Tesl Language — VS Code Extension

Syntax highlighting, live type-checking, go-to-definition, hover types, completions, and occurrence highlighting for the Tesl language.

Tesl is an beta-stage language for building web APIs where validation, auth, and side effects are enforced by the compiler rather than by convention.

Features

• Syntax highlighting — keywords, types, proof annotations (:::), operators, string interpolation
• Live diagnostics — type errors and lint warnings appear inline as you type
• Go-to-definition — jump to where a name is declared
• Hover types — see the inferred type of any expression
• Completions — context-aware field and identifier suggestions
• Occurrence highlighting — all uses of a symbol highlighted on cursor
• Interactive debugging — set breakpoints on .tesl lines, step through execution, and inspect locals with their compile-time proof annotations (e.g. port = 8080 : Int ::: ValidPort port)

Debugging

The extension contributes a tesl debug type. To debug a .tesl file:

• Open the file, set breakpoints in the gutter, and press F5, or
• Right-click the file → Debug Tesl Program / Debug Tesl Tests, or
• Click the ▶ Run / 🐛 Debug CodeLens above any test block.

CodeLens Run is offered for test, api-test, load-test, and doctest blocks; Debug is offered for test, api-test, and doctest blocks. A load-test is run-only — it is a throughput benchmark, so stepping through it is not meaningful.

Behind the scenes the adapter compiles the file with tesl --debug, runs the chosen main (program mode) or test blocks (test mode), and pauses at your breakpoints. The Variables panel shows each local's raw runtime value overlaid with its compile-time type and proof — the proof annotation is recovered from the compiler (it is erased from the runtime, by design), so a proof-carrying value reads e.g. port = 8080 : Int ::: ValidPort port. Records, newtypes, ADTs, tuples, and lists are formatted readably.

A reference launch.json lives in .vscode/launch.json; the same configurations are offered automatically via Run → Add Configuration… → Tesl. Set "mode": "test" plus an optional "testName" to debug a single test block.

For diagnostics, set the TESL_DAP_LOG environment variable to a file path (or stderr) before launching VS Code to capture an adapter trace.

Requirements

The extension needs the Tesl language server (tesl-lsp) to provide diagnostics and editor intelligence.

Recommended: install via Nix

nix profile install github:mtonnberg/tesl

This puts both tesl and tesl-lsp on your PATH. The extension finds tesl-lsp automatically — no configuration needed.

Alternative: repo checkout

If you have the Tesl repository open as your workspace and have run nix develop (or nix-shell), the extension finds tesl-lsp.rkt in the repo automatically.

Extension settings

Known limitations

• Beta stage — breaking changes are expected; the extension tracks the language version
• Nix only — standalone binaries for non-Nix users are on the roadmap
• Linux / macOS only — Windows native is not supported; WSL2 with Nix works

Feedback and issues

github.com/mtonnberg/tesl/issues