Pact Language Server

A Language Server Protocol implementation for Pact, the session-typed extension of AnvilHDL, with extensions for VS Code and Vim/Neovim (coc.nvim). It is derived from the Anvil Language Server.

Note

The language server is experimental. Features that depend on the compiler's AST output (-ast) are enabled automatically when the compiler provides it.

Features

Feature	Support
Diagnostics	Compile errors, including session-type errors
Completion	Keywords with documentation; Unicode session-type operators; endpoints, messages and symbols in context¹
Hover	Syntax help; definitions, types and message timing contracts¹
Navigation	Go to definition and find references across interfaces, pools, protocols, processes, channels, registers and types¹
Session states	The remaining session type of each thread, as inlay hints and on hover¹
Process types	Each process's type above its definition: its declared interfaces, or else the type inferred from its threads, always in mathematical notation; next to it, the data-sensitive type inferred from the process, with the registers and payloads it tracks. Long types are named in the lens; hovering the process shows both over several lines²
Signature help	Function calls, message sends, record initialisers and spawns¹
Highlighting	All Pact keywords and operators (VS Code and Vim)
Timing	Clock-cycle inlay hints and expression lifetimes are not yet available for Pact

¹ Requires a Pact compiler with AST output (-ast). Without it, diagnostics, keyword completion, syntax help and highlighting remain available.

² Requires a Pact compiler that emits process_type; the data-sensitive type needs one with the data-sensitive extension (the hybrid-pact branch). Shown as a code lens; turn it off with pact.showProcessTypes. In coc.nvim, enable codeLens.enable.

See LSP.md for the detailed feature matrix.

Unicode operators

Type a backslash followed by the operator name to insert its symbol. Each symbol has an ASCII equivalent.

Input	Symbol	ASCII	Meaning
\sum	∑	+	Choice
\prod	∏	||	Parallel composition
\and, \wedge	⋀	&	Offer
\xor, \oplus	⨁	^	Exclusive offer
\fxor, \foplus	⨁ᶠ	^^	Fair exclusive offer
\fplus, \fsum	+ᶠ	++	Fair choice
\forall	∀	forall	Quantifier
\in	∈	:	Quantifier range
\mu, \rec	μ	rec	Recursion
\not	̸	not	Range exclusion

The exclusion symbol is a single space followed by U+0338; completing \not replaces any whitespace before it.

Requirements

• Node.js 22 or later.
• The Pact compiler, commit f0115c7 or later. AST output (-ast) is available on the pact-ast branch. The pact submodule pins a version without AST output.

Installation

Compiler

git clone --recurse-submodules <repository-url> pact-lsp
cd pact-lsp/pact
eval $(opam env) && dune build --release && dune install

To use another Pact build, set pact.executablePath to its binary.

VS Code

cd pact-lsp/extensions/vscode
npm install && npm run build

Then run Developer: Install Extension from Location... from the Command Palette and select extensions/vscode.

Vim/Neovim (coc.nvim)

Build extensions/vim with npm install && npm run build and add it to the runtime path:

set rtp^=/path/to/pact-lsp/extensions/vim

The Pact and Anvil extensions both handle .anvil files. Enable only one of them per workspace.

Configuration

Setting	Default	Description
pact.executablePath	pact on PATH	Path to the compiler.
pact.compilerArgs	[]	Additional compiler flags, e.g. ["-disable-lt-checks"].
pact.compatibilityChecks	false	Run pool compatibility checks during diagnostics. They can take minutes on large pools, so they run only when enabled.
pact.compileTimeout	15000	Milliseconds before a compiler run is stopped; 0 disables the limit.
pact.projectRoot	Workspace root	Directory in which the compiler runs.
pact.maxNumberOfProblems	100	Maximum number of diagnostics per file.
pact.showTimingInfo	Condensed inlay hints and hover	Where clock-cycle and lifetime information is shown (not yet emitted by Pact).
pact.showSyntaxHelp	Hover and keyword completions	Where explanations of Pact syntax are shown.
pact.showSessionState	Inlay hints and hover	Where the remaining session type is shown.
pact.asciiOnlyIcons	false	Use ASCII characters instead of Unicode symbols.

Development

./build.sh    # compiler, server and extensions
./test.sh
./format.sh
./update.sh   # pull this repository and resynchronise submodules

Each script accepts component names (pact, server, vscode, vim) to limit it to those components. To test against another compiler build, set PACT_BIN, e.g. cd server && PACT_BIN=/path/to/pact npm test. See MAINTAINERS.md for the architecture and maintenance guide.