How does Atlas handle complex GHC type errors during a Haskell dependency upgrade?

Atlas uses its `bash` tool to run `cabal build`, capturing GHC's detailed type error messages. It then iteratively applies fixes using its `edit` tool, leveraging `lsp`'s `goToDefinition` to inspect the new type signatures and definitions in the upgraded Haskell library, ensuring precise corrections.

Can Atlas upgrade dependencies in a `stack.yaml` project instead of just `.cabal`?

While the primary example focuses on `.cabal`, Atlas's `bash` tool can execute any command. If your Haskell project uses `stack.yaml`, Atlas can run `stack update` and `stack upgrade` commands, then proceed with fixing compile and test failures in the same iterative manner.

How does Atlas ensure my Haskell code style remains consistent after an upgrade?

After resolving compile and test failures, Atlas uses its `bash` tool to run `fourmolu` on all modified Haskell modules. This automatically formats the code according to your project's configuration, ensuring that all changes adhere to your established style guidelines before committing.

What if the upgraded Haskell library has no changelog available via `webfetch`?

If `webfetch` cannot retrieve a changelog, Atlas will rely more heavily on the compiler output from `cabal build` and the `lsp` tool's `goToDefinition` to infer breaking changes from new type signatures and definitions. It will still iteratively fix errors based on GHC's feedback.

How does Atlas integrate with my existing `git` workflow for Haskell projects?

Atlas reads your `git` branches and status, computes unified diffs for all proposed changes, and surfaces them for your approval. Once approved, Atlas can stage the changes and create commits on your behalf, integrating direct with your standard `git` workflow for Haskell development.

Does Atlas understand Haskell-specific constructs like typeclasses or language extensions?

Yes, Atlas is paired with Haskell, allowing it to reason about your type signatures, typeclass instances, and language extensions declared in `default-extensions` or individual module pragmas. It builds its code index using AST declarations via tree-sitter, providing a deep understanding of Haskell's structure.

How does Atlas handle test failures reported by `hspec` after a dependency upgrade?

After a clean `cabal build`, Atlas runs `cabal test` to execute your `hspec` suite. If tests fail, Atlas parses the `hspec` output to identify the failing tests. It then uses its `edit` and `lsp` tools to diagnose and fix the underlying issues in your Haskell code, iterating until all tests pass.

Upgrade a Haskell Dependency and Fix Breakage with Atlas in 2026

Atlas empowers Haskell developers in 2026 to upgrade major library versions and automatically fix resulting compile and test failures by directly interacting with `cabal`, `hspec`, and `fourmolu`. It reads GHC type errors, fetches changelogs, and iteratively applies fixes to your `.cabal` project until all checks pass.

Initiating a Haskell Dependency Upgrade with Atlas

To begin a Haskell dependency upgrade in 2026, Atlas uses `bash` to run `cabal update` and `cabal install` commands, capturing the full output. This initial step ensures your local package index is current and attempts the upgrade, saving any extensive output to a file for later `read` access if it exceeds typical terminal limits.

Atlas integrates directly with your Haskell project's `cabal` package manager. When you instruct Atlas to upgrade a dependency, it first uses its `bash` tool to execute `cabal update` to refresh the package index. Following this, it attempts the actual dependency bump, for example, `cabal install <package-name> --lib --upgrade-dependencies`. Atlas captures all output from these `bash` commands. If the output is particularly verbose, exceeding standard terminal buffer limits, Atlas automatically saves it to a temporary file. This file can then be accessed by Atlas's `read` tool, ensuring that no critical information about the upgrade process or initial breakage is lost, providing a complete context for subsequent repair steps. This approach ensures Atlas works with the real-world behavior of `cabal` rather than making assumptions.

Understanding Breaking Changes with `webfetch`

Before attempting fixes, Atlas leverages its `webfetch` tool to retrieve the library's release notes or changelog, providing crucial context for breaking changes. This ensures that any modifications Atlas proposes to your Haskell codebase, such as adjusting type signatures or function calls, align with the 1:1 documented API changes rather than speculative guesses.

A major version bump in a Haskell library often introduces significant breaking changes, requiring more than just simple syntax adjustments. Atlas understands this and prioritizes context. After initiating the upgrade, Atlas uses its `webfetch` tool to pull the official release notes or changelog for the specific library version being upgraded. This step is vital for Haskell, where type system changes or new typeclass instances can drastically alter how functions are used. By having the actual breaking changes documented, Atlas can reason more effectively about necessary code modifications. For instance, if a function's type signature has changed from `A -> B` to `A -> C -> B`, Atlas will know to look for call sites and introduce the new `C` argument, rather than guessing based solely on compiler errors. This proactive information gathering significantly improves the accuracy and efficiency of the repair process.

Iterative Repair of Haskell Compile Errors

Atlas systematically fixes Haskell compile errors by running `cabal build` with its `bash` tool and then iteratively addressing each GHC type error. It uses the `edit` tool to modify source files and the `lsp` tool's `goToDefinition` to inspect new type signatures in the upgraded package, ensuring precise fixes across your 2026 codebase.

The core of fixing breakage in Haskell lies in resolving GHC's detailed type errors. Atlas uses its `bash` tool to repeatedly execute `cabal build`. Each time `cabal build` fails, Atlas parses the compiler output to identify the specific error messages, file paths, and line numbers. For each error, Atlas employs its `edit` tool to propose and apply changes to the relevant Haskell source files. To ensure these edits are correct, especially when dealing with new major versions, Atlas utilizes the `lsp` tool. By invoking `lsp`'s `goToDefinition` operation, Atlas can jump directly to the definition of a function or type within the newly upgraded library. This allows it to inspect the actual new type signatures, record fields, or typeclass instances, ensuring that its proposed fixes,such as adjusting function arguments, updating record syntax, or adding necessary imports,are perfectly aligned with the upgraded library's API. This iterative process continues, building and fixing, until `cabal build` reports a clean compilation.

