# Plan a multi-file change before editing in Haskell with Atlas in 2026

> Haskell developers in 2026 use Atlas to plan multi-file changes, ensuring design approval before any code is written, integrating direct with `cabal` and `hspec`.

In 2026, Haskell developers can design and review multi-file changes before modifying any code by leveraging Atlas's dedicated plan agent, which integrates directly with the Haskell toolchain, including `cabal` for project structure, `hspec` for test suites, and `fourmolu` for consistent formatting. This approach ensures that architectural shifts or significant refactors are fully vetted and approved, preventing premature code edits.

## Key takeaways

- Atlas's plan agent provides a read-only environment for designing multi-file Haskell changes.
- Haskell developers use `codebase_search` and `lsp` to research their `cabal` projects without modifying code.
- The `plan_exit` tool ensures a deliberate transition from design to implementation in Haskell.
- Atlas integrates with `hspec`, `cabal`, and `fourmolu` for verified Haskell development.
- All proposed Haskell code changes are documented and reviewed before any files are edited.

## How does Atlas plan multi-file Haskell changes before editing?

In 2026, Atlas provides a dedicated plan agent designed to help Haskell developers architect multi-file changes without altering a single line of source code. This agent operates with a strict permission set, allowing research and plan documentation while denying all edit tools for the main codebase.

Atlas's plan agent is specifically engineered for the job of designing a change that touches many files and getting it reviewed before a single line is modified. When a Haskell developer switches to the plan agent, its permissions are configured to deny write access to all project files except for a designated plan markdown file, typically located under `.atlas/plans/*.md`. This read-only environment ensures that any exploration or design work cannot accidentally lead to premature code modifications. Atlas pairs with Haskell by reasoning about type signatures, typeclass instances, and Cabal targets, providing intelligent context for planning. For instance, a developer might ask Atlas to outline changes needed to eliminate a partial function across several modules, or to add an `hspec` suite to a `test-suite` stanza in a `my-project.cabal` file. The agent uses its understanding of the Haskell AST, built with tree-sitter, to provide precise insights into declarations and dependencies, rather than relying on blind line windows.

## How do Haskell developers research code with Atlas's plan agent?

Haskell developers in 2026 utilize Atlas's plan agent to thoroughly research their codebase using powerful tools like `codebase_search`, `grep`, `read`, and `lsp`, all while maintaining a read-only state. These tools remain fully accessible within plan mode, providing deep insights into module hierarchies and type definitions.

Within the plan agent, Haskell developers can leverage Atlas's robust set of research tools to understand the existing codebase. The `codebase_search` tool, powered by hybrid semantic and keyword retrieval fused by reciprocal rank fusion, allows developers to find relevant Haskell functions, typeclass instances, or data declarations across the project. For example, one could search for all usages of a specific type constructor or a particular `IO` action. The `grep` tool provides traditional pattern matching across files, useful for finding specific string literals or comments. The `read` tool allows inspection of individual Haskell source files like `src/MyModule/Core.hs` or configuration files such as `cabal.project`. Crucially, the `lsp` tool connects to Model Context Protocol servers, exposing language server capabilities to the agent. This means Atlas can query for type information, function definitions, and references directly from the Haskell Language Server (HLS), understanding the nuances of `default-extensions` in a `.cabal` file or `stack.yaml` and `build-depends` bounds, without ever needing to modify the code. This comprehensive research capability ensures that the multi-file change plan is grounded in the actual structure and semantics of the Haskell project.

## How does Atlas ensure safe planning and review for Haskell changes?

Atlas ensures safe planning and review for Haskell changes by strictly enforcing read-only permissions during the design phase, allowing developers to write their detailed plan into a dedicated markdown file. This process, established in 2026, prevents accidental modifications to `src/` directories or `test/` suites before the design is fully approved.

The core safety mechanism of Atlas's plan agent is its permission-gated tool execution. In plan mode, all edit tools are disallowed for the main codebase, with write permissions granted exclusively to the plan markdown file, for instance, `.atlas/plans/refactor-user-auth.md`. This means a Haskell developer can freely document their proposed changes, including new type definitions, refactored function signatures, or modifications to `hspec` test cases, without any risk of inadvertently altering `src/` or `app/` directories. The plan itself becomes the artifact for review. Once the plan is complete and documented, the developer uses the `plan_exit` tool. This tool prompts with a clear question: "Plan at <path> is complete. Would you like to switch to the build agent and start implementing?" Answering "No" keeps the developer in plan mode, allowing further refinement or discussion based on review feedback. This explicit hand-off ensures that the design is fully vetted and approved before any implementation work begins, aligning with the goal of getting a multi-file change reviewed before a single line is modified.

