If <T> is only used as an assisted param (and *not* used by any injected
ones), then I think we can get away with just ignoring it. The compiler
will make sure the types are safe, and we can just pretend T is its erased
type. So long as the user supplies us the T and we don't have to worry
about locating anything that references a T... there won't be any unknowns.
sam
On Wed, May 27, 2015, 1:08 AM Nate Bauernfeind <nate.bauernfe...@gmail.com>
wrote:
> Sam, I don't see how it could ever work if T itself had generic
> parameters. I can only see two solutions 1) push the burden onto the client
> of the factory and force them to provide a type literal or 2) the generic
> factory methods only work for a subset of types. In my opinion option 2 is
> useless: you should be able to bind Foo[List[String]] to a different
> implementation than a Foo[List[Int]] for example.
>
> The current solution is to add the type parameter to the factory, which
> requires a one-to-one binding per factory. I thought that people don't like
> this option because they'd like to remove the one-to-one binding
> restriction, or perhaps they don't want to inject `n` different Factories
> someplace. (There might be other advantages I'm being shortsighted about;
> these are just the two that pop out as large costs.)
>
> Removing these particular restrictions is clearly a one-to-many binding
> argument. In fact, a non one-to-many implementation is relatively
> straightforward! Here's an example in scala (for terseness):
>
> object T extends App {
> class Foo[T] (@Assisted t: T) { override def toString = s"Foo($t)" }
> class FooString @Inject()(@Assisted t: String) extends Foo[String](t)
> class FooInt @Inject()(@Assisted t: Integer) extends Foo[Integer](t)
>
> trait F {
> def create(t: String): Foo[String]
> def create(i: Int): Foo[Integer]
> }
>
> val mod = new ScalaModule {
> def configure(): Unit = {
> install(new FactoryModuleBuilder()
> .implement(typeLiteral[Foo[String]], typeLiteral[FooString])
> .implement(typeLiteral[Foo[Integer]], typeLiteral[FooInt])
> .build(typeLiteral[F]))
> }
> }
>
> import net.codingwell.scalaguice.InjectorExtensions._
> val injector = Guice.createInjector(mod)
> println(injector.instance[F].create(42))
> println(injector.instance[F].create("hello world"))
> }
>
> There are a couple of issues that you can't really get around. 1) You need
> to subclass Foo or else the constructor loses the parameter's type
> information due to erasure. 2) you need to enumerate each create method in
> the factory. 3) you need one implement call per T in your factory. And
> subtly, 4) the method `def create[T](t: T): Foo[T]` doesn't exist!
>
> This is three times as much work as would be nice, but due to JVM
> restrictions I don't think you can remove the additional work without
> requiring TypeReferences on the create method.
>
> Does this shed light from my perspective, or am I misunderstanding the
> problem?
>
> On Tue, May 26, 2015 at 6:16 PM Sam Berlin <sber...@gmail.com> wrote:
>
>> The "many to one binding" thing isn't really the reason since it's not a
>> binding, it's just a method call in your factory.
>>
>> For assistedinject, the lack of being able to specify <T> on the method
>> w/o also specifying it on the factory is really just a no one's had the
>> time to sit down and figure out how to do it. See the bug @
>> https://github.com/google/guice/issues/218 .
>>
>> Implementation's @
>> https://github.com/google/guice/blob/master/extensions/assistedinject/src/com/google/inject/assistedinject/FactoryProvider2.java
>> if you want to give it a go.
>>
>> sam
>>
>> On Tue, May 26, 2015 at 6:08 PM Nate Bauernfeind <
>> nate.bauernfe...@gmail.com> wrote:
>>
>>> Assuming you meant:
>>> interface FooFactory {
>>> <T> Foo<T> create(T bar);
>>> }
>>>
>>> No, this does not work. It would require one-to-many bindings, which, if
>>> you read the recent forum's history, is not supported.
>>>
>>> This is the closest you can get. I use it pretty frequently:
>>> interface FooFactory<T> {
>>> Foo<T> create(T bar);
>>> }
>>>
>>> You will need to use type literals to capture the generic types when you
>>> create the assisted inject factories. (See:
>>> http://google.github.io/guice/api-docs/latest/javadoc/index.html?com/google/inject/TypeLiteral.html
>>> )
>>>
>>> Nate
>>>
>>> On Tue, May 26, 2015 at 5:52 PM Brad Micholson <fourofcl...@gmail.com>
>>> wrote:
>>>
>>>> Is there currently any way to use type inference in an assisted inject
>>>> factory?
>>>> For example, is there any way to use an interface like the following as
>>>> an assisted inject factory?
>>>>
>>>>
>>>> interface FooFactory
>>>> {
>>>> Foo<T> create(T bar);
>>>> }
>>>>
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