---

name: build-hs description: Start a ghcid session to monitor the build. Use when working with Haskell projects, starting ghcid, checking build errors, or searching Haskell documentation with hoogle. user-invocable: true

Haskell Build Monitoring with ghcid

Start a ghcid session to monitor the build.

Before starting: Verify which command is actually allowed in .claude/settings.local.json for ghcid and ensure you use it to avoid prompting for permission.

First, check package.yaml to identify the test components:

• tests: section → use the test name with -test suffix (e.g., dota-sage-test)

Then run ghcid with the all target (which includes all libraries and executables, including internal libraries) PLUS all test components using the Bash tool with run_in_background: true:

Copynix develop -c ghcid \
  -c 'cabal repl --enable-multi-repl all <test-name>' \
  --restart=<project>.cabal \
  --test ':!nix develop -c cabal test' \
  --outputfile=build.log \
  --clear

Note: The all target ensures internal libraries are included in the build. Tests must be specified explicitly since all doesn't include them. The --restart flag causes ghcid to automatically restart when the cabal file changes (e.g., after hpack regeneration). The --test flag runs the test suite automatically after a successful build.

To keep context usage reasonable, only the output file build.log should be used to monitor the build status, ideally reading only the first few lines and then more if needed using the Read tool.

Monitoring build status:

• Check for errors: Use Read tool with file_path: "build.log" and limit: 20
• Check if ghcid is running: View background processes in Claude Code UI, or ps aux | grep ghcid
• Kill ghcid: Use KillShell tool with the shell ID, or pkill ghcid

When to restart ghcid:

• If ghcid appears stuck or unresponsive
• When adding a new component (library, executable, test) to package.yaml that needs to be included in the repl command

Note: With the --restart flag, ghcid will automatically restart when the cabal file changes, so manual restarts are not needed after running hpack --force. Always use hpack --force to avoid version mismatch errors when regenerating cabal files.

Important: If you add a new component (library, executable, or test suite) to package.yaml, you must also remind me to update the permission in .claude/settings.local.json to include the new component name in the ghcid command.

Note: Avoid building the project directly with cabal or stack. Let ghcid monitor compilation continuously.

Incremental Compilation Performance:

• The initial ghcid build takes a few seconds to compile all modules
• After that, incremental recompilation is essentially instant (typically <1 second)
• IMPORTANT: Do not add sleep delays between making code changes and checking build.log
• ghcid detects file changes and recompiles immediately, so you can check build.log right after making edits
• If you need to verify compilation completed, just read build.log directly - if ghcid is still compiling, you'll see the status

Common Build Issues

HasField instance errors with OverloadedRecordDot:

• These errors are almost always due to missing imports of the data constructor
• Fix: Import the data constructor explicitly using import Module (Type(..)) rather than just import Module (Type)
• Example: If you see No instance for (GHC.Records.HasField "field" Type ...), check that you're importing Type(..) not just Type

Documentation Search with Hoogle

Use hoogle CLI to search for documentation of project dependencies within the nix develop shell.

Basic search commands:

1. Search for a function by name:

   Copynix develop -c hoogle search "mapMaybe"
   

   Example output: Shows functions from Data.Maybe and other modules

2. Search by type signature:

   Copynix develop -c hoogle search "(a -> Bool) -> [a] -> [a]"
   

   Finds functions like filter, takeWhile, etc.

3. Search for specific package documentation:

   Copynix develop -c hoogle search "aeson" --count=10
   

   Lists top 10 results from the aeson package

4. Get detailed information about a function:

   Copynix develop -c hoogle search "traverse" --info
   

   Shows full documentation with examples

5. Search in specific modules:

   Copynix develop -c hoogle search "module:Data.List take"
   

   Searches only within Data.List module

Advanced usage:

• Limit results: Add --count=N to show only N results
• JSON output: Add --json for machine-readable output
• Exact match: Use quotes for exact function name matching
• Type class search: Search for "Monad m => ..." to find typeclass instances

Common search patterns:

• Find JSON parsing: nix develop -c hoogle search "ByteString -> Maybe Value"
• Find list functions: nix develop -c hoogle search "module:Data.List"
• Find monad operations: nix develop -c hoogle search "Monad m =>"
• Find lens operations: nix develop -c hoogle search "Lens'"

Integration with development:

• When encountering unfamiliar functions in build.log errors, use hoogle to understand their usage
• Use type signature search to discover appropriate functions for your use case
• Explore package APIs by searching the package name to understand available modules and functions before implementing features