For AI agents: the complete documentation index is available at https://silt-db.dev/llms.txt, the full documentation bundle is available at https://silt-db.dev/llms-full.txt, and this page is available as Markdown at https://silt-db.dev/reference/expressions.md.

    Aggregation expressions

    Use the MongoDB expression reference for syntax and examples. This page records supported operators and Silt-specific restrictions. Expressions run in SQLite and preserve JSON types, ordered embedded objects, and the distinction between missing fields and null.

    Unsupported operators throw SILT_UNSUPPORTED; invalid runtime operands fail the SQLite statement. Some operators reject values requiring BSON metadata or producing NaN/infinity. Shared limits are listed in MongoDB compatibility.

    FamilyImplemented expression operators
    Literals and scope$literal, $let, $$ROOT, $$CURRENT, $$REMOVE, user variables, dotted paths
    Boolean and comparison$and, $or, $not, $eq, $ne, $gt, $gte, $lt, $lte, $cmp
    Conditional$cond, $ifNull, $switch
    Arithmetic$add, $subtract, $multiply, $divide, $mod, $pow, $abs, $ceil, $floor, $round, $trunc, $sqrt, $exp, $ln, $log, $log10
    Trigonometry$sin, $cos, $tan, $asin, $acos, $atan, $atan2, $sinh, $cosh, $tanh, $asinh, $acosh, $atanh, $degreesToRadians, $radiansToDegrees
    Array transformation$map, $filter, $reduce, $range, $zip, $sortArray, $reverseArray, $concatArrays, $slice
    Array inspection$isArray, $size, $arrayElemAt, $first, $last, $firstN, $lastN, $minN, $maxN, $in, $indexOfArray
    Array/scalar accumulators$sum, $avg, $min, $max (distinct from pipeline $group accumulators)
    Sets$setUnion, $setIntersection, $setDifference, $setEquals, $setIsSubset, $allElementsTrue, $anyElementTrue
    Objects$objectToArray, $arrayToObject, $mergeObjects, $getField, $setField, $unsetField
    Strings$concat, $split, $toLower, $toUpper, $strLenCP, $strLenBytes, $substrCP, $substrBytes, $substr, $indexOfCP, $indexOfBytes, $strcasecmp, $trim, $ltrim, $rtrim, $replaceOne, $replaceAll, $binarySize
    Types and conversion$type, $isNumber, $toBool, $toString, $toInt, $toLong, $toDouble, $convert
    Regular expressions$regexMatch with a constant pattern and options uses the portable SQL grammar. Dynamic patterns, $regexFind, and $regexFindAll require the optional native PCRE2 extension.

    Numeric conversions use a strict SQL lexical parser rather than SQLite's permissive cast semantics: whitespace, trailing junk, hex, and decimal/exponent syntax for integer targets are rejected. $convert supports literal bool, string, int, long, and double targets (and their BSON type codes), with lazy onNull and onError. Long conversion is restricted to the JavaScript safe-integer range. Decimal128, dates, ObjectId, binary BSON, numeric subtype identity, and dynamic conversion targets are not supported. $type on a number fails because JSON cannot recover its BSON numeric subtype.

    $round and $trunc use SQLite's extended decimal rendering of the binary64 value before decimal quantization. This avoids incorrect results from multiplying by a power of ten first. $substrBytes validates UTF-8 start and end boundaries in SQL. String case conversion follows SQLite's ASCII rules; MongoDB documents these operators as only well-defined for ASCII. Embedded NUL handling is explicitly rejected for trimming and code-point length; byte length and byte slicing retain NUL bytes. Some other SQLite text operations have additional embedded-NUL limitations; this is not full arbitrary BSON string conformance.

    $sortArray ranks each element with correlated comparisons, including embedded arrays/objects. This is quadratic in array length; it is intended for bounded document arrays. $min/$max expression selection also uses correlated comparisons. SQL is executed in SQLite, but complex generated expressions and large single-document arrays still consume SQLite memory and SQL planner resources. Collection streaming does not imply a fixed memory bound for every expression.

    Unsupported families include BSON/date expressions, bitwise numeric-subtype operators, custom JS functions/accumulators, and the newer optional parameters or operators not listed above. $replaceAll supports an empty search string using a recursive SQL walk over Unicode code points. Runtime finite-number and malformed-operand guards do not imply complete MongoDB error precedence parity.

    Validation evidence

    Direct SQLite regressions and captured MongoDB comparisons cover invalid operands, precision/conversion boundaries, field order, and expression behavior. See testing and compatibility evidence for the corpus, recorded results, and reproduction.