SCL Language Support (Siemens)

Language support for Siemens SCL (Structured Control Language), the ST-family language used in Siemens TIA Portal for programming S7 PLCs. This extension runs a bundled Python language server so you get diagnostics, hover info, and completion for .scl files directly in VS Code — no Python installation required.

Features

• Syntax highlighting for SCL keywords, control-flow statements, data types, built-in functions, numeric/typed literals, strings, and comments (via a TextMate grammar, syntaxes/scl.tmLanguage.json).
• Diagnostics, computed from the declaration section and the code after BEGIN:
  • Missing semicolons on assignment statements and END_IF lines (with support for multiline statements/function calls and boolean-operator line continuations).
  • Undefined variables — identifiers used in assignments that were never declared in a VAR/VAR_INPUT/VAR_OUTPUT/VAR_IN_OUT/VAR_TEMP/ CONST block (case-insensitive, matching SCL semantics). Function block call argument names, typed/based numeric literals (T#5s, 16#FF, INT#5, ...), and string literals are correctly excluded.
  • IF/END_IF structure checks: a missing THEN, a missing END_IF, an END_IF missing its trailing semicolon, and empty ELSE blocks.
  • Variable name length/prefix collisions: identifiers longer than 24 characters, and two identifiers in the same scope (global, or within the same STRUCT) that share their first 24 characters and would therefore collide in TIA Portal.
• Hover information for variables and struct fields, including their declared type, default value, and any trailing declaration comment. Hovering over a specific segment of a dotted path (a.b.c) shows info for just that segment.
• Completion for top-level variables and, after a ., for the fields of a struct-typed variable.
• Bracket-match highlighting for (), [], and {}.

Supported platforms

Prebuilt, platform-specific packages are published for:

• Windows (win32-x64)
• macOS (darwin-arm64, via CI)
• Linux (linux-x64, via CI)

Each package bundles a standalone server executable for its platform, so end users never need a separate Python install.

Known limitations

• The parser is regex/line-based, not a real tokenizer or grammar, so it can be tripped up by unusual formatting — particularly multiline function calls, structs, and statements that wrap across several lines.
• Analysis is focused on the declaration section (VAR*/CONST/ STRUCT blocks) and simple statements after BEGIN. Deeper control-flow or data-flow analysis of the executable body is out of scope.
• Diagnostics are heuristic checks, not a full SCL/TIA Portal compiler — treat them as linting aids, not a substitute for compiling in TIA Portal.

Development

Client (TypeScript, run from the repo root):

npm run compile        # type-check + lint + esbuild bundle to dist/extension.js
npm run watch          # esbuild + tsc in watch mode
npm run check-types    # tsc --noEmit
npm run lint           # eslint client/src
npm run package        # production build (minified, no sourcemaps)
npm test               # compiles tests + extension + lints, then runs vscode-test

Server (Python, run from server/scl_server/, this project uses uv for environment/dependency management):

uv venv .venv
uv pip install -p .venv/bin/python -r requirements.txt -r requirements-dev.txt
.venv/bin/pytest                        # run the whole server test suite
.venv/bin/pytest server/tests/test_diagnostics.py -k undefined_variable  # a single test

Building the standalone server binary with PyInstaller (from the repo root):

python -m PyInstaller scl_server.spec   # produces dist/SCLserver/

Packaging the .vsix for a specific platform (after building the matching server binary above):

npx @vscode/vsce package --target win32-x64

See CLAUDE.md for the full architecture overview and the "Publishing" section for how tagged releases are built and published by CI.

Release notes

See CHANGELOG.md for the full history of changes.