sealed abstract class <:<[-From, +To] extends (From) => To with Serializable
An instance of A <:< B
witnesses that A
is a subtype of B
.
Requiring an implicit argument of the type A <:< B
encodes
the generalized constraint A <: B
.
To constrain any abstract type T
that's in scope in a method's
argument list (not just the method's own type parameters) simply
add an implicit argument of type T <:< U
, where U
is the required
upper bound; or for lower-bounds, use: L <:< T
, where L
is the
required lower bound.
In case of any confusion over which method goes in what direction, all the "Co" methods (including
apply) go from left to right in the type ("with" the type), and all the "Contra" methods go
from right to left ("against" the type). E.g., apply turns a From
into a To
, and
substituteContra replaces the To
s in a type with From
s.
In part contributed by Jason Zaugg.
- From
a type which is proved a subtype of
To
- To
a type which is proved a supertype of
From
- Annotations
- @implicitNotFound(msg = "Cannot prove that ${From} <:< ${To}.")
- Source
- typeConstraints.scala
sealed trait Option[+A] { // def flatten[B, A <: Option[B]]: Option[B] = ... // won't work, since the A in flatten shadows the class-scoped A. def flatten[B](implicit ev: A <:< Option[B]): Option[B] = if(isEmpty) None else ev(get) // Because (A <:< Option[B]) <: (A => Option[B]), ev can be called to turn the // A from get into an Option[B], and because ev is implicit, that call can be // left out and inserted automatically. }
- See also
=:= for expressing equality constraints
- Alphabetic
- By Inheritance
- <:<
- Serializable
- Function1
- AnyRef
- Any
- by UnliftOps
- by any2stringadd
- by StringFormat
- by Ensuring
- by ArrowAssoc
- Hide All
- Show All
- Public
- Protected
Abstract Value Members
- abstract def substituteBoth[F[-_, +_]](ftf: F[To, From]): F[From, To]
Substitute
To
forFrom
andFrom
forTo
in the typeF[To, From]
, given thatF
is contravariant in the first argument and covariant in the second.Substitute
To
forFrom
andFrom
forTo
in the typeF[To, From]
, given thatF
is contravariant in the first argument and covariant in the second. Essentially swapsTo
andFrom
inftf
's type.Equivalent in power to each of substituteCo and substituteContra.
This method is impossible to implement without
throw
ing or otherwise "cheating" unlessFrom <: To
, so it ensures that this really represents a subtyping relationship.- returns
ftf
, but with a (potentially) different type
Concrete Value Members
- def andThen[C](r: <:<[To, C]): <:<[From, C]
If
From <: To
andTo <: C
, thenFrom <: C
(subtyping is transitive) - def andThen[C](r: (To) => C): (From) => C
Composes two instances of Function1 in a new Function1, with this function applied first.
- def apply(f: From): To
Coerce a
From
into aTo
. - def compose[C](r: <:<[C, From]): <:<[C, To]
If
From <: To
andC <: From
, thenC <: To
(subtyping is transitive) - def compose[C](r: (C) => From): (C) => To
Composes two instances of Function1 in a new Function1, with this function applied last.
- def liftCo[F[+_]]: <:<[F[From], F[To]]
Lift this evidence over a covariant type constructor
F
. - def liftContra[F[-_]]: <:<[F[To], F[From]]
Lift this evidence over a contravariant type constructor
F
. - def substituteCo[F[+_]](ff: F[From]): F[To]
Substitute the
From
in the typeF[From]
, whereF
is a covariant type constructor, forTo
.Substitute the
From
in the typeF[From]
, whereF
is a covariant type constructor, forTo
.Equivalent in power to each of substituteBoth and substituteContra.
This method is impossible to implement without
throw
ing or otherwise "cheating" unlessFrom <: To
, so it ensures that this really represents a subtyping relationship.- returns
ff
, but with a (potentially) different type
- def substituteContra[F[-_]](ft: F[To]): F[From]
Substitute the
To
in the typeF[To]
, whereF
is a contravariant type constructor, forFrom
.Substitute the
To
in the typeF[To]
, whereF
is a contravariant type constructor, forFrom
.Equivalent in power to each of substituteBoth and substituteCo.
This method is impossible to implement without
throw
ing or otherwise "cheating" unlessFrom <: To
, so it ensures that this really represents a subtyping relationship.- returns
ft
, but with a (potentially) different type
- def toString(): String
Creates a String representation of this object.
- def unlift: PartialFunction[From, B]
Converts an optional function to a partial function.
Converts an optional function to a partial function.
- Implicit
- This member is added by an implicit conversion from <:<[From, To] toUnliftOps[From, B] performed by method UnliftOps in scala.Function1.This conversion will take place only if To is a subclass of Option[B] (To <: Option[B]).
- Definition Classes
- UnliftOps
Unlike Function.unlift, this UnliftOps.unlift method can be used in extractors.
val of: Int => Option[String] = { i => if (i == 2) { Some("matched by an optional function") } else { None } } util.Random.nextInt(4) match { case of.unlift(m) => // Convert an optional function to a pattern println(m) case _ => println("Not matched") }
Example:
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int
,Float
,Array
orOption
which are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collection
and its sub-packages contain Scala's collections frameworkscala.collection.immutable
- Immutable, sequential data-structures such asVector
,List
,Range
,HashMap
orHashSet
scala.collection.mutable
- Mutable, sequential data-structures such asArrayBuffer
,StringBuilder
,HashMap
orHashSet
scala.collection.concurrent
- Mutable, concurrent data-structures such asTrieMap
scala.concurrent
- Primitives for concurrent programming such asFutures
andPromises
scala.io
- Input and output operationsscala.math
- Basic math functions and additional numeric types likeBigInt
andBigDecimal
scala.sys
- Interaction with other processes and the operating systemscala.util.matching
- Regular expressionsOther packages exist. See the complete list on the right.
Additional parts of the standard library are shipped as separate libraries. These include:
scala.reflect
- Scala's reflection API (scala-reflect.jar)scala.xml
- XML parsing, manipulation, and serialization (scala-xml.jar)scala.collection.parallel
- Parallel collections (scala-parallel-collections.jar)scala.util.parsing
- Parser combinators (scala-parser-combinators.jar)scala.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)Automatic imports
Identifiers in the scala package and the
scala.Predef
object are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
List
is an alias forscala.collection.immutable.List
.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
String
is an alias forjava.lang.String
.