Pattern BindingsEdit this page on GitHub
In Scala 2, pattern bindings in
val definitions and
for expressions are
loosely typed. Potentially failing matches are still accepted at compile-time,
but may influence the program's runtime behavior.
From Scala 3.1 on, type checking rules will be tightened so that errors are reported at compile-time instead.
val xs: List[Any] = List(1, 2, 3) val (x: String) :: _ = xs // error: pattern's type String is more specialized // than the right hand side expression's type Any
This code gives a compile-time error in Scala 3.1 (and also in Scala 3.0 under the
-strict setting) whereas it will fail at runtime with a
ClassCastException in Scala 2. In Scala 3.1, a pattern binding is only allowed if the pattern is irrefutable, that is, if the right-hand side's type conforms to the pattern's type. For instance, the following is OK:
val pair = (1, true) val (x, y) = pair
Sometimes one wants to decompose data anyway, even though the pattern is refutable. For instance, if at some point one knows that a list
elems is non-empty one might
want to decompose it like this:
val first :: rest = elems // error
This works in Scala 2. In fact it is a typical use case for Scala 2's rules. But in Scala 3.1 it will give a type error. One can avoid the error by marking the pattern with an @unchecked annotation:
val first :: rest : @unchecked = elems // OK
This will make the compiler accept the pattern binding. It might give an error at runtime instead, if the underlying assumption that
elems can never be empty is wrong.
Analogous changes apply to patterns in
for expressions. For instance:
val elems: List[Any] = List((1, 2), "hello", (3, 4)) for ((x, y) <- elems) yield (y, x) // error: pattern's type (Any, Any) is more specialized // than the right hand side expression's type Any
This code gives a compile-time error in Scala 3.1 whereas in Scala 2 the list
is filtered to retain only the elements of tuple type that match the pattern
The filtering functionality can be obtained in Scala 3 by prefixing the pattern with
for (case (x, y) <- elems) yield (y, x) // returns List((2, 1), (4, 3))
There are two syntax changes relative to Scala 2: First, pattern definitions can carry ascriptions such as
: @unchecked. Second, generators in for expressions may be prefixed with
PatDef ::= ids [‘:’ Type] ‘=’ Expr | Pattern2 [‘:’ Type | Ascription] ‘=’ Expr Generator ::= [‘case’] Pattern1 ‘<-’ Expr
The new syntax is supported in Dotty and Scala 3.0. However, to enable smooth cross compilation between Scala 2 and Scala 3, the changed behavior and additional type checks are only enabled under the
-strict setting. They will be enabled by default in version 3.1 of the language.