I must still be missing the point, Sam. What would you expect the Guice
module to look like?
Is this your understanding of what the feature is:
interface Foo<T> {
...
}
interface FooGuiceFactory {
Foo<Object> create(Object o)
}
Currently, you could then write an instance of the interface you really
want:
class FooFactory {
@Inject private final FooGuiceFactory fooFactory;
<T> Foo<T> create(T t) {
return (Foo<T>)fooFactory.create(t);
}
}
That seems underwhelming compared to what I was imagining. It also seems
like it would be pretty trivial to implement -- If you think people would
use it I would be interested in giving it a shot.
On Wed, May 27, 2015 at 7:58 AM Sam Berlin <[email protected]> wrote:
> 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 <[email protected]>
> 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 <[email protected]> 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 <
>>> [email protected]> 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 <[email protected]>
>>>> 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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