trait Checkable extends AnyRef
On pattern matcher checkability:
The spec says that case _: List[Int] should be always issue an unchecked warning:
> Types which are not of one of the forms described above are > also accepted as type patterns. However, such type patterns > will be translated to their erasure (§3.7). The Scala compiler > will issue an “unchecked” warning for these patterns to flag > the possible loss of type-safety.
But the implementation goes a little further to omit warnings based on the static type of the scrutinee. As a trivial example:
def foo(s: Seq[Int]) = s match { case _: List[Int] => }
need not issue this warning.
Consider a pattern match of this form: (x: X) match { case _: P => }
There are four possibilities to consider: [P1] X will always conform to P [P2] x will never conform to P [P3] X will conform to P if some runtime test is true [P4] X cannot be checked against P
The first two cases correspond to those when there is enough static information to say X <: P or that (x ∈ X) ⇒ (x ∉ P). The fourth case includes unknown abstract types or structural refinements appearing within a pattern.
The third case is the interesting one. We designate another type, XR, which is essentially the intersection of X and |P|, where |P| is the erasure of P. If XR <: P, then no warning is emitted.
We evaluate "X with conform to P" by checking X <: P_wild, where
P_wild is the result of substituting wildcard types in place of
pattern type variables. This is intentionally stricter than
(X matchesPattern P), see scala/bug#8597 for motivating test cases.
Examples of how this info is put to use: sealed trait A[T] ; class B[T] extends A[T] def f(x: B[Int]) = x match { case _: A[Int] if true => } def g(x: A[Int]) = x match { case _: B[Int] => }
f requires no warning because X=B[Int], P=A[Int], and B[Int] <:< A[Int].
g requires no warning because X=A[Int], P=B[Int], XR=B[Int], and B[Int] <:< B[Int].
XR=B[Int] because a value of type A[Int] which is tested to be a B can
only be a B[Int], due to the definition of B (B[T] extends A[T].)
This is something like asSeenFrom, only rather than asking what a type looks like from the point of view of one of its base classes, we ask what it looks like from the point of view of one of its subclasses.
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- trait InferCheckable extends AnyRef
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def
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The applied type of class 'to' after inferring anything possible from the knowledge that 'to' must also be of the type given in 'from'.
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def
formatted(fmtstr: String): String
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- This member is added by an implicit conversion from Checkable to StringFormat[Checkable] performed by method StringFormat in scala.Predef.
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(Since version 2.12.16) Use
formatString.format(value)instead ofvalue.formatted(formatString), or use thef""string interpolator. In Java 15 and later,formattedresolves to the new method in String which has reversed parameters.
The Scala compiler and reflection APIs.