Prior to this change, every evaluation re-parses the expression. The
intermediate form as a parse tree is never stored anywhere. This change
adds a new token type, FUNCALL, which acts as a node in a parse tree, and
adds an (untested) evaluation method which returns the unevaluated parse tree
as opposed to evaluating as it goes.
This is preparation for a following step in which rather than storing a token
sequence for each variety of content, teapot will store a single token
representing the parse tree, allowing for quicker evaluation when the time
comes; no actual parsing will have to occur on evaluation.
Also a small amount of preparation of storing parse trees instead of
linear sequences of tokens got mixed in here, namely a new type of
token called a "FUNCALL".
The new function is X(SRC,REF), which briefly for a cell in the neighborhood
of REF returns the corresponding cell in the neighborhood in SRC. For
further details, see the updated documention.
In developing and documenting this function, I refined some of the
existing error messages (e.g. showing the coordinates when there is an
attempt to obtain a cell with a negative coordinate) and improved the
error propagation to increase the chance that the innermost error will
percolate to the top level.
Glen Whitney