当标量在Map [K，V]和Traversable [T]之间选择时，隐式是模棱两可的

Question

While extending MongoDB serialization library I stumbled upon issue with implicits.

In sample project referred below scalac fails to choose between implicits for Map[K,V] and Traversable[T] . The warning I get after enabling logging is

[info] ambiguous implicit values:
[info]  both method mapSetterStringKey in object BsonWritable of type [V](implicit vw: com.osinka.subset.BsonWritable[V])com.osinka.subset.BsonWritable[Map[String,V]]
[info]  and method seqSetter in object BsonWritable of type [T](implicit w: com.osinka.subset.BsonWritable[T])com.osinka.subset.BsonWritable[Traversable[T]]
[info]  match expected type com.osinka.subset.BsonWritable[scala.collection.immutable.Map[String,List[Double]]]
[info]     val bson = DBO(name -> testMap)()
[info]                         ^

If you uncomment marshallers in object BsonMarshallers it works. But it fails when both implicits are specified in com.osinka.subset.BsonWritable .

Could you please explain why scalac doesn't choose more specific implicit for Map ?

https://github.com/cppexpert/subset2-implicit-issue

Answer 1

If I understand your question, this bit from Programming in Scala, First Edition might explain it.

Non-Ambiguity Rule: An implicit conversion is only inserted if there is no other possible conversion to insert. If the compiler has two options to fix x + y, say using either convert1(x) + y or convert2(x) + y, then it will report an error and refuse to choose between them. It would be possible to define some kind of "best match" rule that prefers some conversions over others. However, such choices lead to really obscure code. Imagine the compiler chooses convert2, but you are new to the file and are only aware of convert1—you could spend a lot of time thinking a different conversion had been applied!

In cases like this, one option is to remove one of the imported implicits so that the ambiguity is removed. If you prefer convert2, then remove the import of convert1. Alternatively, you can write your desired conversion explicitly: convert2(x) + y.