# Review a Pull Request in Haskell with Atlas in 2026

> Atlas helps Haskell developers review pull requests by integrating with cabal, hspec, and fourmolu to catch bugs a line-by-line read would miss.

In 2026, Atlas empowers Haskell developers to review pull requests with the same rigor as a human expert, but at machine speed, by deeply integrating with your existing toolchain. Atlas fetches the branch, reads full Haskell files, and uses lsp to check type signatures and typeclass instances, ensuring that changes don't break callers outside the immediate diff. It then runs your hspec test suites and cabal build to surface GHC type errors, even applying fourmolu for consistent formatting before you approve any commit, providing a comprehensive review that goes beyond line-by-line inspection.

## Key takeaways

- Atlas deeply understands Haskell code by parsing `.cabal` files and module hierarchies.
- `lsp findReferences` in Atlas prevents Haskell type signature changes from breaking callers.
- Atlas runs `cabal build` and `hspec` tests to catch GHC type errors and regressions.
- `fourmolu` integration ensures consistent Haskell code formatting automatically.
- Every Atlas action in Haskell projects is permission-gated for user control.
- Atlas uses full file context, not just diff hunks, for comprehensive Haskell reviews.

## How Atlas Reviews Haskell Pull Requests for Deeper Insights

Atlas revolutionizes Haskell pull request reviews in 2026 by moving beyond simple diffs, adopting a comprehensive strategy that mirrors a meticulous human reviewer. It first fetches the proposed changes, then immediately leaves the raw diff to examine the full context of your Haskell modules, ensuring a holistic understanding of the codebase.

Unlike traditional line-by-line diff tools, Atlas employs a 'leave the diff' approach to review Haskell pull requests. After using `bash` to fetch the feature branch and generate the initial patch, Atlas leverages its `read` tool to pull the complete contents of all changed Haskell files, such as `src/MyModule/Core.hs` or `app/Main.hs`. This allows Atlas to understand the surrounding module hierarchy, `default-extensions`, and `build-depends` bounds defined in your `.cabal` file or `stack.yaml`. By seeing the full file, Atlas can reason about the implications of a change on type signatures, typeclass instances, and overall program logic, catching subtle bugs that a limited diff view would inevitably miss. This deep contextual understanding is crucial for Haskell's strong type system, where a small change can have far-reaching implications across the codebase.

## Verifying Haskell Type Signatures and Callers with Atlas LSP

In 2026, Atlas uses its `lsp` tool to meticulously verify Haskell type signatures and their callers, a critical step for maintaining correctness in complex codebases. For every modified function signature, Atlas performs a `findReferences` operation, ensuring no downstream consumers are inadvertently broken by the change.

Haskell's robust type system is a powerful asset, and Atlas respects this by using the `lsp` tool to perform deep static analysis during pull request reviews. When a developer modifies a function signature in a file like `src/Data/MyType.hs`, Atlas doesn't just look at the changed lines. It specifically invokes `lsp findReferences` for that altered signature. This operation identifies every location in the project where the function is called, even if those call sites are in files not included in the current diff. This capability is vital for Haskell, where changes to a typeclass instance or a data constructor's type can ripple through many modules. Atlas can detect if a change to `myFunction :: Int -> String` to `myFunction :: Text -> String` has broken a caller in `test/Spec.hs` that still expects an `Int`, allowing it to flag potential GHC type errors before `cabal build` is even run.

## Automating Haskell Code Quality Checks with Atlas

Atlas significantly enhances Haskell code quality in 2026 by automating critical checks, ensuring every pull request adheres to project standards. It runs `cabal build` to catch GHC type errors, executes `hspec` test suites, and applies `fourmolu` formatting, all before a human reviewer even begins their work.

Maintaining high code quality in Haskell requires consistent application of tools, and Atlas automates this process during pull request reviews. After analyzing the code context, Atlas uses the `bash` tool to execute `cabal build` within the project. This step is paramount for Haskell, as it immediately surfaces any GHC type errors or warnings introduced by the changes, allowing Atlas to iterate on the diff until the code typechecks successfully. Following a successful build, Atlas runs the project's test suite using `cabal test` or directly invoking `hspec` if configured in the `test-suite` stanza of the `.cabal` file. Finally, to ensure stylistic consistency, Atlas applies `fourmolu` to all changed modules. This ensures that the code adheres to the project's formatting guidelines, eliminating bikeshedding during human review and allowing developers to focus on logic rather than whitespace. Atlas can even be asked to eliminate a partial function, demonstrating its ability to improve code robustness.

## Secure and Controlled Haskell Development with Atlas

Atlas provides a secure and controlled environment for Haskell development in 2026, ensuring that AI assistance never compromises code integrity. Every Atlas tool call, whether running `cabal` or modifying a `.hs` file, is permission-gated, requiring explicit user approval before execution, offering unparalleled transparency.