Validating Fixes with `hspec` and `fourmolu`

After resolving all compile errors, Atlas validates the functional correctness of the upgraded Haskell codebase by running `cabal test` with `hspec` via its `bash` tool. It then applies `fourmolu` to all changed modules to ensure consistent formatting, completing the 100% automated repair cycle before presenting the final diff for approval.

A successful compilation is only half the battle; ensuring the application still behaves correctly is paramount. Once `cabal build` passes, Atlas proceeds to run your test suite. It uses its `bash` tool to execute `cabal test`, which in a typical Haskell project, will invoke `hspec` for unit and integration tests. Atlas monitors the output of `hspec` for any failures. If tests fail, Atlas will re-enter its iterative fix cycle, using `edit` and `lsp` to diagnose and repair issues that manifest at runtime, such as changes in behavior or unexpected data structures. Finally, before presenting the changes for review, Atlas ensures code quality and consistency. It uses `bash` to run `fourmolu` on all modified Haskell modules. This step automatically formats the code according to your project's standards, ensuring that the generated diff is clean, readable, and adheres to established style guides, making the final review process straightforward.

Reviewing and Committing Changes with Atlas

Atlas provides a comprehensive review process for all Haskell dependency upgrades, presenting a unified diff for every file edit before writing. It snapshots file changes as git patches, allowing you to easily diff and roll back edits, and can stage and create commits on your behalf, streamlining the entire 2026 workflow.

Transparency and control are central to Atlas's workflow. Throughout the entire dependency upgrade process, from initial `cabal` commands to final `fourmolu` formatting, Atlas maintains a detailed record of all proposed changes. Before any modifications are written to your file system, Atlas computes a unified diff for every file edit and surfaces it for your approval. This allows Haskell developers to meticulously review every line change, ensuring that the AI-generated fixes align with their understanding of the codebase and the upgraded library. Atlas also snapshots file changes as git patches, providing a robust mechanism to diff and roll back edits if necessary. Once satisfied, Atlas can stage the changes and create a commit message on your behalf, integrating direct with your existing `git` workflow. This ensures that you, the developer, always have the final say and complete oversight over your Haskell project's evolution.

Step by step

01Initiate the upgrade: Ask Atlas to upgrade a specific Haskell dependency using `cabal` via the `bash` tool, capturing the full output.
02Fetch release notes: Instruct Atlas to use `webfetch` to retrieve the changelog or release notes for the new major version of the Haskell library.
03Resolve compile errors: Have Atlas run `cabal build` with `bash` to surface GHC type errors, then iteratively fix each error using `edit` and `lsp`'s `goToDefinition` to inspect new type signatures.
04Run `hspec` tests: Once compilation is clean, ask Atlas to execute `cabal test` with `bash` to run your `hspec` test suite and fix any new failures.
05Apply `fourmolu` formatting: Before committing, instruct Atlas to apply `fourmolu` to all changed Haskell modules using `bash` to ensure consistent code style.
06Review and commit: Review the unified diff presented by Atlas, approve the changes, and let Atlas stage and commit the updates to your `git` repository.

Frequently asked questions

How does Atlas handle complex GHC type errors during a Haskell dependency upgrade?: Atlas uses its `bash` tool to run `cabal build`, capturing GHC's detailed type error messages. It then iteratively applies fixes using its `edit` tool, leveraging `lsp`'s `goToDefinition` to inspect the new type signatures and definitions in the upgraded Haskell library, ensuring precise corrections.
Can Atlas upgrade dependencies in a `stack.yaml` project instead of just `.cabal`?: While the primary example focuses on `.cabal`, Atlas's `bash` tool can execute any command. If your Haskell project uses `stack.yaml`, Atlas can run `stack update` and `stack upgrade` commands, then proceed with fixing compile and test failures in the same iterative manner.
How does Atlas ensure my Haskell code style remains consistent after an upgrade?: After resolving compile and test failures, Atlas uses its `bash` tool to run `fourmolu` on all modified Haskell modules. This automatically formats the code according to your project's configuration, ensuring that all changes adhere to your established style guidelines before committing.
What if the upgraded Haskell library has no changelog available via `webfetch`?: If `webfetch` cannot retrieve a changelog, Atlas will rely more heavily on the compiler output from `cabal build` and the `lsp` tool's `goToDefinition` to infer breaking changes from new type signatures and definitions. It will still iteratively fix errors based on GHC's feedback.
How does Atlas integrate with my existing `git` workflow for Haskell projects?: Atlas reads your `git` branches and status, computes unified diffs for all proposed changes, and surfaces them for your approval. Once approved, Atlas can stage the changes and create commits on your behalf, integrating direct with your standard `git` workflow for Haskell development.
Does Atlas understand Haskell-specific constructs like typeclasses or language extensions?: Yes, Atlas is paired with Haskell, allowing it to reason about your type signatures, typeclass instances, and language extensions declared in `default-extensions` or individual module pragmas. It builds its code index using AST declarations via tree-sitter, providing a deep understanding of Haskell's structure.
How does Atlas handle test failures reported by `hspec` after a dependency upgrade?: After a clean `cabal build`, Atlas runs `cabal test` to execute your `hspec` suite. If tests fail, Atlas parses the `hspec` output to identify the failing tests. It then uses its `edit` and `lsp` tools to diagnose and fix the underlying issues in your Haskell code, iterating until all tests pass.

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