CODEWITHSUNDEEP
scalasyntax
Harsh Gupta
Harsh Gupta

Reputation: 331

Understanding the val declaration syntax for type Some in scala

While going through Spray.io examples library I came across this declaration of val in FileUploadHandler example of routing app.

val Some(HttpHeaders.Content-Type(ContentType(multipart: MultipartMediaType, _))) = header[HttpHeaders.Content-Type]

As per my understanding the variable declaration goes as val <identifier> = ...

Please help in understanding this paradigm of syntax.

Upvotes: 2

Views: 212

Answers (3)

Roman Kh
Roman Kh

Reputation: 2735

val is a bit more complex than just an assignment operator.

A definition

val p = e

where p is not just a variable name, is expanded to

val x = e match { case p => x }

Take a loot at the simplest example:

val Some(s) = Some(5)

As a result, s would be equal 5.

In your example header[HttpHeaders.Content-Type] is matched against Some(...).

Upvotes: 2

Martin Senne
Martin Senne

Reputation: 6059

According to Scala language spec: Value definitions can alternatively have a pattern as left-hand side. Watch out for PatDef in the document.

Section "Patterns in Value Definitions" of Daniel Westheide's Blog gives a nice overview on the usage.

Upvotes: 1

nikiforo
nikiforo

Reputation: 424

You're looking for extractors/mattern matching in scala, please see http://www.scala-lang.org/old/node/112.

You need a simple form of it, take a look at this snippet:

scala> val Some(t) = Some("Hello")
t: String = Hello

Upvotes: 0

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