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An extractor is no longer required to return Option[T], and
can instead return anything which directly contains methods
with these signatures:
def isEmpty: Boolean
def get: T
If the type of get contains methods with the names of
product selectors (_1, _2, etc.) then the type and arity
of the extraction is inferred from the type of get. If
it does not contain _1, then it is a single value
extractor analogous like Option[T].
This has significant benefits and opens new territory:
- an AnyVal based Option-like class can be used which
leverages null as None, and no allocations are necessary
- for primitive types the benefit is squared (see below)
- the performance difference between case classes and
extractors should now be largely eliminated
- this in turn allows us to recapture great swaths of
memory which are currently squandered (e.g. every
TypeRef has fields for pre and args, even though these
are more than half the time NoPrefix and Nil)
Here is a primitive example:
final class OptInt(val x: Int) extends AnyVal {
def get: Int = x
def isEmpty = x == Int.MinValue // or whatever is appropriate
}
// This boxes TWICE: Int => Integer => Some(Integer)
def unapply(x: Int): Option[Int]
// This boxes NONCE
def unapply(x: Int): OptInt
As a multi-value example, after I contribute some methods to TypeRef:
def isEmpty = false
def get = this
def _1 = pre
def _2 = sym
def _3 = args
Then it's extractor becomes
def unapply(x: TypeRef) = x
Which, it need hardly be said, involves no allocations.
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A Typed Pattern (_: T) is more than `.isInstanceOf`: if `T` is a
path dependent type, the scrutinee's $outer reference is also
compared against the prefix of `T`.
The code that synthesises this is split into two places.
`needsOuterCheck` determines whether to add this check, based on
the type `T`, and the type of the scrutinee. If it gives the
go-ahead, `treeCondStrategy.outerCheck` synthesizes the check.
The new test case demonstrates the problems caused by the failure
to dealias in `needsOuterCheck`: it could either wrongly lead to
synthesis of an outer test (which would crash), or wrongly omit
the outer test (meaning overly liberal matching.)
A simple `dealias` remedies this. `dealiasWiden` is *not*
appropriate here; we need to keep hold of singleton types.
I'll also note that there is already a little slack between these
methods, as commented:
> ExplicitOuter replaces `Select(q, outerSym) OBJ_EQ expectedPrefix`
> by `Select(q, > outerAccessor(outerSym.owner)) OBJ_EQ expectedPrefix`
> if there's an outer accessor, otherwise the condition becomes `true`
> TODO: can we improve needsOuterTest so there's always an outerAccessor?
So this is probably a fragile area that warrants a careful review
with a view to design improvements.
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