function with "multiple arguments" is actually a chain of functions.

```elm
add : Int -> Int -> Int
add a b = a + b

-- This is actually:
add : Int -> (Int -> Int)
-- A function that takes an Int and returns a function that takes an Int
```

### Partial Application

You can apply some arguments now and the rest later:

```elm
add : Int -> Int -> Int
add a b = a + b

addFive : Int -> Int
addFive = add 5  -- Partially applied!

addFive 3   -- 8
addFive 10  -- 15
```

**JavaScript equivalent:**
```javascript
const add = a => b => a + b;
const addFive = add(5);
addFive(3);  // 8
```

### Practical Example

```elm
-- A formatting function
format : String -> String -> String
format prefix text =
    prefix ++ ": " ++ text

-- Create specialized formatters
formatError : String -> String
formatError = format "ERROR"

formatWarning : String -> String
formatWarning = format "WARNING"

formatError "File not found"    -- "ERROR: File not found"
formatWarning "Low memory"      -- "WARNING: Low memory"
```

## Anonymous Functions (Lambdas)

```elm
-- JavaScript
// const double = x => x * 2;
// numbers.map(x => x * 2);

-- Elm
double = \x -> x * 2
List.map (\x -> x * 2) numbers
```

The `\` represents the Greek letter lambda (λ).

### Multiple Parameters

```elm
-- JavaScript
// const add = (a, b) => a + b;

-- Elm
add = \a b -> a + b
-- Or equivalently
add = \a -> \b -> a + b
```

## Higher-Order Functions

Higher-order functions take or return other functions.

### List.map (like Array.map)

```elm
-- JavaScript
// [1, 2, 3].map(x => x * 2)

-- Elm
List.map (\x -> x * 2) [1, 2, 3]  -- [2, 4, 6]

-- Or with a named function
double x = x * 2
List.map double [1, 2, 3]  -- [2, 4, 6]
```

### List.filter (like Array.filter)

```elm
-- JavaScript
// [1, 2, 3, 4, 5].filter(x => x > 3)

-- Elm
List.filter (\x -> x > 3) [1, 2, 3, 4, 5]  -- [4, 5]
```

### List.foldl / List.foldr (like Array.reduce)

```elm
-- JavaScript
// [1, 2, 3].reduce((sum, x) => sum + x, 0)

-- Elm
List.foldl (\x sum -> sum + x) 0 [1, 2, 3]  -- 6

-- Or using the (+) operator as a function
List.foldl (+) 0 [1, 2, 3]  -- 6
```

Note: Arguments are in different order than JavaScript!
- `foldl` goes left-to-right
- `foldr` goes right-to-left

### Other Useful Functions

```elm
List.head [1, 2, 3]           -- Just 1 (returns Maybe!)
List.tail [1, 2, 3]           -- Just [2, 3]
List.take 2 [1, 2, 3, 4]      -- [1, 2]
List.drop 2 [1, 2, 3, 4]      -- [3, 4]
List.reverse [1, 2, 3]        -- [3, 2, 1]
List.sort [3, 1, 2]           -- [1, 2, 3]
List.member 2 [1, 2, 3]       -- True
List.length [1, 2, 3]         -- 3
```

## The Pipe Operator |>

The pipe operator makes chains of functions readable:

```elm
-- Without pipes (hard to read, inside-out)
String.toUpper (String.trim (String.reverse "  hello  "))

-- With pipes (clear data flow)
"  hello  "
    |> String.reverse
    |> String.trim
    |> String.toUpper
-- Result: "OLLEH"
```

**How it works:** `x |> f` is the same as `f x`

### JavaScript Comparison

```javascript
// JavaScript method chaining (only works with methods)
"  hello  ".split("").reverse().join("").trim().toUpperCase()

// JavaScript pipe (proposed, not standard)
"  hello  " |> reverse |> trim |> toUpperCase
```

### Complex Example

```elm
-- Get the names of users over 21, sorted
users
    |> List.filter (\user -> user.age > 21)
    |> List.map .name
    |> List.sort
```

