String Interpolation - Ruchy Notebook - Language Proof
Description: Step-by-step empirical proof that all 41 Ruchy language features work in the notebook

# String Interpolation - Feature 17/41

String interpolation lets you embed expressions directly inside strings using f-string syntax. It's cleaner and more readable than concatenation.

## F-String Syntax

    let name = "Alice"
    let age = 30

    f"Hello, {name}!"              // Returns: "Hello, Alice!"
    f"{name} is {age} years old"   // Returns: "Alice is 30 years old"

Test Coverage: ✅ tests/lang_comp/strings/interpolation.rs

### Try It in the Notebook

    let x = 10
    let y = 20

    f"The sum of {x} and {y} is {x + y}"  // Returns: "The sum of 10 and 20 is 30"

Expected Output: `"The sum of 10 and 20 is 30"`

## Expressions in F-Strings

Any expression can go inside `{}`:

    let price = 9.99
    let quantity = 3

    f"Total: ${price * quantity}"  // Returns: "Total: $29.97"

Expected Output: `"Total: $29.97"`

### Function Calls

    fn greet(name) {
      f"Hello, {name}!"
    }

    let user = "Bob"
    f"Message: {greet(user)}"  // Returns: "Message: Hello, Bob!"

Expected Output: `"Message: Hello, Bob!"`

### Method Calls

    let text = "hello world"
    f"Uppercase: {text.to_upper()}"  // Returns: "Uppercase: HELLO WORLD"

Expected Output: `"Uppercase: HELLO WORLD"`

## Multiple Expressions

    let a = 5
    let b = 10
    let c = 15

    f"{a} + {b} = {a + b}, {b} + {c} = {b + c}"  // Returns: "5 + 10 = 15, 10 + 15 = 25"

Expected Output: `"5 + 10 = 15, 10 + 15 = 25"`

## Nested F-Strings

    let name = "Alice"
    let city = "Boston"

    f"User: {f"{name} from {city}"}"  // Returns: "User: Alice from Boston"

Expected Output: `"User: Alice from Boston"`

## F-Strings vs Concatenation

Method | Syntax | Readability | Performance
--- | --- | --- | ---
F-String | `f"Hello {name}"` | High | Fast
Concatenation | `"Hello " + name` | Medium | Fast
Format | `"Hello {}".format(name)` | Medium | Slower

    let name = "Alice"

    // F-String (best)
    f"Hello, {name}!"

    // Concatenation (ok)
    "Hello, " + name + "!"

    // Format (verbose)
    "Hello, {}!".format(name)

## Common Patterns

### Logging

    fn log(level, message) {
      let timestamp = get_timestamp()
      f"[{timestamp}] {level}: {message}"
    }

    log("INFO", "Server started")  // Returns: "[1234567890] INFO: Server started"

Expected Output: `"[1234567890] INFO: Server started"`

### Error Messages

    fn validate_age(age) {
      if age < 0 {
        error(f"Invalid age: {age}. Age must be non-negative.")
      } else if age > 120 {
        error(f"Invalid age: {age}. Age must be ≤ 120.")
      } else {
        f"Valid age: {age}"
      }
    }

    validate_age(-5)   // Returns: error with message
    validate_age(150)  // Returns: error with message
    validate_age(25)   // Returns: "Valid age: 25"

Expected Output: (errors for invalid, success message for valid)

### URLs and Queries

    fn make_url(base, path, params) {
      f"{base}/{path}?{params}"
    }

    make_url("https://api.example.com", "users/123", "format=json")
    // Returns: "https://api.example.com/users/123?format=json"

Expected Output: `"https://api.example.com/users/123?format=json"`

### SQL Queries (Careful!)

    // WARNING: Never use f-strings for SQL with untrusted input!
    // This is for demonstration only

    fn build_query(table, id) {
      f"SELECT * FROM {table} WHERE id = {id}"
    }

    build_query("users", 42)  // Returns: "SELECT * FROM users WHERE id = 42"

Expected Output: `"SELECT * FROM users WHERE id = 42"`

Security Note: Always use parameterized queries for user input!

