CODEWITHSUNDEEP
scalatypes
MaatDeamon
MaatDeamon

Reputation: 9761

What is the Unit Type Behavior in Scala

Why does this code works:

val f: String => Unit = {(_:String) => 2}

//or more succinctly

val f: String => Unit = _ => 2

Clearly the body of my function return an int. However the type expected is Unit

But if i do

implicitly[String <:< Unit]

//Cannot prove that String <:< Unit.

Hence i am a bit confused as to how the cast is happening ? Why is it even allowed in the first place

Upvotes: 0

Views: 289

Answers (2)

J&#246;rg W Mittag
J&#246;rg W Mittag

Reputation: 369458

This is called Value Discarding, see 6.26.1 Value Conversions, sub-section Value Discarding of the Scala Language Specification:

If 𝑒 has some value type and the expected type is Unit, 𝑒 is converted to the expected type by embedding it in the term { 𝑒; () }.

In other words, your code is compiled as-if you had written

val f: String => Unit = {(_:String) => { 2; () }}

//or more succinctly

val f: String => Unit = _ => { 2; () }

I guess the intention behind this feature is familiarity for programmers coming from imperative programming languages, where it is often possible to discard values.

E.g. in C, it is legal to write

printf("Hello");

and discard the return value, even though printf returns an int. This goes together with the idea of expression statements and statement expressions that many imperative programming languages have.

Upvotes: 4

Dmytro Mitin
Dmytro Mitin

Reputation: 51658

In Scala any type A can be transformed into type Unit. It's not subtyping, it's transforming. Like type A means some possible side effect plus returning A while Unit means we disregard return value but keep the side effect.

Upvotes: 1

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