## Function Composition

Combine functions into new functions:

```elm
-- The >> operator (left to right)
addOne = (+) 1
double = (*) 2

addOneThenDouble : Int -> Int
addOneThenDouble = addOne >> double

addOneThenDouble 5  -- 12 (5+1=6, 6*2=12)

-- The << operator (right to left)
doubleThenAddOne : Int -> Int
doubleThenAddOne = addOne << double

doubleThenAddOne 5  -- 11 (5*2=10, 10+1=11)
```

### Pipe vs Composition

```elm
-- Pipe: Apply to a specific value
5 |> addOne |> double  -- 12

-- Composition: Create a new function
addOneThenDouble = addOne >> double
addOneThenDouble 5  -- 12
```

## Let Expressions

Use `let...in` for local bindings:

```elm
-- JavaScript
// function circleArea(radius) {
//   const pi = 3.14159;
//   const squared = radius * radius;
//   return pi * squared;
// }

-- Elm
circleArea : Float -> Float
circleArea radius =
    let
        pi = 3.14159
        squared = radius * radius
    in
    pi * squared
```

You can also define helper functions:

```elm
pythagoras : Float -> Float -> Float
pythagoras a b =
    let
        square x = x * x
    in
    sqrt (square a + square b)
```

## Exercise 3.1: Basic Functions

Write these functions:

1. `isEven : Int -> Bool` - Returns True if number is even
2. `exclaim : String -> String` - Adds "!" to the end of a string
3. `greet : String -> String -> String` - Takes a greeting and a name

<details>
<summary>Solution</summary>

```elm
isEven : Int -> Bool
isEven n =
    modBy 2 n == 0


exclaim : String -> String
exclaim text =
    text ++ "!"


greet : String -> String -> String
greet greeting name =
    greeting ++ ", " ++ name ++ "!"
```

</details>

## Exercise 3.2: Higher-Order Functions

Given this list:
```elm
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
```

Use List functions to:
1. Get only the odd numbers
2. Square each number
3. Get the sum of all numbers

<details>
<summary>Solution</summary>

```elm
-- 1. Odd numbers
List.filter (\n -> modBy 2 n /= 0) numbers
-- [1, 3, 5, 7, 9]

-- 2. Squared
List.map (\n -> n * n) numbers
-- [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

-- 3. Sum
List.foldl (+) 0 numbers
-- Or: List.sum numbers
-- 55
```

</details>

## Exercise 3.3: Pipe Operator

Rewrite this using the pipe operator:

```elm
String.fromInt (List.length (List.filter isEven (List.range 1 100)))
```

<details>
<summary>Solution</summary>

```elm
List.range 1 100
    |> List.filter isEven
    |> List.length
    |> String.fromInt
-- "50"
```

</details>

## Exercise 3.4: Partial Application

Create a `greetUser` function by partially applying this:

```elm
greet : String -> String -> String
greet greeting name =
    greeting ++ ", " ++ name ++ "!"
```

So that `greetUser "Alice"` returns `"Hello, Alice!"`

<details>
<summary>Solution</summary>

```elm
greetUser : String -> String
greetUser = greet "Hello"

greetUser "Alice"  -- "Hello, Alice!"
greetUser "Bob"    -- "Hello, Bob!"
```

</details>

## Key Takeaways

1. **All Elm functions are pure** - Same input always gives same output
2. **All functions are curried** - Multi-arg functions are chains of single-arg functions
3. **Partial application** lets you create specialized functions
4. **The pipe operator `|>`** makes data transformation chains readable
5. **Higher-order functions** like `map`, `filter`, `foldl` are your primary tools
6. **`let...in`** provides local bindings within functions

## Common Patterns

```elm
-- Transform a list
items |> List.map transform

-- Filter and transform
items
    |> List.filter condition
    |> List.map transform

-- Find something
items
    |> List.filter condition
    |> List.head

-- Aggregate
items
    |> List.foldl combiner initial
```

## What's Next?

In [Lesson 4](04-tea.md), we'll learn The Elm Architecture (TEA):
- Model: Your application state
- View: Turning state into HTML
- Update: Handling messages and updating state

This is the heart of Elm applications and inspired Redux!

---

[← Previous: Lesson 2](02-basics.md) | [Next: Lesson 4 - The Elm Architecture →](04-tea.md)