Security and control are paramount when integrating AI into a Haskell development workflow. Atlas is designed with robust safeguards, ensuring that developers retain full oversight. Every single tool call, from executing `cabal build` to applying `fourmolu` or modifying a Haskell source file, is permission-gated. This means Atlas will always ask for explicit `allow` or `deny` approval before running any command that could alter your codebase or execute external processes. Before making any changes, Atlas drafts a plan in a read-only plan agent, which you review. When it switches to a build agent, it computes a unified diff for every proposed file edit and surfaces it for your approval before writing. Furthermore, Atlas snapshots file changes as git patches, allowing edits to be easily diffed and rolled back if necessary. This level of transparency and control ensures that Atlas acts as a powerful assistant, not an autonomous agent, in your Haskell development process.

## Steps

1. Fetch the feature branch and generate the initial diff: Use `bash` to pull the remote branch and produce the raw patch, providing Atlas with the initial scope of changes in your Haskell project.
2. Read full Haskell files for complete context: Atlas uses its `read` tool to pull the entire contents of changed `.hs` files, such as `src/MyModule.hs`, rather than just diff hunks, to understand the full module hierarchy and `default-extensions`.
3. Verify Haskell type signatures with `lsp findReferences`: For every modified function signature in your Haskell code, Atlas invokes `lsp findReferences` to check all callers, ensuring no type errors are introduced outside the immediate diff.
4. Grep for stale patterns in Haskell code: Atlas uses `grep` to search for old constant names, stale copies, or feature flags that the change should have updated but might have missed across your Haskell codebase.
5. Run `cabal build` to surface GHC type errors: Atlas executes `bash -c "cabal build"` to compile the Haskell project, identifying and reporting any GHC type errors or warnings introduced by the pull request.
6. Execute `hspec` test suites: Atlas runs `bash -c "cabal test --test-suite=my-hspec-suite"` or `bash -c "hspec"` to validate the changes against your project's `hspec` tests, reporting findings as a `todowrite` list.
7. Apply `fourmolu` formatting to changed modules: Atlas uses `bash -c "fourmolu --mode=inplace src/ChangedModule.hs"` to automatically format the modified Haskell files, ensuring consistent code style before commit approval.

## FAQ

### How does Atlas handle Haskell type errors during a PR review?

Atlas proactively handles Haskell type errors by executing `cabal build` using its `bash` tool. This command compiles your project, surfacing any GHC type errors or warnings introduced by the pull request. Atlas can then iterate on the proposed changes, refining the diff until the code typechecks successfully, ensuring that only valid Haskell code is committed.

### Can Atlas apply `fourmolu` formatting to my Haskell code?

Yes, Atlas fully integrates with `fourmolu`, the standard Haskell formatter. During a pull request review, Atlas can apply `fourmolu` to all changed modules, such as `src/MyFeature.hs`, using a `bash` command like `fourmolu --mode=inplace`. This ensures that your code adheres to consistent style guidelines, reducing friction during human review and maintaining a clean, readable codebase.

### What Haskell project files does Atlas read for context?

Atlas reads crucial Haskell project files to build a comprehensive understanding of your codebase. This includes `.cabal` files or `stack.yaml` to understand dependencies, `default-extensions`, and `test-suite` stanzas. It also parses your module hierarchy and individual `.hs` source files, leveraging tree-sitter for AST declarations, to reason about type signatures and typeclass instances.

### How does Atlas run `hspec` tests for a Haskell pull request?

Atlas runs `hspec` tests by executing `cabal test` or directly invoking `hspec` via its `bash` tool. It identifies the relevant `test-suite` stanzas in your `.cabal` file, such as `test-suite my-hspec-suite`, and then runs the tests. Atlas reports the findings as a `todowrite` list, ordered by severity, allowing you to quickly see any regressions or failures introduced by the changes.

### Is Atlas safe to use with my Haskell codebase?

Yes, Atlas is designed with safety and control as core principles for Haskell development. Every Atlas tool call, including those involving `cabal`, `fourmolu`, or file modifications, is permission-gated. Atlas drafts a read-only plan, asks for approval before switching to a build agent, and presents a unified diff for every proposed edit before writing, ensuring you have full oversight and can roll back changes with git patches.

### How does Atlas find references for Haskell functions?

Atlas uses its `lsp` tool to find references for Haskell functions. When a function signature changes, Atlas invokes `lsp findReferences` to identify all call sites across your project, even those outside the immediate diff. This capability is essential for Haskell's strong type system, allowing Atlas to detect potential breaking changes to type signatures or typeclass instances before they cause GHC type errors.

### Can Atlas help eliminate partial functions in Haskell?

Yes, Atlas can assist in improving the robustness of your Haskell code by identifying and suggesting ways to eliminate partial functions. By understanding your module hierarchy and type signatures through its AST indexing, Atlas can reason about potential runtime errors from partial functions and propose safer alternatives, such as using `Maybe` or `Either` types, and then iterate on the diff to implement the changes.

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Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.