This document discusses immutability in Scala. It recommends immutability to avoid unexpected values, concurrent state issues, and most container types are immutable by default. It provides examples of using vals for immutable variables, creating new instances instead of modifying existing ones, using the copy method for case classes, and hiding vars to prevent external modification of mutable state. It cautions being mindful of modifying immutable nested structures, fields initialized each instance, and closing over mutable state.

1. Scala:
immutability

2. Why
immutability?
• Keep
control
of
your
state
– Avoid
unexpected
values
• Avoid
concurrent
state
issues
• Most
container
types
are
immutable
by
default
(collec>ons,
for
example)

3. Use
vals
// Create immutable variable with a value
val temperature = BigInt(-10)
// Does not compile, cannot reassign to val
now = BigInt(5)

4. Don't
modify,
create
// Create immutable variable with a value
val temperature = BigInt(-10)
// Get the absolute value as a new instance
val absolute = temperature.abs

5. Use
copy
// Case classes give you a copy method for free
case class User(name: String, email: String)
// Use to create new copies of the same class
val bilbo = User("Bilbo", "bilbo@shi.re")
val newBilbo = bilbo.copy(email = "bilbo@valin.or")

6. Hide
your
vars
class User(name: String) {
// Public by default, hide them!
private var status = Satus.Initial
def transition(s: Status) {
status = s
}
}

7. Be
mindful
of
• Modifying
immutable
nested
structures
• Fields
that
get
ini>alized
with
each
new
instance
• Mutable
state
that
is
not
labeled
as
private
• Closing
over
mutable
state

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