A Visual Studio Code extension that provides language support for EXPRESS (ISO 10303-11), the schema definition language used in STEP and building-information models such as IFC.

Open any .exp file to get syntax highlighting, semantic analysis, and IDE navigation powered by a Langium-based language server.

Features

Syntax highlighting

EXPRESS source is highlighted using a TextMate grammar. Keywords, literals, comments, and identifiers are distinguished in the editor. The extension recognizes both uppercase and lowercase keyword spellings common in EXPRESS schemas.

Editor integration

Bracket matching for (), [], and {}
Auto-closing pairs for brackets and string quotes
Line and block comments (// and /* */)

Language Server Protocol (LSP)

The extension launches a dedicated language server when you open an EXPRESS file. Standard LSP features are provided through Langium:

Feature	Description
Diagnostics	Unresolved references and other link errors appear in the Problems panel
Go to definition	Jump from a reference to its declaration (entity, type, attribute, function, local variable, etc.)
Document symbols	Browse declarations in the current file
Completion	Context-aware suggestions while typing

Semantic linking

Beyond basic name resolution, the language server understands EXPRESS-specific scoping and type structure:

Declarations — entities, types, functions, constants, and enumeration items
Attributes — direct attributes, inherited attributes, inverse relations, and redeclared supertype attributes
Local scope — variables in algorithms, RULE bodies, QUERY expressions, REPEAT loop indices, and ALIAS statements
Expression context — attribute access on SELF, subtype operands, function-call results, and indexed qualifiers (e.g. U[2].DirectionRatios)
DERIVE / WHERE rules — references inside constraint and derivation expressions, including nested QUERY and USEDIN aggregates
Enumerations — enumeration literals, including case-insensitive matching where applicable
Document-scoped lookup — entity and type resolution prefers symbols in the current schema document to avoid cross-file ambiguity in large workspaces

The implementation is validated against real-world schemas, including large IFC EXPRESS files.

Getting started

Install EXPRESS Language from the VS Code Extensions view, then open any file with the .exp extension.

Supported file types

Extension	Language ID
`.exp`	`express`

Requirements

Visual Studio Code 1.67.0 or newer

Known limitations

Performance — Very large schemas (tens of thousands of lines) may take noticeable time to analyze on each edit. Optimization is planned for future runtime improvements.
Validation — Custom semantic validation rules are minimal today; most Problems panel feedback comes from unresolved references during linking.