Pyfun for VS Code

Language support for Pyfun, an F#-inspired, functional-first language that compiles to readable Python. The extension is a thin client for the Pyfun language server, so every feature below is computed by the compiler itself.

Requirements

This extension needs the Pyfun compiler, which provides the language server (pyfun lsp). Install it from PyPI (Python 3.12+):

pip install pyfun-lang

Once pyfun is on your PATH, the extension launches the server automatically the first time you open a .pyfun file. If your pyfun lives somewhere else, point the extension at it with the pyfun.server.path setting.

Features

Diagnostics — type, effect, unit, and exhaustiveness errors inline, updated as you type.
Hover — the inferred type of the expression or binding under the cursor, with latent effects shown on arrows (e.g. string ->{io} unit). Pyfun types are never written, so hover is how you see what the compiler inferred. Doc comments (##) render below the type.
Go-to-definition and find-references — across files, for locals, parameters, pattern variables, top-level and module values, constructors, and type names.
Rename — project-wide, for values, constructors, and types.
Completion — module members (List.map, Map.tryFind), prelude builtins, constructors, type names, and keywords.
Document and workspace symbols — the file outline (including in-file module members) and a project-wide symbol search.

All of the above run over resilient analysis: the lexer and parser both recover, so a half-typed file still hovers, navigates, and completes.

Syntax colours

The TextMate grammar tags tokens by their semantic role, and the extension pins a colour per role so the palette reads the same across themes. The dark values are Monokai's (plus bracket-pair gold); light themes get darkened, same-hue variants so each role stays legible on white (WCAG-AA contrast). Pink/magenta is reserved exclusively for the mutability and FFI escape hatches.

Role	Tokens	Dark	Light
Declarations (introduces a name)	`let` `fun` `type` `measure` `module`	cyan `#66D9EF`	teal `#0E7490`
Escape hatches (mutation + FFI)	`mut` `<-` `extern`	pink `#F92672`	magenta `#C71D6C`
Computation-expression builders	`async` `seq` `result`	purple `#AE81FF`	violet `#7C3AED`
Units of measure	`<m>`, `<m/s^2>`	orange `#FD971F`	burnt orange `#C2410C`
Control flow + logical	`if` `then` `else` `elif` `match` `case` `with` `return` `yield` `do` `in` · `and` `or` `not`	gold `#FFD700`	amber `#9A6700`

Everything else (identifiers, type names, constructors, strings, numbers, operators) follows your active theme. To override a pin, add your own editor.tokenColorCustomizations rule for the scope (e.g. keyword.control.pyfun) in user settings; it wins over the extension default.

Settings

Setting	Default	Description
`pyfun.server.path`	`pyfun`	Path to the `pyfun` executable used to launch the language server. Supports `${workspaceFolder}`.

Learn more

Pyfun on GitHub — README, examples, and the design document.
pyfun-lang on PyPI — the compiler this extension drives.

Building the extension from source, or hacking on the client, is covered in DEVELOPMENT.md.

License

Apache-2.0.