This package contains classes responsible for evaluation of FLWOR expressions, in particular, those clauses of FLWOR expressions that are supported in Saxon-HE.

FLWOR expressions are implemented as a pipeline of clauses, much as described in the specification. The pipeline can be evaluated in push or pull mode: in push mode, the supplier of tuples activates the consumer of tuples when a tuple is ready to be processed, while in pull mode, the consumer of tuples calls the supplier to request the next tuple. In both cases the "tuple" is not actually passed as an argument or result of this call, but is represented by the state of local variables in the XPathContext stack on completion of the call. The only time tuples are represented as real objects is when the processing is not pipelined, for example when tuples need to be sorted or grouped.

Simple "for" and "let" expressions do not use this mechanism: instead, they are compiled to a ForExpression or LetExpression.

Michael H. Kay
Saxonica Limited
28 November 2011