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Asked: 2026-05-13T13:07:34+00:00

In Scala, what does trait A <: B mean? Is it just the same

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In Scala, what does

trait A <: B

mean? Is it just the same as

trait A extends B

?

Edited to add: I’m familiar with the syntax for type parameters, and what <: means in that context. However, in the above example it would seem to me that A is the name of the trait being declared, not a type parameter.

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  1. Editorial Team

    NOTE As of Scala 2.12.5 using <: for extends is deprecated

    scala -deprecation -e 'trait B; trait A <: B'
/var/folders/0w/kb0d3rqn4zb9fcc91pxhgn8w0000gn/T/scalacmd2374381600671257557.scala:1: warning: Using `<:` for `extends` is deprecated
trait B; trait A <: B
                 ^
one warning found

    Seems to compile to the same thing.

     ~/code/scratch: scala -Xprint:typer -e 'trait B; trait A <: B'
          // snip
          abstract trait B extends scala.AnyRef;
          abstract trait A extends java.lang.Object with this.B

 ~/code/scratch: scala -Xprint:typer -e 'trait B; trait A extends B'
          // snip
          abstract trait B extends scala.AnyRef;
          abstract trait A extends java.lang.Object with this.B

    The spec doesn’t explain this in “5.3.3 Traits”. But the Syntax Summary does mention this.

    TraitDef ::= id [TypeParamClause] TraitTemplateOpt 
TraitTemplateOpt ::= Extends TraitTemplate | [[Extends] TemplateBody]
Extends ::= ‘extends’ | ‘<:’

    UPDATE It was introduced in r14632. With the compiler option -Xexperimental it marks the trait as abstract, for use with a proposed language feature Virtual Traits. Without -Xexperimental, it is a synonym for ‘extends’ that is allowed only for traits.

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