How does Atlas understand my Haskell project's dependencies and structure?

Atlas reads your `.cabal` file, including `build-depends` bounds and `default-extensions`, to build its understanding of your Haskell module hierarchy and language extensions. It indexes code by AST declarations using tree-sitter, not blind line windows.

Can Atlas help me understand complex Haskell type signatures and typeclass instances?

Yes, Atlas indexes code by AST declarations, allowing it to reason about your Haskell type signatures and typeclass instances. The `lsp` tool's `goToDefinition` operation helps you work through these complexities directly within the codebase.

What if I want to explore a Haskell codebase without making any changes?

Atlas's `explore` subagent is permissioned read-only by default. It allows you to `grep`, `glob`, `read`, `bash`, `webfetch`, and `websearch` across your Haskell project without modifying any files, ensuring safe investigation.

How does Atlas ensure code quality and formatting in Haskell projects?

Atlas can apply `fourmolu` to changed Haskell modules before you approve a commit, ensuring consistent formatting. It also surfaces a unified diff for every file edit, which you must approve before any changes are written to your Haskell project.

Can Atlas run my Haskell tests or help fix compilation errors?

Yes, Atlas can be instructed to run your `hspec` test suites to verify changes. It can also run `cabal build` to surface GHC type errors and then iterate on the diff until your Haskell code typechecks correctly.

Is my Haskell code sent to third-party servers for indexing or processing?

No, Atlas can build its code index with local Ollama embeddings, keeping your Haskell code entirely off third-party servers. Your codebase remains local and secure during the entire onboarding process.

Onboard to an Unfamiliar Haskell Codebase with Atlas in 2026

In 2026, Atlas helps Haskell developers build a working mental model of unfamiliar codebases by leveraging its semantic index, understanding `cabal` project structures, and integrating with tools like `hspec` and `fourmolu` to quickly grasp module hierarchies and type signatures without reading every file. Atlas starts from meaning, not filenames, to guide you through complex Haskell projects.

How Atlas Begins Haskell Codebase Exploration

In 2026, Atlas begins onboarding to a Haskell codebase by understanding its semantic meaning, not just file names. It uses `codebase_search` to query a semantic index built from your local Haskell project, ensuring relevant snippets are surfaced immediately.

Atlas starts by building a robust code index using AST declarations via tree-sitter, which allows it to reason about Haskell type signatures, typeclass instances, and `cabal` targets. This indexing can be performed with local Ollama embeddings, keeping your proprietary Haskell code off third-party servers. When you ask `codebase_search` a plain-language question, such as "how requests are authenticated in this Haskell service?", Atlas queries this semantic index. It returns ranked snippets of Haskell code along with their file paths, providing an answer-first approach to understanding complex logic. This initial step bypasses the need to manually navigate directories or guess at file contents, directly pointing you to the most relevant `.hs` files within the project.

Navigating Haskell Project Structure with Atlas

After identifying key areas, Atlas helps you understand the overall layout of a Haskell project in 2026. The `glob` tool provides a high-level view of the directory shape and naming conventions, crucial for grasping a `cabal` package's organization.

Understanding the directory structure is fundamental to onboarding to any Haskell project. Atlas's `glob` tool allows you to quickly inspect the top-level directories, revealing the typical `src/`, `app/`, `test/`, and `data/` layouts common in `cabal` projects. This gives you an immediate sense of where application logic, executables, test suites, and static assets reside. For instance, running `glob` might show you `my-project.cabal`, `src/`, `app/`, `test/`, and `flake.nix`. This quick overview helps you infer package boundaries and module hierarchies before you even open a single `.hs` file. Combined with the initial semantic search, you gain both a conceptual understanding and a structural map of the unfamiliar Haskell codebase.

Deep Diving into Haskell Code with LSP and Read

Once the initial layout is clear, Atlas facilitates a deeper dive into specific Haskell modules in 2026. It uses the `read` tool to inspect high-ranking files and the `lsp` tool's `goToDefinition` operation to follow module imports and understand type definitions.

With the most relevant Haskell files identified by `codebase_search` and the project structure understood via `glob`, Atlas guides you into the actual code. You can use the `read` tool to view the contents of the two or three `.hs` files that were ranked highest. From there, Atlas leverages the `lsp` tool, which connects to Model Context Protocol servers, to expose powerful operations like `goToDefinition`. This allows you to direct navigate through module imports, jump to the definition of a specific type, function, or typeclass instance, and trace data flow across different Haskell modules. This capability is invaluable for understanding complex type signatures and how various components interact within a large Haskell application, all without having to manually search for definitions across the entire repository.

