add Haskell skill

modules/home/claude-code/default.nix

@@ -17,8 +17,22 @@ let
         (builtins.readFile (commandsDir + "/${fileName}"))
     )
     (builtins.readDir commandsDir);
+
+  skillsDir = ./skills;
+  skillDirs = lib.filterAttrs (_: type: type == "directory") (builtins.readDir skillsDir);
 in
 {
+  # Link skill directories to ~/.claude/skills/
+  # (home-manager module doesn't support skills yet, so we link manually)
+  home.file = lib.mapAttrs'
+    (skillName: _:
+      lib.nameValuePair ".claude/skills/${skillName}" {
+        source = skillsDir + "/${skillName}";
+        recursive = true;
+      }
+    )
+    skillDirs;
+
   home.packages = [
     pkgs.tree
     pkgs.python313Packages.markitdown

modules/home/claude-code/skills/haskell/SKILL.md

@@ -0,0 +1,47 @@
+---
+name: haskell
+description: Expert Haskell development assistance. Use when working with Haskell code, .hs files, Cabal, ghcid, or when user mentions Haskell, functional programming, or type-level programming.
+---
+
+# Haskell Development
+
+Expert assistance for Haskell programming.
+
+## Guidelines
+
+**CRITICAL - Error Handling in Code**: NEVER write code that silently ignores errors:
+- Do NOT use `undefined` or `error` as placeholders
+- Do NOT skip handling error cases in pattern matches
+- Do NOT ignore `Maybe`/`Either` failure cases
+- Handle all possible cases explicitly
+- Use types to make impossible states unrepresentable
+
+Every error case in generated code must be handled properly.
+
+**Code Quality**:
+- Write type signatures for all top-level definitions
+- Write total functions (avoid `head`, `tail`, `!!`)
+- Prefer pure functions over IO when possible
+- Use explicit exports in modules
+- Leverage type system for safety
+- Favor composition over complex functions
+- Write Haddock documentation for public APIs
+
+**Idiomatic Patterns**:
+- Prefer `Text` over `String`
+- Use `newtype` wrappers for domain types
+- Apply smart constructors for validation
+- Use lenses for record manipulation when appropriate
+- Use `Applicative` and `Monad` appropriately
+
+## Testing
+
+- Use QuickCheck for property-based testing
+- Use HUnit or Hspec for unit tests
+- Provide good examples in documentation
+
+## Build instructions
+
+As you make code changes, start a subagent in parallel to resolve any compile errors in `ghcid.log`.
+
+**IMPORTANT**: Do not run build commands yourself. The human runs ghcid on the terminal, which then updates `ghcid.log` with any compile error or warning (if this file does not exist, or if ghcid has stopped, remind the human to address it). You should read `ghcid.log` (in _entirety_) after making code changes; this file updates near-instantly. Don't rely on VSCode diagnostics.