Scala Cheat Sheet 2026

The 30 highest-yield Scala facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. How do you mix a trait into a class in Scala? → class Foo with Bar
2. What is `flatMap` used for with Scala Futures? → Chaining asynchronous operations where each step depends on the previous result
3. In the REPL, how do you halt execution? → Pressing Ctrl+C
4. How is pattern matching performed in Scala? → using match-case
5. What is a `for-comprehension` over Futures equivalent to in Scala? → Chaining flatMap and map calls for sequential async composition
6. What happens when the Scala compiler cannot find a required implicit value in scope? → It produces a compile-time error
7. Which Scala collection type guarantees uniqueness of elements? → Set
8. Which method on a Scala Future converts it to a blocking `Try`? → Await.ready then future.value
9. What is variance in Scala generics? → How subtyping relationships of type parameters relate to subtyping of parameterized types
10. What does `unapplySeq` enable compared to `unapply` in Scala extractors? → It enables variable-length sequence extraction in patterns
11. What does the `filter` method do on a Scala collection? → Returns only elements satisfying a predicate
12. What does `scanLeft` do in Scala compared to `foldLeft`? → scanLeft returns all intermediate accumulator values, not just the final result
13. In Scala 3, what is `summon[T]` equivalent to in Scala 2? → implicitly[T]
14. What does `partition` return on a Scala collection? → A tuple of two collections: elements satisfying a predicate and those that don't
15. What best describes a type class in Scala? → A pattern using traits and implicits to achieve ad-hoc polymorphism
16. How are anonymous functions (lambdas) defined in Scala? → (x: Int) => x + 1
17. How do you pattern match a Tuple2 in Scala? → case (a, b) =>
18. Which Scala method combines all elements of a collection into a single value? → fold or reduce
19. Which of the following claims about a functor in Scala is incorrect? → It is a built-in construct in Scala
20. How do you match on a List's head and tail in Scala? → case h :: t =>
21. Which Scala feature lets you add a condition to a pattern match case? → Pattern guard using `if`
22. What is an algebraic data type (ADT) in Scala? → A type formed by combining other types using sealed traits and case classes
23. What is a type class in Scala? → A pattern using traits and implicits to add behavior to existing types without inheritance
24. What is the `Nothing` type in Scala? → The type of exceptions; it is a subtype of every type
25. How do you merge two immutable Maps in Scala? → map1 ++ map2
26. What does `implicit` mean when applied to a parameter in Scala? → The compiler automatically finds and passes a matching value from implicit scope
27. Can case objects be used in pattern matching in Scala? → Yes, case objects are matched by reference identity
28. What does `distinct` do on a Scala Seq? → Returns only unique elements in their original order
29. Which keyword is used to declare an immutable variable in Scala? → val
30. What is a Scala `Future`? → A delayed computation that may complete asynchronously with a value or failure