Package operators provides aggregation operators.

Package stages provides aggregation stages.

GetPushdownQuery gets pushdown query ($match and $sort) for aggregation.

NewExpression returns Expression from dollar sign $ prefixed string.

SumNumbers accumulate numbers and returns the result of summation.

ErrEmptyFieldPath indicates that field path expression is empty.

ErrEmptyVariable indicates that variable name is empty.

ErrInvalidExpression indicates that expression is invalid.

ErrNotExpression indicates that field is not an expression.

ErrUndefinedVariable indicates that variable name is not defined.

Expression represents a value that needs evaluation.

ExpressionError describes an error that occurs while evaluating expression.

Stage is a common interface for all aggregation stages.

ExpressionErrorCode represents Expression error code.