Pattern Guards - Feature 21/41

Pattern guards add conditional logic to pattern matching using if expressions. They enable more precise pattern matching beyond structural patterns alone.

Basic Guards

let value = 42

match value {
  n if n < 0 => "Negative",
  n if n == 0 => "Zero",
  n if n > 100 => "Large",
  n => "Normal"
}
// Returns: "Normal"

Test Coverage: ✅ tests/lang_comp/pattern_matching/guards.rs

Try It in the Notebook

let age = 25

match age {
  n if n < 18 => "Minor",
  n if n >= 18 && n < 65 => "Adult",
  n => "Senior"
}
// Returns: "Adult"

Expected Output: "Adult"

Guards with Destructuring

let point = (10, 20)

match point {
  (x, y) if x == y => "On diagonal",
  (x, y) if x > y => "Above diagonal",
  (x, y) => "Below diagonal"
}
// Returns: "Below diagonal"

Expected Output: "Below diagonal"

Enum Guards

enum Status {
  Active { id: i32, priority: i32 },
  Pending { id: i32 },
  Completed
}

fn describe(status) {
  match status {
    Status::Active { id, priority } if priority > 5 => f"High priority task {id}",
    Status::Active { id, priority } => f"Normal task {id} (priority {priority})",
    Status::Pending { id } => f"Task {id} is pending",
    Status::Completed => "Task completed"
  }
}

describe(Status::Active { id: 1, priority: 8 })  // Returns: "High priority task 1"

Expected Output: "High priority task 1"

Common Patterns

Range Checking

fn categorize_score(score) {
  match score {
    n if n >= 90 => "A",
    n if n >= 80 => "B",
    n if n >= 70 => "C",
    n if n >= 60 => "D",
    n => "F"
  }
}

categorize_score(85)  // Returns: "B"

Expected Output: "B"

Validation

fn validate_user(user) {
  match user {
    { age, name } if age < 0 => Err("Invalid age"),
    { age, name } if age > 120 => Err("Age too high"),
    { age, name } if name.len() == 0 => Err("Name required"),
    { age, name } => Ok({ age, name })
  }
}

validate_user({ age: 25, name: "Alice" })  // Returns: Ok({ age: 25, name: "Alice" })

Expected Output: Ok({ age: 25, name: "Alice" })

Complex Conditions

let data = { x: 10, y: 20, z: 30 }

match data {
  { x, y, z } if x + y == z => "Sum equals z",
  { x, y, z } if x * y == z => "Product equals z",
  { x, y, z } if x < y && y < z => "Ascending order",
  { x, y, z } => "No pattern"
}
// Returns: "Ascending order"

Expected Output: "Ascending order"

Option Guards

let maybe_value = Some(42)

match maybe_value {
  Some(n) if n > 100 => "Large value",
  Some(n) if n < 0 => "Negative value",
  Some(n) => f"Value: {n}",
  None => "No value"
}
// Returns: "Value: 42"

Expected Output: "Value: 42"

Result Guards

let result = Ok(42)

match result {
  Ok(n) if n > 100 => f"Success: large {n}",
  Ok(n) => f"Success: {n}",
  Err(e) if e.contains("timeout") => "Retry later",
  Err(e) => f"Error: {e}"
}
// Returns: "Success: 42"

Expected Output: "Success: 42"

Multiple Guards

let value = (10, 20, 30)

match value {
  (x, y, z) if x == y && y == z => "All equal",
  (x, y, z) if x == y || y == z || x == z => "Some equal",
  (x, y, z) if x < y && y < z => "Ascending",
  (x, y, z) if x > y && y > z => "Descending",
  _ => "Mixed"
}
// Returns: "Ascending"

Expected Output: "Ascending"

Guards vs Nested If

With Guards (Good)

match value {
  Some(n) if n > 100 => "Large",
  Some(n) if n < 0 => "Negative",
  Some(n) => "Normal",
  None => "Empty"
}

Nested If (Bad)

match value {
  Some(n) => {
    if n > 100 {
      "Large"
    } else if n < 0 {
      "Negative"
    } else {
      "Normal"
    }
  },
  None => "Empty"
}

Best Practices

✅ Use Guards for Value Checks

// Good: Clear, declarative
match age {
  n if n < 18 => "Minor",
  n if n >= 65 => "Senior",
  n => "Adult"
}

// Bad: Nested conditionals
match age {
  n => {
    if n < 18 { "Minor" }
    else if n >= 65 { "Senior" }
    else { "Adult" }
  }
}

✅ Keep Guards Simple

// Good: Simple condition
match point {
  (x, y) if x == y => "Diagonal",
  (x, y) => "Off diagonal"
}

// Bad: Complex logic
match point {
  (x, y) if (x * x + y * y) < 100 && abs(x - y) > 5 => "Complex",
  (x, y) => "Simple"
}
// Better: Extract to function
fn is_complex(x, y) {
  (x * x + y * y) < 100 && abs(x - y) > 5
}

match point {
  (x, y) if is_complex(x, y) => "Complex",
  (x, y) => "Simple"
}

✅ Order Guards Carefully

// Good: Most specific first
match score {
  n if n == 100 => "Perfect!",
  n if n >= 90 => "A",
  n if n >= 80 => "B",
  n => "Lower"
}

// Bad: Generic first (unreachable code)
match score {
  n if n >= 80 => "B or higher",  // Catches 90-100
  n if n >= 90 => "A",            // Never reached!
  n => "Lower"
}

Summary

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

Pattern guards add conditional logic to match expressions using if, enabling precise pattern matching beyond structural patterns.

Key Takeaways:

• Syntax: pattern if condition => result
• Works with all pattern types
• Guards evaluated after pattern matches
• Keep guards simple and readable
• Order guards from specific to generic
• Prefer guards over nested if statements

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