# Internal changes

Information about the type hierarchy is now cached in a new data structure (called `TypeHierarchy`) referenced from the `NamePool`. The aim is to improve the performance of type checking both at compile time and at run-time.

For some time (since before Saxon 6.5) Saxon has evaluated an expression such as `A < B` as `min(A) lt max(B)`. This optimization is now done as a compile-time rewrite, rather than being decided at run-time. The expansion `min(A) lt max(B)` is now used only if neither operand is known to be a singleton. In an expression such as `PRICE > 3.50`, where it is known that the second operand is a singleton, the expansion used is now `some $p in PRICE satisfies $p gt 3.50`; this gives a better chance of finishing early than the min/max comparison. An important benefit of doing these rewrites at compile time, apart from simplifying the run-time code, is that the rewritten expression is a candidate for further optimization, for example it may be possible to move the computation of `min(A)` out of a loop.

A new optimization has been introduced to ensure that a recursive function that builds a sequence by incremental append operations now has linear performance: the time taken, and the memory used, are both proportional to the length of the sequence (and to the number of recursive calls). Note that it is useful to write such functions to be tail recursive. The optimization typically operates when a function returns a result that is obtained by appending to the sequence supplied as the argument. This optimization makes use of a new sequence representation (`ShareableSequence`) that allows many sequences to share the same underlying list in memory, provided that the value of each of the sequences is a leading sublist of this underlying list.

In an expression such as `A and f:call(P)`, the call on `f:call()` is now a candidate for tail-call optimization. This is achieved by rewriting the expression `A and B` as `if (A) then B else false()`.

The `XMLChar` module, originally taken from Xerces 2.5.0, has been replaced by the version from Xerces 2.7.1. This offers improved speed of initialization. The `XML11Char` from Xerces 2.7.1 is also now included. All checking of XML names is now done using a `NameChecker` obtained from the `Configuration` object; this uses the XML 1.0 or XML 1.1 rules depending on the XML version setting.

Next