Context parameters can be declared by-name to avoid a divergent inferred expansion. Example:
trait Codec[T]:
def write(x: T): Unit
given intCodec: Codec[Int] = ???
given optionCodec: [T] => (ev: => Codec[T]) => Codec[Option[T]]:
def write(xo: Option[T]) = xo match
case Some(x) => ev.write(x)
case None =>
val s = summon[Codec[Option[Int]]]
s.write(Some(33))
s.write(None)
As is the case for a normal by-name parameter, the argument for the context parameter ev is evaluated on demand. In the example above, if the option value x is None, it is not evaluated at all.
The synthesized argument for a context parameter is backed by a local val if this is necessary to prevent an otherwise diverging expansion.
The precise steps for synthesizing an argument for a by-name context parameter of type => T are as follows.
Create a new given of type T:
where lv is an arbitrary fresh name.
This given is not immediately available as candidate for argument inference (making it immediately available could result in a loop in the synthesized computation). But it becomes available in all nested contexts that look again for an argument to a by-name context parameter.
If this search succeeds with expression E, and E contains references to lv, replace E by
Otherwise, return E unchanged.
In the example above, the definition of s would be expanded as follows.
val s = summon[Test.Codec[Option[Int]]](
using optionCodec[Int](using intCodec)
)
No local given instance was generated because the synthesized argument is not recursive.
ReferenceFor more information, see Issue #1998 and the associated Scala SIP.
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