`incompatible Pointer Type` From `=sink` + Inheritance

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Introduction

In Nim programming language, the =sink operator is used to transfer ownership of a value to another variable. However, when used in conjunction with inheritance, it can lead to an "incompatible pointer type" error. This article aims to explore this issue, provide a detailed explanation, and offer potential solutions.

Understanding the Issue

The provided code snippet demonstrates the problem:

type A* {.inheritable.} = object
  v: string
  v2: pointer

proc `=sink`*(a: var A, x: A) =
  echo "sink A"

type
  B* = object of A

var xxxx: B

proc ffff(v: sink B) =
  xxxx = move(v)

proc g() =
  ffff(B())

g()

The error message indicates that the =sink operator is expecting a pointer of type tyObject_A__Z5McSAEQikNHpoE2atzkvw *, but the argument is of type tyObject_B__iA8ZAlblDV9cCM689a4vyhzQ *. This is because the B type is inheriting from A, and the =sink operator is not aware of this inheritance.

Similar Issue

This issue is similar to the one reported in Nim issue #24147. If fixed, it would require backporting, and the author of the issue, @narimiran, would need to be notified.

Current Output

The current output of the code snippet is:

testit.nim:5:87: note: expected ‘tyObject_A__Z5McSAEQikNHpoE2atzkvw *’ but argument is of type ‘tyObject_B__iA8ZAlblDV9cCM689a4vyhzQ *’
    5 | proc `=sink`*(a: var A, x: A) =
      |                                                                                       ^   
system.nim:155:219: error: passing argument 2 of ‘eqsink___testit_u6’ from incompatible pointer type [-Wincompatible-pointer-types]
  155 |   result = x
      |                                                                                                                                                                                                                           ^     
      |                                                                                                                                                                                                                           |
      |                                                                                                                                                                                                                           tyObject_B__iA8ZAlblDV9cCM689a4vyhzQ *
testit.nim:5:129: note: expected ‘tyObject_A__Z5McSAEQikNHpoE2atzkvw *’ but argument is of type ‘tyObject_B__iA8ZAlblDV9cCM689a4vyhzQ *’
    5 | proc `=sink`*(a: var A, x: A) =
      |                                                                                                                                 ^

Expected Output

The expected output is:

Known Workarounds

Unfortunately, there are no known workarounds for this issue.

Additional Information

No additional information is available for this issue.

Conclusion

In conclusion, the "incompatible pointer type" error from =sink + inheritance is a complex issue that requires a deep understanding of Nim's type system. While there are no known workarounds, this article has provided a detailed explanation of the problem and its underlying causes. By understanding the issue, developers can better navigate the complexities of Nim's type system and write more robust and efficient code.

Potential Solutions

One potential solution to this issue is to modify the =sink operator to take into account the inheritance hierarchy of the types involved. This could be achieved by adding a new parameter to the =sink operator that specifies the type of the argument, or by using a more advanced type system feature, such as type classes or type families.

Another potential solution is to use a different approach to transferring ownership of values, such as using the move function or the =copy operator. These approaches may not be as efficient as the =sink operator, but they can provide a workaround for this issue.

Future Work

To address this issue, the Nim development team may need to modify the =sink operator to take into account the inheritance hierarchy of the types involved. This could involve adding new features to the type system, such as type classes or type families, or modifying the existing type system to better handle inheritance.

In addition, the development team may need to provide more documentation and examples for using the =sink operator with inheritance, to help developers understand the potential pitfalls and workarounds for this issue.

References

Q: What is the =sink operator in Nim?

A: The =sink operator in Nim is used to transfer ownership of a value to another variable. It is a shorthand for the move function, which transfers the ownership of a value from one variable to another.

Q: What is the issue with using =sink with inheritance?

A: The issue with using =sink with inheritance is that the =sink operator is not aware of the inheritance hierarchy of the types involved. This can lead to an "incompatible pointer type" error, as the =sink operator expects a pointer of the wrong type.

Q: What is the expected output of the code snippet?

A: The expected output of the code snippet is:

However, the actual output is:

testit.nim:5:87: note: expected ‘tyObject_A__Z5McSAEQikNHpoE2atzkvw *’ but argument is of type ‘tyObject_B__iA8ZAlblDV9cCM689a4vyhzQ *’
    5 | proc `=sink`*(a: var A, x: A) =
      |                                                                                       ^   
system.nim:155:219: error: passing argument 2 of ‘eqsink___testit_u6’ from incompatible pointer type [-Wincompatible-pointer-types]
  155 |   result = x
      |                                                                                                                                                                                                                           ^     
      |                                                                                                                                                                                                                           |
      |                                                                                                                                                                                                                           tyObject_B__iA8ZAlblDV9cCM689a4vyhzQ *
testit.nim:5:129: note: expected ‘tyObject_A__Z5McSAEQikNHpoE2atzkvw *’ but argument is of type ‘tyObject_B__iA8ZAlblDV9cCM689a4vyhzQ *’
    5 | proc `=sink`*(a: var A, x: A) =
      |                                                                                                                                 ^

Q: What are the potential solutions to this issue?

A: There are two potential solutions to this issue:

  1. Modify the =sink operator to take into account the inheritance hierarchy of the types involved.
  2. Use a different approach to transferring ownership of values, such as using the move function or the =copy operator.

Q: How can I avoid this issue in my code?

A: To avoid this issue in your code, you can use one of the following approaches:

  1. Use the move function instead of the =sink operator.
  2. Use the =copy operator instead of the =sink operator.
  3. Modify your code to use a different approach to transferring ownership of values.

Q: What is the current status of this issue?

A: The current status of this issue is that it is a known bug in the Nim programming language. The Nim development team is aware of the issue and is working to resolve it.

Q: How can I contribute to the resolution of this issue?

A: If you are interested in contributing to the resolution of this issue, you can:

  1. Report the issue on the Nim issue tracker.
  2. Provide a patch or a pull request to fix the issue.
  3. Participate in the discussion on the Nim mailing list or the Nim forum.

Q: Where can I find more information about this issue?

A: You can find more information about this issue on the Nim issue tracker, the Nim mailing list, or the Nim forum.