fix(stdlib): align Json.getCases return type with Python output (#279)

The F# return type `string array` for `getCases` was inaccurate — Fable
union types' `cases()` static method returns `list[str]`, not an
FSharpArray. Pyright flagged the resulting `cases: Array[str] = ...`
assignment as a type error in the generated Python (issue #278).

Change the return type to `ResizeArray<string>` (which maps to Python
`list`) and update the call site to use `.Count` instead of `.Length`.
Both compile to `len(...)` so runtime behavior is unchanged.

Also wire up pyright in CI to catch future regressions: add a `just
pyright` recipe scoped to `build/stdlib`, run it in the build-and-test
workflow, and bump pyright to 1.1.409. Fable codegen-artifact rules
(`reportUnusedImport`, `reportInvalidTypeVarUse`) are disabled since
those are emitted by the compiler, not the bindings.

Closes #278

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dbrattli

.github/workflows/build-and-test.yml

@@ -45,5 +45,8 @@ jobs:
       - name: Build
         run: just build
 
+      - name: Pyright type check
+        run: just pyright
+
       - name: Test
         run: just test

justfile

@@ -57,6 +57,10 @@ test-python: build
     {{fable}} {{test_path}} --lang Python --outDir {{build_path}}/tests
     uv run pytest {{build_path}}/tests
 
+# Type-check the generated Python bindings with pyright (see issue #278)
+pyright: build
+    uv run pyright
+
 # Create NuGet package with version from CHANGELOG.md
 pack:
     #!/usr/bin/env bash

pyproject.toml

@@ -8,6 +8,7 @@ readme = "README.md"
 license = "MIT"
 dependencies = [
     "fable-library==5.0.0",
+    "pyright>=1.1.409",
 ]
 
 [project.urls]
@@ -67,10 +68,16 @@ indent-style = "space"
 line-ending = "auto"
 
 [tool.pyright]
+# Type-check the Fable-generated Python bindings in build/stdlib so the shipped
+# output stays clean for downstream pyright users (see issue #278).
+include = ["build/stdlib"]
 reportMissingTypeStubs = false
 reportMissingImports = false
 reportUnnecessaryTypeIgnoreComment = true
-reportUnusedImport = true
+# Disabled: Fable emits unused `UNIT` imports and single-occurrence TypeVars
+# from F# generics. Those are codegen artifacts, not bindings bugs.
+reportUnusedImport = false
+reportInvalidTypeVarUse = false
 reportUnusedVariable = true
 reportUnnecessaryIsInstance = true
 reportUnnecessaryComparison = true
@@ -83,4 +90,4 @@ reportOverlappingOverload = true
 reportInconsistentConstructor = true
 reportImplicitStringConcatenation = true
 pythonVersion = "3.12"
-typeCheckingMode = "basic"
+typeCheckingMode = "standard"

src/stdlib/Json.fs

@@ -88,7 +88,7 @@ let private slotsToDict (o: obj) : obj = nativeOnly
 let private unionToList (o: obj) (caseName: string) : obj = nativeOnly
 
 [<Emit("getattr(type($0), 'cases', lambda: [])()")>]
-let private getCases (o: obj) : string array = nativeOnly
+let private getCases (o: obj) : ResizeArray<string> = nativeOnly
 
 [<Emit("(_ for _ in ()).throw(TypeError(f'Object of type {type($0).__name__} is not JSON serializable'))")>]
 let private raiseTypeError (o: obj) : obj = nativeOnly
@@ -136,7 +136,7 @@ let fableDefault (o: obj) : obj =
             let tag: int = getattr o "tag" :?> int
 
             let caseName =
-                if tag < cases.Length then
+                if tag < cases.Count then
                     cases.[tag]
                 else
                     "Case" + string tag