Fun (FLS) for VS Code

June 10, 2026 · View on GitHub

Official VS Code support for the Fun language. This extension provides syntax highlighting and connects VS Code to the fls language server.

Features

  • Syntax highlighting for .fn files
  • Language Server support via fls (hover, completion, diagnostics, go-to-definition, formatting, etc. as provided by fls)
  • ▶ Run and ⚙ Debug code lenses above every fun main( for single-action run and debug workflows
  • Full debugger experience: breakpoints on .fn files, call stack, variables panel with Fun type names
  • Output channel: Fun Language Server (useful for debugging startup issues)
  • Bundled color theme: Fun Web (matches the reference website palette)

Requirements

You need the Fun tooling installed:

  • fls language server
  • fun compiler (required for Run / Debug)

For the Debug feature you also need:

  • A native debugger extension — CodeLLDB (recommended on macOS/Linux/Windows) or C/C++ (cpptools)
  • A system C compiler: cc/clang/gcc on macOS/Linux, clang-cl or cl.exe on Windows

By default the extension will try, in order:

  1. Workspace-local binaries: zig-out/bin/fls(.exe) and zig-out/bin/fun(.exe)
  2. Your system PATH (e.g. fls, fun)

Getting Started

  1. Install the extension.
  2. Open a .fn file.
  3. If the server doesn’t start, open View → Output and select Fun Language Server.

For local development builds, the extension prefers workspace binaries from zig-out/bin before falling back to executables on PATH.

Optional: set Preferences → Theme → Color Theme → Fun Web to use the same code palette as the website.

Settings

These settings live under Settings → Extensions → Fun:

  • fun.fls.path
    • Path to the fls executable.
    • Default: fls (falls back to zig-out/bin/fls when available)
  • fun.fls.funPath
    • Optional path to the fun executable.
    • When set to a valid executable, it is passed to fls via the FLS_FUN_PATH environment variable.
  • fun.fls.stdlibDir
    • Optional stdlib root (folder containing std/).
    • Passed to fls via FUN_STDLIB_DIR and takes precedence over derived paths.
  • fun.debugger.type
    • Debugger to use for ⚙ Debug. One of lldb (CodeLLDB) or cppdbg (cpptools).
    • Leave empty for auto-detection.

Notes:

  • Settings support ${workspaceFolder} / ${workspaceRoot}.
  • On Windows, %VAR% environment variables inside paths are expanded (best-effort).

Run and Debug

Every .fn file that defines fun main( shows two buttons above it:

  • ▶ Run — compiles and runs the file in an integrated terminal (equivalent to fun -in file.fn).
  • ⚙ Debug — compiles with debug info, then launches the native debugger.

Debug experience

Breakpoints are set directly on .fn source lines. When a breakpoint is hit:

  • Call stack shows Fun function names and .fn file/line numbers.
  • Variables panel shows variable names from your Fun code with Fun type names (num, str, bin, dec, raw*, etc.) instead of C equivalents.
  • Watch and Debug Console expressions also display Fun types.
  • Internal C boilerplate frames (e.g. async helpers) are marked as secondary and collapsed by default.

Debugging async functions

await expressions lower to a chain of C trampoline functions (__fun_async_call_, __fun_async_spawn_, __fun_async_entry_, then the real function body). The actual function call goes through pthread_create (or CreateThread on Windows) — which is opaque C runtime code the debugger cannot step through at the Fun source level.

This means step into on an await line will not automatically land inside the called async function. Instead the debugger follows the C runtime path:

await to_consumer.send_async(out);   // step into → enters trampoline C code

The right way to debug async calls:

  1. Set a breakpoint inside the async function you want to inspect (e.g. a line inside send_async in channel.fn). The debugger will break there when the spawned thread executes it, and you can step normally from that point.
  2. Alternatively, set a breakpoint on the line after the await to resume once the call has returned.

Step-over (F10) on an await line works correctly — it blocks until the async call completes and advances to the next Fun source line.

How it works

  1. The compiler is invoked with -g which embeds #line N "file.fn" directives in the generated C. These tell the C compiler to attribute all DWARF symbols to the original Fun source locations.
  2. The C file is compiled with -g (or /Zi on Windows) to produce a native binary with full DWARF/CodeView debug info.
  3. VS Code launches the native debugger (CodeLLDB or cpptools) against the binary. Because DWARF points to .fn files, the debugger shows Fun source natively — no extra mapping layer.
  4. A DAP message tracker intercepts variable type strings and remaps them from C types back to Fun types in real time.
  5. Temp files (.c and compiled binary) are cleaned up automatically when the debug session ends.

Debugger auto-detection

The extension picks the debugger in this order:

  1. fun.debugger.type setting (if set explicitly)
  2. CodeLLDB (vadimcn.vscode-lldb) if installed
  3. cpptools (ms-vscode.cpptools) if installed
  4. CodeLLDB on macOS/Linux, cpptools on Windows (best-effort fallback)

On Windows the extension tries clang-cl first (produces DWARF-compatible debug info for CodeLLDB), then cl.exe (MSVC, works with cpptools).

Commands

Open the Command Palette and run:

  • Fun: Restart Language Server
  • Fun: Show Language Server Output
  • Fun: Run — run the current file
  • Fun: Debug — debug the current file

Troubleshooting

No hover / completions / diagnostics

  • Verify fls is found:
    • Either ensure it’s on PATH, or set fun.fls.path to the full path.
  • If you’re building Fun from source, ensure zig-out/bin exists (or point settings at the built executables).

Standard library isn’t found

  • The extension attempts to derive FUN_STDLIB_DIR from the directory containing fls / fun.
  • Check the Fun Language Server output for the resolved paths.

Developing This Extension

From this folder (editors/vscode):

  1. npm install
  2. npm run compile
  3. Press F5 in VS Code (Extension Development Host)

Packaging a .vsix:

npm install
npm run package
code --install-extension fun-language-*.vsix

See CONTRIBUTING.md for repo-wide contribution guidelines.