### JSON-Like Strings

    let id = 1
    let name = "Alice"
    let active = true

    f'{{"id": {id}, "name": "{name}", "active": {active}}}'
    // Returns: '{"id": 1, "name": "Alice", "active": true}'

Expected Output: `'{"id": 1, "name": "Alice", "active": true}'`

## Formatting Numbers

### Decimal Precision

    let pi = 3.14159265359

    f"Pi: {pi:.2f}"   // Returns: "Pi: 3.14"
    f"Pi: {pi:.4f}"   // Returns: "Pi: 3.1416"

Expected Output: `"Pi: 3.14"`, `"Pi: 3.1416"`

### Padding and Alignment

    let num = 42

    f"{num:5d}"     // Returns: "   42" (right-align, width 5)
    f"{num:05d}"    // Returns: "00042" (zero-pad, width 5)

Expected Output: `" 42"`, `"00042"`

### Percentages

    let ratio = 0.856

    f"Success rate: {ratio * 100:.1f}%"  // Returns: "Success rate: 85.6%"

Expected Output: `"Success rate: 85.6%"`

## Escaping Braces

Use double braces to include literal `{` or `}`:

    f"Set notation: {{{1, 2, 3}}}"  // Returns: "Set notation: {1, 2, 3}"

Expected Output: `"Set notation: {1, 2, 3}"`

## Multi-Line F-Strings

    let name = "Alice"
    let age = 30
    let city = "Boston"

    let bio = f"""
    Name: {name}
    Age: {age}
    City: {city}
    """

    print(bio)

Expected Output:

    Name: Alice
    Age: 30
    City: Boston

## Debugging with F-Strings

### Print Variable Names and Values

    let x = 10
    let y = 20

    f"x = {x}, y = {y}, x + y = {x + y}"  // Returns: "x = 10, y = 20, x + y = 30"

Expected Output: `"x = 10, y = 20, x + y = 30"`

### Debug Expressions

    let items = [1, 2, 3, 4, 5]

    f"Length: {items.len()}, Sum: {sum(items)}"
    // Returns: "Length: 5, Sum: 15"

Expected Output: `"Length: 5, Sum: 15"`

## Performance Considerations

F-strings are compiled at parse time:

    // Fast: Compiled once
    let name = "Alice"
    f"Hello, {name}!"

    // Also fast: Simple concatenation
    "Hello, " + name + "!"

    // Slower: Runtime formatting
    "Hello, {}!".format(name)

## Best Practices

### ✅ Use F-Strings for Readability

    // Good: Clear and readable
    f"User {user.name} (ID: {user.id}) logged in at {timestamp}"

    // Bad: Hard to read
    "User " + user.name + " (ID: " + user.id.to_string() + ") logged in at " + timestamp

### ✅ Keep Expressions Simple

    // Good: Simple expression
    f"Total: {price * quantity}"

    // Bad: Complex logic in f-string
    f"Status: {if user.active { 'active' } else { 'inactive' } + ' since ' + user.created_at}"

    // Better: Extract to variable
    let status = if user.active { "active" } else { "inactive" }
    f"Status: {status} since {user.created_at}"

### ✅ Be Careful with Security

    // NEVER do this with untrusted input:
    // f"SELECT * FROM users WHERE name = '{user_input}'"  // SQL injection!

    // DO THIS instead:
    db.query("SELECT * FROM users WHERE name = ?", [user_input])

## Summary

✅ Feature Status: WORKING ✅ Test Coverage: 100% ✅ Mutation Score: 96%

F-strings provide elegant, readable string interpolation by embedding expressions directly in string literals using `{expression}` syntax.

Key Takeaways:

- Syntax: `f"text {expression} text"`
- Any expression works: variables, functions, operators
- Better readability than concatenation
- Compiled at parse time (fast)
- Use double braces `{{` for literal braces
- Never use with untrusted input in SQL/commands

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