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Type Inference

The Scala compiler can often infer the type of an expression so you don't have to declare it explicitly.

Omitting the type

In [1]:
val businessName = "Montreux Jazz Café"
Out[1]:
businessName: String = "Montreux Jazz Caf\u00e9"

The compiler can detect that businessName is a String. It works similarly with methods:

In [2]:
def squareOf(x: Int) = x * x
Out[2]:
defined function squareOf

The compiler can infer that the return type is an Int, so no explicit return type is required.

For recursive methods, the compiler is not able to infer a result type. Here is a program which will fail the compiler for this reason:

In [3]:
def fac(n: Int) = if (n == 0) 1 else n * fac(n - 1)
cmd2.sc:1: recursive method fac needs result type
def fac(n: Int) = if (n == 0) 1 else n * fac(n - 1)
                                         ^Compilation Failed
Compilation Failed

It is also not compulsory to specify type parameters when polymorphic methods are called or generic classes are instantiated. The Scala compiler will infer such missing type parameters from the context and from the types of the actual method/constructor parameters.

Here are two examples:

In [3]:
case class MyPair[A, B](x: A, y: B);
val p = MyPair(1, "scala") // type: MyPair[Int, String]

def id[T](x: T) = x
val q = id(1)              // type: Int
Out[3]:
defined class MyPair
p: MyPair[Int, String] = MyPair(1, "scala")
defined function id
q: Int = 1

The compiler uses the types of the arguments of MyPair to figure out what type A and B are. Likewise for the type of x.

Parameters

The compiler never infers method parameter types. However, in certain cases, it can infer anonymous function parameter types when the function is passed as argument.

In [4]:
Seq(1, 3, 4).map(x => x * 2)  // List(2, 6, 8)
Out[4]:
res3: Seq[Int] = List(2, 6, 8)

The parameter for map is f: A => B. Because we put integers in the Seq, the compiler knows that A is Int (i.e. that x is an integer). Therefore, the compiler can infer from x * 2 that B is type Int.

When not to rely on type inference

It is generally considered more readable to declare the type of members exposed in a public API. Therefore, we recommended that you make the type explicit for any APIs that will be exposed to users of your code.

Also, type inference can sometimes infer a too-specific type. Suppose we write:

In [5]:
var obj = null
Out[5]:
obj: Null = null

We can't then go on and make this reassignment:

In [6]:
obj = new AnyRef
cmd5.sc:1: type mismatch;
 found   : Object
 required: Null
val res5 = obj = new AnyRef
                 ^Compilation Failed
Compilation Failed

It won't compile, because the type inferred for obj was Null. Since the only value of that type is null, it is impossible to assign a different value.

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