Safe and Wide Exploration of Haskell Codebases

To ensure safe and read-only exploration of a Haskell codebase, Atlas delegates wide sweeps to its `explore` subagent in 2026. This subagent operates with a deny-by-default permission set, preventing accidental modifications while allowing comprehensive investigation.

When you need to perform broad searches or gather information across a large Haskell repository without the risk of altering anything, Atlas's `explore` subagent is the ideal tool. Defined with a strict deny-by-default permission set, the `explore` subagent is only allowed to use tools like `grep`, `glob`, `read`, `bash`, `webfetch`, and `websearch`. This means you can confidently task it with operations such as `task explore "grep for all instances of 'unsafePerformIO' across the Haskell codebase"` or `task explore "list all modules that import Control.Monad.Reader"`. The `explore` subagent can fan out work to parallel background sessions, efficiently gathering information about language extensions in `default-extensions` or specific `build-depends` bounds without ever writing to your Haskell project, providing a secure environment for initial understanding.

Recording Insights and Ensuring Haskell Code Quality

As a Haskell developer builds a mental model, Atlas helps record findings and refine code in 2026. The `todowrite` tool captures open questions, and Atlas integrates with `cabal` and `fourmolu` to ensure type safety and consistent formatting.

Building a mental model often involves identifying areas for further investigation or specific tasks. Atlas's `todowrite` tool allows you to record what you learned and capture open questions, ensuring that insights survive into the next turn of your onboarding process. For instance, you might note, "Investigate the usage of `TypeFamilies` in `src/Domain/Types.hs`." for making actual changes, Atlas connects directly with the Haskell toolchain. You can ask Atlas to run `cabal build` to surface GHC type errors, then let it iterate on the diff until the code typechecks correctly. Before you approve any commit, Atlas can apply `fourmolu` to the changed modules, ensuring your Haskell code adheres to formatting standards. Every Atlas tool call is permission-gated, and every file edit generates a unified diff for your approval, providing complete control over your Haskell codebase.

Step by step

01Ask `codebase_search` a plain-language question, for example, "how requests are authenticated in this Haskell service?", to query the semantic index and return ranked `.hs` snippets with file paths.
02Run `glob` on the top-level directories to see the `cabal` package layout and naming conventions, looking for `src/`, `app/`, `test/`, and `data/` directories before opening anything.
03Read the two or three `.hs` files `codebase_search` ranked highest using the `read` tool, then follow module imports and type definitions with the `lsp` tool's `goToDefinition` operation.
04Delegate wide sweeps to the `explore` subagent through the `task` tool, for example, `task explore "grep for all instances of 'unsafePerformIO' across the Haskell codebase"`, leveraging its read-only permissions.
05Record what you learned as a `todowrite` list, noting open questions about specific `TypeFamilies` or `GHC.Generics` usage within the Haskell project.
06Have Atlas run `cabal build` to surface GHC type errors, then let it iterate on the diff until the Haskell code typechecks.
07Let Atlas apply `fourmolu` to the changed `.hs` modules before you approve the commit, ensuring consistent Haskell code formatting.

Frequently asked questions

How does Atlas understand my Haskell project's dependencies and structure?: Atlas reads your `.cabal` file, including `build-depends` bounds and `default-extensions`, to build its understanding of your Haskell module hierarchy and language extensions. It indexes code by AST declarations using tree-sitter, not blind line windows.
Can Atlas help me understand complex Haskell type signatures and typeclass instances?: Yes, Atlas indexes code by AST declarations, allowing it to reason about your Haskell type signatures and typeclass instances. The `lsp` tool's `goToDefinition` operation helps you work through these complexities directly within the codebase.
What if I want to explore a Haskell codebase without making any changes?: Atlas's `explore` subagent is permissioned read-only by default. It allows you to `grep`, `glob`, `read`, `bash`, `webfetch`, and `websearch` across your Haskell project without modifying any files, ensuring safe investigation.
How does Atlas ensure code quality and formatting in Haskell projects?: Atlas can apply `fourmolu` to changed Haskell modules before you approve a commit, ensuring consistent formatting. It also surfaces a unified diff for every file edit, which you must approve before any changes are written to your Haskell project.
Can Atlas run my Haskell tests or help fix compilation errors?: Yes, Atlas can be instructed to run your `hspec` test suites to verify changes. It can also run `cabal build` to surface GHC type errors and then iterate on the diff until your Haskell code typechecks correctly.
Is my Haskell code sent to third-party servers for indexing or processing?: No, Atlas can build its code index with local Ollama embeddings, keeping your Haskell code entirely off third-party servers. Your codebase remains local and secure during the entire onboarding process.

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