=:=

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 Tos in a type with Froms.

Type parameters:

From: a type which is proved equal to To
To: a type which is proved equal to From

See also:

<:< for expressing subtyping constraints

Example:

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.
}

Source:

typeConstraints.scala

class From <:< To

trait Serializable

trait From => To

class Object

trait Matchable

class Any

Value members

Abstract methods

Substitute To for From and From for To in the type F[To, From], given that F is a type constructor of two arguments.

Substitute To for From and From for To in the type F[To, From], given that F is a type constructor of two arguments. Essentially swaps To and From in ftf's type.

Equivalent in power to each of substituteCo and substituteContra.

This method is impossible to implement without throwing or otherwise "cheating" unless From = To, so it ensures that this really represents a type equality.

Returns:: ftf, $sameDiff
Definition Classes: <:<
Source:: typeConstraints.scala

Concrete methods

If From = To and To = C, then From = C (equality is transitive)

Source:: typeConstraints.scala

Coerce a From into a To.

Coerce a From into a To. This is guaranteed to be the identity function.

This method is often called implicitly as an implicit A =:= B doubles as an implicit view A => B.

Definition Classes: <:< -> Function1
Source:: typeConstraints.scala

If From = To and C = From, then C = To (equality is transitive)

Source:: typeConstraints.scala

If From = To then To = From (equality is symmetric)

Source:: typeConstraints.scala

Lift this evidence over any type constructor F.

Definition Classes: <:<
Source:: typeConstraints.scala

Lift this evidence over the type constructor F, but flipped.

Definition Classes: <:<
Source:: typeConstraints.scala

Substitute the From in the type F[From], where F is any type constructor, for To.

Equivalent in power to each of substituteBoth and substituteContra.

This method is impossible to implement without throwing or otherwise "cheating" unless From = To, so it ensures that this really represents a type equality.

Returns:: ff, $sameDiff
Definition Classes: <:<
Source:: typeConstraints.scala

Substitute the To in the type F[To], where F is any type constructor, for From.

Equivalent in power to each of substituteBoth and substituteCo.

This method is impossible to implement without throwing or otherwise "cheating" unless From = To, so it ensures that this really represents a type equality.