build: add more compile-time checks

.github/workflows/stack.yml

@@ -14,14 +14,39 @@ jobs:
 
       - id: matrix
         uses: freckle/stack-action/generate-matrix@v5
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: haskell-actions/hlint-setup@v2
+        with:
+          version: "3.10"
+
+      - uses: haskell-actions/hlint-run@v2
+        with:
+          path: '["src/", "test/", "effectful-data-cache-opentelemetry/src/", "effectful-data-cache-opentelemetry/test/"]'
+          fail-on: warning
+
+  format:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: haskell-actions/run-fourmolu@v13
+        with:
+          version: "0.21.0.0"
+          pattern: |
+            src/**/*.hs
+            test/**/*.hs
+            effectful-data-cache-opentelemetry/src/**/*.hs
+            effectful-data-cache-opentelemetry/test/**/*.hs
 
   build:
     needs: generate-matrix
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-latest, macos-15]
         stack-yaml: ${{ fromJSON(needs.generate-matrix.outputs.stack-yamls) }}
     steps:
       - uses: actions/checkout@v6

.hlint.yaml

@@ -0,0 +1,2 @@
+- warn: {lhs: map, rhs: fmap, name: Use fmap}
+- warn: {lhs: a ++ b, rhs: a <> b, name: Use <>}

AGENTS.md

@@ -0,0 +1,18 @@
+# AGENTS
+
+Haskell library (stack + hpack). Edit `package.yaml`, never generated `.cabal`.
+
+## Commands
+
+- build and test (GHC 9.10): `stack build --test --ghc-options=-Werror`
+- format: `fourmolu -i src test effectful-data-cache-opentelemetry/{src,test}` (fourmolu 0.21.0.0, config in `fourmolu.yaml`)
+- lint: `hlint src test effectful-data-cache-opentelemetry/{src,test}` (rules in `.hlint.yaml`; HLS has no hlint plugin on GHC 9.10)
+
+CI (`.github/workflows/stack.yml`) runs all of the above; keep it green.
+
+## Style
+
+- `fmap` over `map`, `<>` over `++` (enforced by hlint)
+- curried constraints: `Read a => Ord a => a`, not `(Read a, Ord a) => a`;
+  not enforced by tooling
+- `-Wredundant-constraints` on: only add `Show e` for `throwError` where needed

effectful-data-cache-opentelemetry/test/Effectful/Cache/OpenTelemetrySpec.hs

@@ -40,7 +40,7 @@ attr
   :: ImmutableSpan
   -> Text
   -> Maybe Attribute
-attr s k = lookupAttribute (spanAttributes s) k
+attr s = lookupAttribute (spanAttributes s)
 
 spec :: Spec
 spec = do
@@ -49,7 +49,7 @@ spec = do
       insert @String @Int "a" (1 :: Int)
       _ <- lookup @String @Int "a"
       purge @String @Int
-    map spanName spans `shouldBe` ["cache.insert", "cache.lookup", "cache.purge"]
+    fmap spanName spans `shouldBe` ["cache.insert", "cache.lookup", "cache.purge"]
 
   it "records hits and misses" $ do
     (_, spans) <- traced $ do

fourmolu.yaml

@@ -0,0 +1,16 @@
+---
+column-limit: 90
+comma-style: leading
+function-arrows: leading
+haddock-style: single-line
+haddock-style-module: single-line
+import-export-style: diff-friendly
+indentation: 2
+let-style: inline
+respectful: false
+single-constraint-parens: never
+single-deriving-parens: never
+sort-constraints: true
+sort-derived-classes: true
+sort-deriving-clauses: true
+...