sealed abstract class =:=[From, To] extends <:<[From, To] with Serializable
An instance of A =:= B
witnesses that the types A
and B
are equal. It also acts as a A <:< B
,
but not a B <:< A
(directly) due to restrictions on subclassing.
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.
- From
a type which is proved equal to
To
- To
a type which is proved equal to
From
- Annotations
- @implicitNotFound(msg = "Cannot prove that ${From} =:= ${To}.")
- Source
- typeConstraints.scala
An in-place variant of scala.collection.mutable.ArrayBuffer#transpose
implicit class BufOps[A](private val buf: ArrayBuffer[A]) extends AnyVal { def inPlaceTranspose[E]()(implicit ev: A =:= ArrayBuffer[E]) = ??? // Because ArrayBuffer is invariant, we can't make do with just a A <:< ArrayBuffer[E] // Getting buffers *out* from buf would work, but adding them back *in* wouldn't. }
- See also
<:< for expressing subtyping 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 a type constructor of two arguments.Substitute
To
forFrom
andFrom
forTo
in the typeF[To, From]
, given thatF
is a type constructor of two arguments. 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 type equality.- 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
(equality is transitive) - def andThen[C](r: <:<[To, C]): <:<[From, C]
If
From <: To
andTo <: C
, thenFrom <: C
(subtyping is transitive)If
From <: To
andTo <: C
, thenFrom <: C
(subtyping is transitive)- Definition Classes
- <:<
- 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
(equality is transitive) - def compose[C](r: <:<[C, From]): <:<[C, To]
If
From <: To
andC <: From
, thenC <: To
(subtyping is transitive)If
From <: To
andC <: From
, thenC <: To
(subtyping is transitive)- Definition Classes
- <:<
- def compose[C](r: (C) => From): (C) => To
Composes two instances of Function1 in a new Function1, with this function applied last.
- def flip: =:=[To, From]
If
From = To
thenTo = From
(equality is symmetric) - def liftCo[F[_]]: =:=[F[From], F[To]]
Lift this evidence over any type constructor
F
. - def liftContra[F[_]]: =:=[F[To], F[From]]
Lift this evidence over the type constructor
F
, but flipped. - def substituteCo[F[_]](ff: F[From]): F[To]
Substitute the
From
in the typeF[From]
, whereF
is any type constructor, forTo
.Substitute the
From
in the typeF[From]
, whereF
is any 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 type equality.- 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 any type constructor, forFrom
.Substitute the
To
in the typeF[To]
, whereF
is any 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 type equality.- 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
.