Type inference in Scala

The compiler reads Int, Double, String and List types off the values you write.

// The compiler reads the type off the right-hand side.
val potCount = 6         // Int
val soilLiters = 2.5     // Double
val species = "monstera" // String

// The element type comes from the values in the list.
val wateringDays = List(1, 4, 8, 11) // List[Int]

// Numeric literals harmonize, so this is not List[AnyVal].
val doses = List(0.5, 1, 1.75) // List[Double]

// Annotate what other code depends on: a return type is part
// of the signature, so state it even when it is obvious.
def litersFor(pots: Int): Double = pots * soilLiters

def plantSummary: String = species + " x " + potCount

def careDemo(): Unit =
  println(s"$plantSummary: ${litersFor(wateringDays.length)}")
  println(doses.sum)

How it works

1. val potCount = 6 infers Int, val soilLiters = 2.5 infers Double, and val species = "monstera" infers String.
2. val wateringDays = List(1, 4, 8, 11) takes its element type from the values, and val doses = List(0.5, 1, 1.75) harmonizes the literals into a List[Double].
3. def litersFor(pots: Int): Double still annotates its return type, because the signature is what other code depends on.

Keywords and builtins used here

• Double
• Int
• List
• String
• Unit
• careDemo
• def
• litersFor
• plantSummary
• val

The run, in numbers

Lines

20

Characters to type

704

Tokens

98

Three-star pace

80 tpm

At the three-star pace of 80 tokens a minute, this run takes about 74 seconds.

Type this snippet

Step 2 of 3 in Values & types, step 2 of 27 in Language basics.

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