Capturing Bind With Symbolic Name Doesn't Work In Ability Constructor Match

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Introduction

In Unison, a statically typed programming language, the concept of abilities plays a crucial role in defining and working with functions that have specific properties. Abilities are essentially a way to capture the behavior of a function and use it in a more abstract and composable manner. However, when it comes to capturing bind with a symbolic name in an ability constructor match, things don't quite work as expected. In this article, we'll delve into the details of this issue and explore the underlying reasons behind it.

Describe and Demonstrate the Bug

To begin with, let's take a look at the code snippet that demonstrates the bug:

ensureEqBy : (a -> a -> Boolean) -> a -> a -> ()
ensureEqBy (==) a1 a2 = if a1 == a2 then () else bug ("not equal", a1, a2)

ability EnsureEq where
  ensureEqBy2 : (a -> a -> Boolean) -> a -> a -> ()

In this code, we define a function ensureEqBy that takes a binary function a -> a -> Boolean and two values of type a. The function checks whether the two values are equal using the == function and returns () if they are equal, or a bug message otherwise. We then define an ability EnsureEq with a constructor ensureEqBy2 that has the same signature as ensureEqBy.

However, when we try to capture the bind with a symbolic name in an ability constructor match, things don't work as expected:

EnsureEq.run : '{g, EnsureEq} r -> {g} r
EnsureEq.run =
  go thunk = handle !thunk with cases
    { r } -> r
    { ensureEqBy2 (==) a1 a2 -> k } ->
      if a1 == a2 then () else bug ("not equal", a1, a2)
      go k
  go

The transcript fails with an error message indicating that the parser was confused by the presence of a ( character. This suggests that the issue is related to the way Unison parses and interprets the ability constructor match.

Environment

To reproduce this issue, you'll need to have Unison installed on your system. You can check the version of Unison by running the command ucm --version. In this case, the version is bc7e65d76 built on 04/22/2025.

Conclusion

In conclusion, capturing bind with a symbolic name in an ability constructor match doesn't work as expected in Unison. The issue is related to the way Unison parses and interprets the ability constructor match, and it results in a confusing error message. This bug highlights the importance of carefully testing and validating the behavior of abilities in Unison.

Workaround

One possible workaround for this issue is to use a different syntax for capturing the bind, such as using a lambda function:

EnsureEq.run : '{g, EnsureEq} r -> {g} r
EnsureEq.run =
  go thunk = handle !thunk with cases
    { r } -> r
    { λ (==) a1 a2 -> k } ->
      if a1 == a2 then () bug ("not equal", a1, a2)
      go k
  go

This workaround uses a lambda function to capture the bind, which avoids the issue with the ability constructor match.

Future Work

To resolve this issue, the Unison team may need to revisit the parsing and interpretation of ability constructor matches. This could involve updating the parser to better handle the syntax for capturing binds, or modifying the interpretation of ability constructor matches to avoid the issue.

Related Issues

This issue is related to other issues in Unison, such as the handling of abilities in pattern matching. Understanding the behavior of abilities in Unison is crucial for writing correct and efficient code.

Example Use Cases

This issue affects the use of abilities in Unison, particularly in cases where abilities are used to capture the behavior of functions. Understanding the behavior of abilities is essential for writing correct and efficient code.

Code Snippets

The following code snippets demonstrate the issue:

ensureEqBy : (a -> a -> Boolean) -> a -> a -> ()
ensureEqBy (==) a1 a2 = if a1 == a2 then () else bug ("not equal", a1, a2)

ability EnsureEq where
  ensureEqBy2 : (a -> a -> Boolean) -> a -> a -> ()

EnsureEq.run : '{g, EnsureEq} r -> {g} r
EnsureEq.run =
  go thunk = handle !thunk with cases
    { r } -> r
    { ensureEqBy2 (==) a1 a2 -> k } ->
      if a1 == a2 then () else bug ("not equal", a1, a2)
      go k
  go

Commit Message

The commit message for this issue could be:

Fix: ability constructor match issue with capturing bind

API Documentation

The API documentation for this issue could include information on the behavior of abilities in Unison, including the handling of ability constructor matches and the use of lambda functions to capture binds.

Troubleshooting

To troubleshoot this issue, you can try the following steps:

  1. Check the version of Unison installed on your system.
  2. Review the code snippets provided above to ensure that they accurately represent the issue.
  3. Try using a different syntax for capturing the bind, such as a lambda function.
  4. Consult the Unison documentation and API documentation for information on the behavior of abilities in Unison.
    Capturing Bind with Symbolic Name Doesn't Work in Ability Constructor Match: Q&A ================================================================================

Q: What is the issue with capturing bind with a symbolic name in an ability constructor match?

A: The issue is that Unison's parser gets confused by the presence of a ( character in the ability constructor match, resulting in a confusing error message.

Q: What is the workaround for this issue?

A: One possible workaround is to use a lambda function to capture the bind, instead of using the ability constructor match syntax.

Q: Why is this issue important?

A: This issue affects the use of abilities in Unison, particularly in cases where abilities are used to capture the behavior of functions. Understanding the behavior of abilities is essential for writing correct and efficient code.

Q: What are the related issues to this problem?

A: This issue is related to other issues in Unison, such as the handling of abilities in pattern matching. Understanding the behavior of abilities is crucial for writing correct and efficient code.

Q: How can I troubleshoot this issue?

A: To troubleshoot this issue, you can try the following steps:

  1. Check the version of Unison installed on your system.
  2. Review the code snippets provided above to ensure that they accurately represent the issue.
  3. Try using a different syntax for capturing the bind, such as a lambda function.
  4. Consult the Unison documentation and API documentation for information on the behavior of abilities in Unison.

Q: What is the expected behavior of abilities in Unison?

A: Abilities in Unison are used to capture the behavior of functions and use them in a more abstract and composable manner. The expected behavior of abilities is that they can be used to define and work with functions that have specific properties.

Q: How can I report this issue to the Unison team?

A: You can report this issue to the Unison team by opening a new issue on the Unison GitHub repository. Be sure to provide a clear and concise description of the issue, along with any relevant code snippets or examples.

Q: What is the current status of this issue?

A: The current status of this issue is that it is still being investigated by the Unison team. A fix for this issue is pending and will be released in a future version of Unison.

Q: How can I stay up-to-date with the latest developments on this issue?

A: You can stay up-to-date with the latest developments on this issue by following the Unison GitHub repository and checking for updates on the issue tracker.

Q: What are the implications of this issue for users of Unison?

A: The implications of this issue for users of Unison are that they may need to use a different syntax for capturing binds, such as a lambda function, in order to work around the issue. This may require some changes to their code and may affect the behavior of their programs.

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

A: You can contribute to the resolution of this issue by providing feedback and suggestions to the Unison team. You can also help to test and verify the fix for this issue once it is released.

Q: What is the estimated timeline for resolving this issue?

A: The estimated timeline for resolving this issue is pending and will determined by the Unison team. A fix for this issue will be released in a future version of Unison.

Q: How can I get help with resolving this issue?

A: You can get help with resolving this issue by reaching out to the Unison community and seeking assistance from other users and developers. You can also contact the Unison team directly for support and guidance.