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Questions: Implicit search

{% include JB/setup %}

Implicits01

  • While searching for the argument for implicit parameter tc, are each of the following implicits firstImplicit, secondImplicit, thirdImplicit, fourthImplicit, fifthImplicit and sixthImplicit in the implicit search scope?
  • Which implicit has been applied as an implicit argument for tc?
  • Is there more than one implicit that the compiler verified to be correct for that position? If so, why was the one you mentioned above selected and there was no ambiguity?

Implicits02

In this example we would like to use implicits to safely do matrix-matrix, matrix-vector etc. products. The test provides multiple combinations (not all of them making sense).

Your task is to say which of the implicits in MultiplicationComp are verified by the compiler to be correct as arguments for implicit parameter c in multiplyMe application. If there is more than one applicable, specify how does the typechecker select the most specific one or why it is not able to (on failure).

Questions:

  • Why cannot it resolve ambiguity in 2) but succeeded in 1)?
  • Name all the ambiguous implicits in 2)
  • Why cannot it resolve ambiguity in 4) but succeeded in 3)?
  • Name all the ambiguous implicits in 4)
  • Why cannot it find the implicit for 6) but succeeded in 5?

Implicits03

test01

  • What type is inferred for T in the first universalComp application?
  • Is aOrdering in scope? If yes, why is it not picked by the implicit search?
  • There will be other implicits that potentially apply as implicit arguments given their generic type but compiler will (correctly) fail verify them. Name them and explain why are they rejected (hint: this question does not ask about implicit imported from scala.Predef).
  • How would you fix the type error?

test02

  • How many implicits are verified by the compiler to be compatible with the search type for the implicit parameter?
  • If there is more than one, name them and explain why there is no ambiguity.
  • What is the implicit argument that the compiler applied to the application?

Implicits04

Tests that follow experiment with chaining the implicits.

test01

  • Is there a direct implicit view of type Int => SumTuple3 (in order for the selection 1.sum to succeed)?

  • During the initial search for an implicit view, to satisfy member selection, typechecker will verify two potential possibilities. Name them.

  • What implicits (and in what sequence) are used in order to satisfy member selection 1.sum?

  • Which implicit(s) are being verified by the compiler during the first implicit conversion of(2, 5)?

  • Which implicits (and in what sequence) are used in order to satisfy the typechecking of (2, 5).sum?

  • Why does the compiler complain about missing view (Int, Int, Int) => (Int, Int) in (1, 2, 3).sum?

Implicits05

Implicit search can often return an error message related to diverging implicits. It is relatively easy to construct this problem yourself (as we will show) but it can also appear while using normal Scala collections.

test01

Consider application of doWork:

  • Which implicits arguments are considered as valid possibilities and will be verified as initial arguments for process parameter?
  • Which parameter of the transitive implicit diverges?
  • Are implicits pre2Main and main2Final are considered as valid possibilities and verified as arguments of transitive?
  • How would the diverging expansion in this particular case look like?

test02

The signature of the sorted method is def sorted[B >: A](implicit ord: math.Ordering[B]): List[A].

  • Which implicit argument is applied as an argument to the sorted method of listOfFoo?

  • There is more than one implicit possible as an argument to the sorted method? Why does the compiler select the one you stated in your previous answer?

  • Which implicit arguments are taken into account when searching for implicit argument of the sorted method of listOfBar?

  • Can the typechecker use the same implicit argument as in the case of listOfFoo? Why?

  • What is the sequence of implicits that causes the divergence?

  • Give a possible fix for the underlying problem (so that you can call sorted method on listOfBar)?

test03

Consider a SortedSet trait available from the standard library: http://www.scala-lang.org/archives/downloads/distrib/files/nightly/docs/library/index.html#scala.collection.SortedSet

  • What is the type of set1?

  • Does the application ++ present in set1 involve finding implicits at any point of typechecking the body of the value?

  • What is the type of the member selection SortedSet.empty only (in set2)?

  • Give an example of a sequence of implicit arguments which would lead to the divergence?

Implicits06 (source at implicits/Implicits2.scala and implicits/package.scala)

Notice that you are required to compile the package object file as well (it contains some implicits).

Implicits06

Consider implicits packageObjectFAction (in implicits/package.scala), basicImpl, generalImpl, otherGeneralImpl, and someImpl.

  • For each of the implicits say whether they are applicable or not, as implicit arguments for the application of foo(1.0) context. Give a one sentence explanation in each case.

  • Which implicit argument is applied and why?

  • For each of the implicits say whether they are applicable or not, as implicit arguments for the application of identity(foo(1)) context. Give a one sentence explanation in each case.

  • Which implicit argument is applied and why?

  • For each of the implicits say whether they are applicable or not, as implicit arguments for the application of bar(foo(1)) context. Give a one sentence explanation in each case.

  • Which implicit argument is applied and why?