What does "trait A <: B" mean?
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
开发者_C百科 is the name of the trait being declared, not a type parameter.
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.
The <: syntax is reserved for future use in virtual classes (which are not implemented yet).
Looking at the Scala Language Specification, it seems to mean the same thing. The description for trait only mentions the trait A extends B
syntax. But the Scala syntax summary uses extends
and <:
interchangeably for trait definitions:
TraitTemplateOpt ::= Extends TraitTemplate | [[Extends] TemplateBody]
Extends ::= ‘extends’ | ‘<:’
Yes, well, almost, see this article for a little more information. From the language spec, we see the following definition:
We define two relations between types.
Type equivalence T ≡ U T and U are interchangeable in all contexts.
Conformance T <: U Type T conforms to type U .
Edit: Looking into the language spec it appears that <:
and extends
are the same, in particular it is defined as:
ClassTemplateOpt ::= Extends ClassTemplate | [[Extends] TemplateBody]
TraitTemplateOpt ::= Extends TraitTemplate | [[Extends] TemplateBody]
Extends ::= ‘extends’ | ‘<:’
Please note that as of Scala 2.12.5:
Using
<:
forextends
is deprecated
$ scala -deprecation -e 'trait B; trait A <: B'
/var/folders/0w/kb0d3rqn4zb9fcc91pxhgn8w0000gn/T/scalacmd4147407032094171597.scala:1: warning: Using `<:` for `extends` is deprecated
trait B; trait A <: B
^
one warning found
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