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lsp

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How the atopile Language Server works (pygls), how it builds per-document graphs for completion/hover/defs, and the invariants for keeping it fast and crash-proof.

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How to use this skill

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Source SKILL.md: https://github.com/atopile/atopile/blob/HEAD/.claude/skills/lsp/SKILL.md

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LSP Module

The lsp module (located in src/atopile/lsp/) implements the Language Server Protocol for atopile. It provides IDE features like autocomplete, go-to-definition, and diagnostics (error reporting) for ato files.

Quick Start

Run the server on stdio (what editors expect):

python -m atopile.lsp.lsp_server

Relevant Files

  • Server implementation: src/atopile/lsp/lsp_server.py
    • owns global LSP_SERVER (pygls LanguageServer)
    • maintains per-document DocumentState (graph/typegraph/build_result)
    • implements completion/hover/definition/diagnostics handlers
  • Utilities: src/atopile/lsp/lsp_utils.py
  • Optional debugging helper: src/atopile/lsp/_debug_server.py

Dependants (Call Sites)

  • VSCode Extension: The designated client for this server.
  • Compiler: The LSP invokes the compiler (often in a partial or fault-tolerant mode) to understand the code structure.

How to Work With / Develop / Test

Core Concepts

  • Partial Compilation: Unlike the CLI build, the LSP must handle broken or incomplete code without crashing.
  • Latency: Features must be fast (<50ms for typing, <200ms for completion).
  • Per-document graphs: each open document has an isolated GraphView + TypeGraph stored in DocumentState.
  • Keep last good build: the server keeps the last successful BuildFileResult to power completion/hover even when the current edit has errors.

Development Workflow

  1. Edit handlers/helpers in src/atopile/lsp/lsp_server.py.
  2. Run completion tests (fast loop) and verify GraphView cleanup paths.

Testing

  • Integration-style tests: ato dev test --llm test/test_lsp_completion.py -q

Best Practices

  • Robustness: Never let the server crash. Catch all exceptions in handlers and log them.
  • Debouncing: Don't trigger expensive operations on every keystroke.

Core Invariants (easy to regress)

  • Always destroy old graphs on rebuild/reset (DocumentState.reset_graph calls GraphView.destroy()).
  • Do not assume builds succeed; most features must handle:
    • syntax errors (ANTLR)
    • partial typegraphs
    • exceptions from linking/deferred execution