Refactoring 3/35: Replace global state with parameter

.claude/skills/refactoring-entry/LESSONS.md

@@ -161,3 +161,40 @@ brief.md or workflow.js and deleted here. Keep the calibration table current.
   marker-ended arrows entirely inside that gutter, and low-alpha highlight fills with
   no stroke. Render the actual SVG at desktop and mobile widths before assembly.
   → folded into workflow.js's diagram prompt.
+
+## 2026-09-04 — 03 Replace global state with parameter
+
+- **Toolchain.** The system gem dir is not writable on this machine: `bundle install`
+  fails building native extensions. `bundle config set --local path vendor/bundle`
+  works; `/vendor/` added to `.gitignore` in this PR. Headless
+  `google-chrome-stable` exists for layout screenshots; a scratch
+  `python3 -m http.server` must be started with `setsid nohup ... &` or it dies
+  when the terminal command returns (first screenshot attempt was
+  ERR_CONNECTION_REFUSED).
+- **Rule.** In scala-hedgehog `withTests` is a member of `Test`, not `Property`:
+  `property("name", prop).withTests(500)`. The brief's old line misleads into a
+  compile error. -> reworded in brief.md.
+- **Rule.** Haskell model of a global mutable cell: top-level `IORef` CAF
+  (`{-# NOINLINE x #-}` + `unsafePerformIO (newIORef ...)`). Before's entry point
+  lives in IO, After's is pure; the spec compares projections (label lists), per
+  the lens entry's compare-results rule. A spec whose properties touch the same
+  top-level cell must use `checkSequential` — `checkParallel` races the cell.
+- **Harness.** `edit_file` cannot create files inside a fresh untracked directory
+  tree (VFS error "path changed from ... to ..."); write sources with a terminal
+  heredoc and `git add` them immediately — the mirror then tracks them. Recorded
+  so the next run loses no time to it.
+- **Reviewer (self-run mutation round).** Every mutant caught, none survived:
+  ex1 fee ignoring its parameter (caught by the purpose property only), fee
+  applied twice (agreement); ex2 `<` vs `<=` in both languages (shrunk to
+  `List(-30)` within 5 tests), limit hard-coded (purpose property only); ex3
+  increment-by-2 and no-increment (Scala), right-subtree-first numbering
+  (Haskell), forcing the `Lit` payload (Haskell, laziness probe only).
+- **Citation skeptic.** martinfowler.com/bliki GlobalVariables.html and
+  Singleton.html are 404; refactoring.com/catalog/globalData.html is 404 — the
+  smells live in the 2e book, cite the book. misko.hevery.com is unreachable —
+  cite the Google Testing Blog post that quotes the same claims. "State-passing
+  style" does not occur verbatim in State in Haskell; it says "state
+  transformer" — write it that way.
+- **Time.** Single agent, no batch runner (workflow.js is Claude-Code-only):
+  research + citation checks ~15 min, examples + specs ~25 min, mutation round
+  ~15 min, page + diagrams + screenshots ~30 min.

.claude/skills/refactoring-entry/brief.md

@@ -72,7 +72,8 @@ pages/refactorings/<slug>/
         val r = Property.check(t.withConfig(PropertyConfig.default), t.result, Seed.fromTime())
         println(Test.renderReport("Props", t, r, ansiCodesSupported = false)); r.status }
       if !results.forall(_ == Status.ok) then sys.exit(1)
-  `.withTests(500)` on a property raises its count.
+  Bump a property's count with `property("name", prop).withTests(500)`:
+  `withTests` is a member of `Test`, not `Property`.
 - Haskell: GHC 9.8.4 + hedgehog. No ghc on this machine; docker image `cp-hedgehog` (run.sh builds it if missing):
     docker run --rm -v "$PWD/pages/refactorings/<slug>:/w" -w /w cp-hedgehog runghc -iNN-ex NN-ex/Spec.hs
   (from the repo root). Spec.hs: `main = do ok <- checkParallel (Group "Props" [...]); unless ok exitFailure`.
@@ -86,6 +87,10 @@ pages/refactorings/<slug>/
 - A second property per example is expected when natural (the extracted piece on its own; a law it obeys).
 - In Haskell probe laziness/strictness with `undefined` where the refactoring could change what is forced;
   in Scala reason about evaluation count (def vs val, by-name) and say so in a comment when it matters.
+- If an example needs a global MUTABLE cell in Haskell: a top-level `IORef` CAF
+  (`{-# NOINLINE x #-}` + `unsafePerformIO (newIORef ...)`); Before's entry point lives
+  in IO, After's is pure — the type change is the point, say so on the page. Compare
+  projections (the lens rule), and use `checkSequential` when properties touch the cell.
 
 ## Style
 - Site prose: crisp, editorial, no hype; sentences, not bullets, in body text (the caveats section may use a list).

.gitignore

@@ -74,3 +74,6 @@ dist-newstyle/
 
 # Local linting venv (see README "Lint")
 /.venv/
+
+# Bundler local gem path (bundle config set --local path vendor/bundle)
+/vendor/

_config.yml

@@ -45,3 +45,4 @@ plugins: [jekyll-paginate, jekyll-seo-tag, jekyll-feed, jekyll-remote-theme]
 exclude:
   - pages/refactorings/extract-method/
   - pages/refactorings/replace-mutable-fields-with-lenses/
+  - pages/refactorings/replace-global-state-with-parameter/

_data/refactorings.yml

@@ -6,7 +6,7 @@
   items:
     - { name: "Extract / Inline method", slug: extract-method }
     - { name: "Replace mutable fields with lenses", slug: replace-mutable-fields-with-lenses }
-    - { name: "Replace global state with parameter" }
+    - { name: "Replace global state with parameter", slug: replace-global-state-with-parameter }
     - { name: "Introduce ReaderT (Kleisli)" }
     - { name: "Replace loop with fold" }
     - { name: "Replace recursion with fold" }