Packages

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 Tos in a type with Froms.

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
Example:

  1. scala.Option#flatten

    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

Linear Supertypes
java.io.Serializable, (From) => To, AnyRef, Any
Known Subclasses
=:=
Type Hierarchy
Ordering
  1. Alphabetic
  2. By Inheritance
Inherited
  1. <:<
  2. Serializable
  3. Function1
  4. AnyRef
  5. Any
Implicitly
  1. by UnliftOps
  2. by any2stringadd
  3. by StringFormat
  4. by Ensuring
  5. by ArrowAssoc
  1. Hide All
  2. Show All
Visibility
  1. Public
  2. Protected

Abstract Value Members

  1. abstract def substituteBoth[F[-_, +_]](ftf: F[To, From]): F[From, To]

    Substitute To for From and From for To in the type F[To, From], given that F is contravariant in the first argument and covariant in the second.

    Substitute To for From and From for To in the type F[To, From], given that F is contravariant in the first argument and covariant in the second. 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 subtyping relationship.

    returns

    ftf, but with a (potentially) different type

Concrete Value Members

  1. final def !=(arg0: Any): Boolean

    Test two objects for inequality.

    Test two objects for inequality.

    returns

    true if !(this == that), false otherwise.

    Definition Classes
    AnyRef → Any
  2. final def ##: Int

    Equivalent to x.hashCode except for boxed numeric types and null.

    Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.

    returns

    a hash value consistent with ==

    Definition Classes
    AnyRef → Any
  3. def +(other: String): String
    Implicit
    This member is added by an implicit conversion from <:<[From, To] toany2stringadd[<:<[From, To]] performed by method any2stringadd in scala.Predef.
    Definition Classes
    any2stringadd
  4. def ->[B](y: B): (<:<[From, To], B)
    Implicit
    This member is added by an implicit conversion from <:<[From, To] toArrowAssoc[<:<[From, To]] performed by method ArrowAssoc in scala.Predef.This conversion will take place only if To is a subclass of Option[Nothing] (To <: Option[Nothing]).
    Definition Classes
    ArrowAssoc
    Annotations
    @inline()
  5. final def ==(arg0: Any): Boolean

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  6. def andThen[C](r: <:<[To, C]): <:<[From, C]

    If From <: To and To <: C, then From <: C (subtyping is transitive)

  7. def andThen[C](r: (To) => C): (From) => C

    Composes two instances of Function1 in a new Function1, with this function applied first.

    Composes two instances of Function1 in a new Function1, with this function applied first.

    returns

    a new function f such that f(x) == g(apply(x))

    Definition Classes
    <:< → Function1
  8. def apply(f: From): To

    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.

    f

    some value of type From

    returns

    f, but with a (potentially) different type

    Definition Classes
    <:< → Function1
  9. final def asInstanceOf[T0]: T0

    Cast the receiver object to be of type T0.

    Cast the receiver object to be of type T0.

    Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

    returns

    the receiver object.

    Definition Classes
    Any
    Exceptions thrown

    ClassCastException if the receiver object is not an instance of the erasure of type T0.

  10. def clone(): AnyRef

    Create a copy of the receiver object.

    Create a copy of the receiver object.

    The default implementation of the clone method is platform dependent.

    returns

    a copy of the receiver object.

    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.CloneNotSupportedException]) @native()
    Note

    not specified by SLS as a member of AnyRef

  11. def compose[C](r: <:<[C, From]): <:<[C, To]

    If From <: To and C <: From, then C <: To (subtyping is transitive)

  12. def compose[C](r: (C) => From): (C) => To

    Composes two instances of Function1 in a new Function1, with this function applied last.

    Composes two instances of Function1 in a new Function1, with this function applied last.

    returns

    a new function f such that f(x) == apply(g(x))

    Definition Classes
    <:< → Function1
  13. def ensuring(cond: (<:<[From, To]) => Boolean, msg: => Any): <:<[From, To]
    Implicit
    This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  14. def ensuring(cond: (<:<[From, To]) => Boolean): <:<[From, To]
    Implicit
    This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  15. def ensuring(cond: Boolean, msg: => Any): <:<[From, To]
    Implicit
    This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  16. def ensuring(cond: Boolean): <:<[From, To]
    Implicit
    This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  17. final def eq(arg0: AnyRef): Boolean

    Tests whether the argument (that) is a reference to the receiver object (this).

    Tests whether the argument (that) is a reference to the receiver object (this).

    The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

    • It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
    • For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
    • null.eq(null) returns true.

    When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).

    returns

    true if the argument is a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  18. def equals(arg0: AnyRef): Boolean

    The equality method for reference types.

