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backport of 1d3ec4e708154ec05554f540d7d68ed55dc12426
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This brings all the files into line with the .gitattributes
settings, which should henceforth be automatically maintained
by git.
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The new name for AbsTypeTag was a matter of a lengthy discussion:
http://groups.google.com/group/scala-internals/browse_thread/thread/fb2007e61b505c4d
I couldn't decide until having fixed SI-6323 today, which is about
trying to reflect against a local class using typeOf.
The problem with local classes is that they aren't pickled, so their metadata
isn't preserved between Scala compilation runs. Sure, we can restore some of
that metadata with Java reflection, but you get the idea.
Before today typeOf of a local class created a free type, a synthetic symbol,
with a bunch of synthetic children that remember the metadata, effectively
creating a mini symbol table. That might be useful at time, but the problem is
that this free type cannot be reflected, because the global symbol table of
Scala reflection doesn't know about its mini symbol table.
And then it struck me. It's not the presence of abs types (type parameters and
abs type members) that differentiates arbitrary type tags from good type tags.
It's the presence of types that don't map well on the runtime world - ones that
can't be used to instantiate values, ones that can't be reflected.
So we just need a name for these types. Phantom types are compile-time only
concept, whereas our types can have partial correspondence with the runtime.
"Weak types" sound more or less okish, so let's try them out.
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For the common case when someone hasn't quite grokked
the significance of lower case in a pattern match. I'd
like to make all the unreachables errors, not warnings,
but there may be a bug or two to clear out first.
class A {
def badEquals(x: Any, y: Any) = x match {
case y => true
case _ => false
}
}
a.scala:3: warning: patterns after a variable pattern cannot match (SLS 8.1.1)
If you intended to match against parameter y of method badEquals, you must use backticks, like: case `y` =>
case y => true
^
a.scala:4: warning: unreachable code due to variable pattern 'y' on line 3
case _ => false
^
two warnings found
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The original idea was to support both both TypeTags and ConcreteTypeTags as
context bounds on macro implementations.
Back then TypeTags were the implied default flavor of type tags. Basically
because "TypeTag" is shorter than "ConcreteTypeTag" everyone jumped onto
them and used them everywhere.
That led to problems, because at that time TypeTags could reify unresolved type
parameters ("unresolved" = not having TypeTag annotations for them). This
led to a series of creepy errors, when one forgets to add a context bound
in the middle of a chain of methods that all pass a type tag around, and then
suddenly all the tags turn into pumpkins (because that unlucky method just
reifies TypeRef(NoPrefix, <type parameter symbol>, Nil and passes it down
the chain).
Hence we decided to rename ConcreteTypeTag => TypeTag & TypeTag => AbsTypeTag,
which makes a lot of sense from a reflection point of view.
Unfortunately this broke macros (in a sense), because now everyone writes
TypeTag context bounds on macro implementations, which breaks in trivial
situations like: "def foo[T](x: T) = identity_macro(x)" (the type of x
is not concrete, so macro expansion will emit an error when trying to
materialize the corresponding TypeTag).
Now we restore the broken balance by banning TypeTag from macro impls.
This forces anyone to use AbsTypeTags, and if someone wants to check the input
for presence of abstract types, it's possible to do that manually.
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Implements SIP 16: Self-cleaning macros: http://bit.ly/wjjXTZ
Features:
* Macro defs
* Reification
* Type tags
* Manifests aliased to type tags
* Extended reflection API
* Several hundred tests
* 1111 changed files
Not yet implemented:
* Reification of refined types
* Expr.value splicing
* Named and default macro expansions
* Intricacies of interaction between macros and implicits
* Emission of debug information for macros (compliant with JSR-45)
Dedicated to Yuri Alekseyevich Gagarin
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