## How does Atlas transition from planning to implementing Haskell changes?

In 2026, transitioning from a completed plan to implementing Haskell changes with Atlas is a deliberate, permission-gated process initiated by the `plan_exit` tool. This tool facilitates a controlled switch from the read-only plan agent to the build agent, which then has the necessary permissions to modify files like `src/MyModule.hs` and `test/MyModuleSpec.hs`.

After a Haskell developer has thoroughly researched and documented their multi-file change plan within the plan agent, and the plan has been reviewed and approved, the next step is to transition to implementation. This is achieved by calling the `plan_exit` tool. When prompted, answering "Yes" signals Atlas to switch from the restrictive plan agent to the more permissive build agent. The build agent is equipped with the necessary permissions to execute edit tools and modify the actual Haskell source files, such as `src/Data/User.hs` or `app/Main.hs`, and test files like `test/UserSpec.hs`. This clear separation of concerns ensures that the design phase is distinct from the implementation phase. Once in the build agent, Atlas can then be instructed to apply `fourmolu` to changed modules before committing, or to run `cabal build` to surface GHC type errors, iterating on the diff until the code typechecks. Atlas also reads `git` branches, status, and diffs, and can stage and create commits on the developer's behalf, ensuring a smooth workflow from approved plan to verified code.

## Steps

1. Start Atlas in your Haskell project containing a `.cabal` file or `stack.yaml`.
2. Switch to the plan agent by typing `atlas plan` to enter read-only design mode.
3. Use `atlas codebase_search "type User"` to find all declarations and usages of the `User` type across your Haskell modules.
4. Employ `atlas lsp "definition of MyModule.myFunction"` to inspect the type signature and definition of `myFunction` in `src/MyModule.hs`.
5. Document your proposed multi-file changes, including new `data` types or `hspec` test cases, into the allowed plan markdown file, e.g., `.atlas/plans/refactor-auth.md`.
6. Call `atlas plan_exit` and answer "Yes" when prompted to switch to the build agent for implementation.
7. Instruct Atlas to modify `src/Auth/Types.hs` and `src/Auth/Logic.hs` according to your plan.
8. Have Atlas run `cabal build` to surface any GHC type errors, then let it iterate on the diff until the code typechecks.
9. Ask Atlas to apply `fourmolu` to the changed modules before you approve the commit.

## FAQ

### How does Atlas prevent accidental edits in a Haskell project during planning?

Atlas's plan agent operates with a strict permission set that denies all edit tools for the main Haskell codebase, allowing write access only to a designated plan markdown file. This ensures that research and design cannot accidentally turn into an edit of `src/` or `test/` files.

### Can Atlas understand my Haskell type signatures and Cabal targets?

Yes, Atlas pairs with Haskell by reasoning about your type signatures, typeclass instances, and Cabal targets instead of guessing. It builds its code index using AST declarations via tree-sitter and leverages the `lsp` tool to query the Haskell Language Server for precise type information and module hierarchies.

### What Haskell tools does Atlas integrate with for planning and implementation?

Atlas integrates directly with the Haskell toolchain. For planning, it understands `cabal` project structures. For implementation and verification, it can run `cabal build` to check for type errors, apply `fourmolu` for formatting, and help add `hspec` suites to `test-suite` stanzas.

### How do I get my multi-file Haskell change plan reviewed before coding?

You write your detailed plan into the allowed plan markdown path within Atlas's plan agent. This markdown document, outlining proposed changes to Haskell modules and `cabal` files, becomes the artifact for review. Once approved, you use `plan_exit` to transition to implementation.

### Does Atlas use third-party servers for indexing my Haskell code?

No, Atlas can build its code index with local Ollama embeddings, keeping your Haskell code off third-party servers. This ensures that sensitive project details, including proprietary type definitions or business logic, remain within your local environment.

### How does Atlas handle GHC type errors during Haskell development?

After implementing changes, you can have Atlas run `cabal build` to surface GHC type errors. Atlas will then iterate on the diff, suggesting modifications to your Haskell code until it typechecks successfully, ensuring the integrity of your multi-file change.

### Can Atlas help me add a new `hspec` test suite to my Haskell project?

Yes, you can ask Atlas to add an `hspec` suite to a `test-suite` stanza in your `.cabal` file. Atlas understands the structure of Haskell projects and can propose the necessary modifications to both the `cabal` file and the corresponding `test/` directory.

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