The Type Hierarchy & Type Inference

Local type inference: parameter types of methods must be declared.

10.1 Unified type hierarchy

Any: supertype of everything (equals, hashCode, toString).

AnyVal: value types (unboxed where possible).

AnyRef: reference types.

Nothing: subtype of everything; type of throw and non-terminating expressions.

Null: type of null. With -Yexplicit-nulls, reference types are non-nullable and you write String | Null.

Unit: single value ().

Any
              /         \
          AnyVal        AnyRef (java.lang.Object)
   Int Double Boolean     String, List, your classes...
   Char Unit Long ...          |
              \            Null (only with explicit-nulls off)
               \          /
                 Nothing

val things: List[Any] = List(1, "two", 3.0, 'c', true, () => 5)
def fail(msg: String): Nothing = throw RuntimeException(msg)

10.2 Type inference

Local type inference: parameter types of methods must be declared.

val x = 42                         // Int
val xs = List(1, 2, 3)             // List[Int]
val mixed = List(1, "a")           // List[Int | String] in Scala 3 (Any in Scala 2)
def sq(x: Int) = x * x             // return type inferred: Int

10.3 Type aliases

type UserId = Long
type Callback[A] = A => Unit

10.4 Tuples

val t = (1, "Ann", true)          // (Int, String, Boolean)
t._1, t._2                        // 1, "Ann"
t(0)                              // 1 (Scala 3 index access)
val (id, name, active) = t        // destructuring
t.toList, t.size, t ++ (4.0, 'x'), t.head, t.tail

// Named tuples (Scala 3.7+)
val p = (name = "Ann", age = 30)
p.name

🧪 Part I Exercises

Write @main def fizzbuzz(n: Int) using match on (i % 3, i % 5).

Write a tail-recursive power(base: Double, exp: Int): Double.

Write isPalindrome(s: String): Boolean ignoring case and non-letters.

Create a Vec class with +, -, * (scalar), dot, and unary -.

Write compose(fs: List[Int => Int]): Int => Int.

Write a method retry[A](times: Int)(block: => A): A using by-name parameters.

Write collatz(n: Int): List[Int].

Using for/yield, generate all Pythagorean triples with sides ≤ 30.

Write countWords(text: String): Map[String, Int].

Convert temperatures with default and named parameters: convert(value, from = "C", to = "F").