An instance ofA <:< B witnesses thatA is a subtype ofB. Requiring an implicit argument of the typeA <:< B encodes the generalized constraintA <: B.
To constrain any abstract typeT that's in scope in a method's argument list (not just the method's own type parameters) simply add an implicit argument of typeT <:< U, whereU is the required upper bound; or for lower-bounds, use:L <:< T, whereL is the required lower bound.
In case of any confusion over which method goes in what direction, all the "Co" methods (includingapply) 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 aFrom into aTo, andsubstituteContra replaces theTos in a type withFroms.
In part contributed by Jason Zaugg.
Type parameters
- From
a type which is proved a subtype of
To- To
a type which is proved a supertype of
From
Attributes
- See also
=:= for expressing equality constraints
- Example
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.}- Companion
- object
- Source
- typeConstraints.scala
- Graph
- Supertypes
- Known subtypes
Members list
Value members
Abstract methods
SubstituteTo forFrom andFrom forTo in the typeF[To, From], given thatF is contravariant in the first argument and covariant in the second.
SubstituteTo 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 ofsubstituteCo andsubstituteContra.
This method is impossible to implement withoutthrowing or otherwise "cheating" unlessFrom <: To, so it ensures that this really represents a subtyping relationship.
Attributes
- Returns
ftf, but with a (potentially) different type- Source
- typeConstraints.scala
Concrete methods
Composes two instances ofFunction1 in a newFunction1, with this function applied first.
Composes two instances ofFunction1 in a newFunction1, with this function applied first.
Type parameters
- A
the result type of function
g
Value parameters
- g
a function R => A
Attributes
- Returns
a new function
fsuch thatf(x) == g(apply(x))- Definition Classes
- Source
- typeConstraints.scala
IfFrom <: To andTo <: C, thenFrom <: C (subtyping is transitive)
IfFrom <: To andTo <: C, thenFrom <: C (subtyping is transitive)
Attributes
- Source
- typeConstraints.scala
Coerce aFrom into aTo.
Coerce aFrom into aTo. This is guaranteed to be the identity function.
This method is often called implicitly as an implicitA <:< B doubles as an implicit viewA => B.
Value parameters
- f
some value of type
From
Attributes
- Returns
f, but with a (potentially) different type- Definition Classes
- Source
- typeConstraints.scala
Composes two instances ofFunction1 in a newFunction1, with this function applied last.
Composes two instances ofFunction1 in a newFunction1, with this function applied last.
Type parameters
- A
the type to which function
gcan be applied
Value parameters
- g
a function A => T1
Attributes
- Returns
a new function
fsuch thatf(x) == apply(g(x))- Definition Classes
- Source
- typeConstraints.scala
IfFrom <: To andC <: From, thenC <: To (subtyping is transitive)
IfFrom <: To andC <: From, thenC <: To (subtyping is transitive)
Attributes
- Source
- typeConstraints.scala
Lift this evidence over a covariant type constructorF.
Lift this evidence over a contravariant type constructorF.
Substitute theFrom in the typeF[From], whereF is a covariant type constructor, forTo.
Substitute theFrom in the typeF[From], whereF is a covariant type constructor, forTo.
Equivalent in power to each ofsubstituteBoth andsubstituteContra.
This method is impossible to implement withoutthrowing or otherwise "cheating" unlessFrom <: To, so it ensures that this really represents a subtyping relationship.
Attributes
- Returns
ff, but with a (potentially) different type- Source
- typeConstraints.scala
Substitute theTo in the typeF[To], whereF is a contravariant type constructor, forFrom.
Substitute theTo in the typeF[To], whereF is a contravariant type constructor, forFrom.
Equivalent in power to each ofsubstituteBoth andsubstituteCo.
This method is impossible to implement withoutthrowing or otherwise "cheating" unlessFrom <: To, so it ensures that this really represents a subtyping relationship.
Attributes
- Returns
ft, but with a (potentially) different type- Source
- typeConstraints.scala
Inherited methods
Returns a string representation of the object.
Returns a string representation of the object.
The default representation is platform dependent.
Attributes
- Returns
a string representation of the object.
- Definition Classes
- Inherited from:
- Function1
- Source
- Function1.scala