    The equality method for reference types. Default implementation delegates to eq.

    See also equals in scala.Any.

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  19. def finalize(): Unit

    Called by the garbage collector on the receiver object when there are no more references to the object.

    Called by the garbage collector on the receiver object when there are no more references to the object.

    The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.Throwable])
    Note

    not specified by SLS as a member of AnyRef

  20. def formatted(fmtstr: String): String

    Returns string formatted according to given format string.

    Returns string formatted according to given format string. Format strings are as for String.format (@see java.lang.String.format).

    Implicit
    This member is added by an implicit conversion from <:<[From, To] toStringFormat[<:<[From, To]] performed by method StringFormat in scala.Predef.
    Definition Classes
    StringFormat
    Annotations
    @inline()
  21. final def getClass(): Class[_ <: AnyRef]

    Returns the runtime class representation of the object.

    Returns the runtime class representation of the object.

    returns

    a class object corresponding to the runtime type of the receiver.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  22. def hashCode(): Int

    The hashCode method for reference types.

    The hashCode method for reference types. See hashCode in scala.Any.

    returns

    the hash code value for this object.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  23. final def isInstanceOf[T0]: Boolean

    Test whether the dynamic type of the receiver object is T0.

    Test whether the dynamic type of the receiver object is T0.

    Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

    returns

    true if the receiver object is an instance of erasure of type T0; false otherwise.

    Definition Classes
    Any
  24. def liftCo[F[+_]]: <:<[F[From], F[To]]

    Lift this evidence over a covariant type constructor F.

  25. def liftContra[F[-_]]: <:<[F[To], F[From]]

    Lift this evidence over a contravariant type constructor F.

  26. final def ne(arg0: AnyRef): Boolean

    Equivalent to !(this eq that).

    Equivalent to !(this eq that).

    returns

    true if the argument is not a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  27. final def notify(): Unit

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Annotations
    @native()
    Note

    not specified by SLS as a member of AnyRef

  28. final def notifyAll(): Unit

    Wakes up all threads that are waiting on the receiver object's monitor.

    Wakes up all threads that are waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Annotations
    @native()
    Note

    not specified by SLS as a member of AnyRef

  29. def substituteCo[F[+_]](ff: F[From]): F[To]

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

    Substitute the From in the type F[From], where F is a covariant 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 subtyping relationship.

    returns

    ff, but with a (potentially) different type

  30. def substituteContra[F[-_]](ft: F[To]): F[From]

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

    Substitute the To in the type F[To], where F is a contravariant 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 subtyping relationship.

    returns

    ft, but with a (potentially) different type

  31. final def synchronized[T0](arg0: => T0): T0

    Executes the code in body with an exclusive lock on this.

    Executes the code in body with an exclusive lock on this.

    returns

    the result of body

    Definition Classes
    AnyRef
  32. def toString(): String

    Creates a String representation of this object.

    Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

    returns

    a String representation of the object.

    Definition Classes
    Function1 → AnyRef → Any
  33. 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
    Example:

    1. 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")
}
  34. final def wait(): Unit

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--.

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--.

    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException])
    Note

    not specified by SLS as a member of AnyRef

  35. final def wait(arg0: Long, arg1: Int): Unit

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-int-

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-int-

    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException])
    Note

    not specified by SLS as a member of AnyRef

  36. final def wait(arg0: Long): Unit

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-.

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-.

    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException]) @native()
    Note

    not specified by SLS as a member of AnyRef

Deprecated Value Members

  1. def →[B](y: B): (<:<[From, To], B)
    Implicit
    This member is added by an implicit conversion from <:<[From, To] toArrowAssoc[<:<[From, To]] performed by method ArrowAssoc in scala.Predef.This conversion will take place only if To is a subclass of Option[Nothing] (To <: Option[Nothing]).
    Definition Classes
    ArrowAssoc
    Annotations
    @deprecated
    Deprecated

    (Since version 2.13.0) Use -> instead. If you still wish to display it as one character, consider using a font with programming ligatures such as Fira Code.

Inherited from java.io.Serializable

Inherited from (From) => To

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion UnliftOps from<:<[From, To] to UnliftOps[From, B]

Inherited by implicit conversion any2stringadd from<:<[From, To] to any2stringadd[<:<[From, To]]

Inherited by implicit conversion StringFormat from<:<[From, To] to StringFormat[<:<[From, To]]

Inherited by implicit conversion Ensuring from<:<[From, To] to Ensuring[<:<[From, To]]

Inherited by implicit conversion ArrowAssoc from<:<[From, To] to ArrowAssoc[<:<[From, To]]

Ungrouped