<:< (original) (raw)
scala.<:<
See the<:< companion object
sealed abstract class <:<[-From, +To] extends From => To, 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.
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
Source
Graph
Supertypes
Known subtypes
Members list
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.
Attributes
Returns
ftf, but with a (potentially) different type
Source
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.
Type parameters
A
the result type of function g
Value parameters
g
a function R => A
Attributes
Returns
a new function f such that f(x) == g(apply(x))
Definition Classes
Source
If From <: To and To <: C, then From <: C (subtyping is transitive)
If From <: To and To <: C, then From <: C (subtyping is transitive)
Attributes
Source
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.
Value parameters
f
some value of type From
Attributes
Returns
f, but with a (potentially) different type
Definition Classes
Source
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.
Type parameters
A
the type to which function g can be applied
Value parameters
g
a function A => T1
Attributes
Returns
a new function f such that f(x) == apply(g(x))
Definition Classes
Source
If From <: To and C <: From, then C <: To (subtyping is transitive)
If From <: To and C <: From, then C <: To (subtyping is transitive)
Attributes
Source
Lift this evidence over a covariant type constructor F.
Lift this evidence over a contravariant type constructor F.
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.
Attributes
Returns
ff, but with a (potentially) different type
Source
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.
Attributes
Returns
ft, but with a (potentially) different type
Source
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:
Source
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