Instruction file imported from knime-oss/knime-base-expressions (.github/instructions/core.instructions.md). Copyright stays with the author.

Core expression language implementation (Java)

Key Architectural Patterns

Expression Function Builder Pattern: All functions use a fluent builder requiring name, description, examples, keywords, category, args, returnType, and impl in sequence:

public static final ExpressionFunction MAKE_DATE = functionBuilder()
    .name("make_date")
    .description("Creates a LOCAL_DATE from year, month, day...")
    .examples("* `make_date(1970, 1, 1)` returns `1970-01-01`")
    .keywords("make", "date", "local_date", "create")
    .category(CATEGORY_CREATE_EXTRACT)
    .args(
        arg("year", "The year to use for the date.", isIntegerOrOpt()),
        arg("month", "The month to use (1-12).", isIntegerOrOpt()),
        arg("day", "The day to use (1-31).", isIntegerOrOpt())
    )
    .returnType("A LOCAL_DATE...", RETURN_LOCAL_DATE_MISSING, args -> OPT_LOCAL_DATE)
    .impl(TemporalCreateExtractFunctions::makeDateImpl)
    .build();

Function names must be snake_case (enforced by regex). Categories use two-level hierarchy (e.g., "String – General").

AST Design: All expression nodes implement sealed Ast interface. Use the Visitor pattern (Ast.AstVisitor) for traversal. Each AST node has a mutable data() map for attaching compilation/type information.

Parser Integration: Parser.java wraps ANTLR-generated KnimeExpressionParser/KnimeExpressionLexer. Parse errors are collected via custom ErrorListener and converted to ExpressionCompileException.