[fix] isolate REPL behind a build tag so the core compiles for js/wasm (LET-30)

[vt128]

Please review — do not merge yet. Opening for a second pair of eyes on the portability issue and the build-tag approach.

What

Machine.REPL() (in run.go, always compiled) imports go.starlark.net/repl, which transitively pulls in github.com/chzyer/readline — a terminal library that does not compile for GOOS=js (it references State, ClearScreen, GetScreenWidth, … which don't exist on js).

Because run.go is part of the always-compiled core, this forced chzyer/readline into every consumer's build and made the whole starlet package fail to cross-compile for js/wasm — even though no js code path uses the REPL. This blocks, for example, a browser WASM playground that runs starlet's pure modules (json/regex/serial/…) client-side.

The root cause is upstream-rooted (go.starlark.net/repl → readline), but the right place to contain it is at our boundary: an interactive terminal REPL is meaningless in a browser, so it shouldn't drag a terminal dependency into the js build.

Fix — build-tag isolation (no fork, no upstream change, no behavior change off-js)

Move REPL() out of run.go into two build-tagged files:

file	tag	content
run.go	—	drop the repl import + the REPL() method
run_repl.go	//go:build !js	the real REPL() + import "go.starlark.net/repl"
run_repl_js.go	//go:build js	a no-op stub REPL() (keeps the API present so consumer code still compiles for wasm)

• Non-js builds (linux/macos/windows): run_repl.go is compiled → REPL and readline behave exactly as before.
• js/wasm builds: run_repl.go is excluded by !js → repl/readline are never in the import graph → the package cross-compiles for js/wasm.

chzyer/readline stays in go.mod (the non-js REPL still needs it) but is no longer compiled for js — GOOS=js GOARCH=wasm go list -deps . no longer lists readline/repl.

Test-first

TestBuildsForJSWASM cross-compiles the package with GOOS=js GOARCH=wasm go build . and asserts success. It fails on the old code (readline undefined-symbol errors) and passes after the fix — a regression guard against re-importing a terminal dependency into the core.

Verification

go test -race -count=2 ./..., go vet, gofmt -l clean, and Docker golang:1.19 go test -race all green (the new test cross-builds for js under go1.19 too). Existing TestMachine_REPL_OK/Error still pass (real REPL on the native build).

Why a PR if "don't merge"

This surfaced while proving a WASM playground for the Star* ecosystem site is feasible (it is — starlet runs in the browser after this fix). Flagging the portability issue + approach for review before merging.

Requirement: LET-30

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

🟢 Metrics 8 complexity · 0 duplication

Metric	Results
Complexity	8
Duplication	0

View in Codacy

🟢 Coverage 100.00% diff coverage · +0.00% coverage variation

Metric	Results
Coverage variation	✅ +0.00% coverage variation (-1.00%)
Diff coverage	✅ 100.00% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (5de5dbf)	7773	7357	94.65%
Head commit (d91a3cd)	7773 (+0)	7357 (+0)	94.65% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#169)	6	6	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

_{NEW Get contextual insights on your PRs based on Codacy's metrics, along with PR and Jira context, without leaving GitHub. Enable AI reviewer
TIP This summary will be updated as you push new changes.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (5de5dbf) to head (d91a3cd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #169   +/-   ##
=======================================
  Coverage   93.44%   93.44%           
=======================================
  Files          49       50    +1     
  Lines        6222     6222           
=======================================
  Hits         5814     5814           
  Misses        259      259           
  Partials      149      149           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.