Complex Patterns and Advanced Features

Due to the complex inner workings of the Notation package it is helpful to outline some of the more advanced features and structures of the Mathematica front end and how they relate to the Notation package. The following sections give a small overview of the functionality of tag boxes the specific tags used by the Notation package and the tag box option SyntaxForm.

The reader should be familiar with the concepts in Textual Input and Output and moreover understand the following tutorials: The Representation of Textual Forms, The Interpretation of Textual Forms, Representing Textual Forms by Boxes, String Representation of Boxes, Converting between Strings, Boxes, and Expressions, and Low-Level Input and Output Rules.

A TagBox is a box structure just like RowBox, SubscriptBox, GridBox. It is used to change the structure of an expression or indicate a grouping or interpretation of a subexpression at an underlying level. To illustrate tag boxes consider the following input which contains an embedded TagBox.

All Mathematica input and output is made up of box structures at a low level. When Mathematica receives input, these box structures are parsed into internal expressions, which can be thought of as full form expressions. Internal evaluation then takes place, and finally the internal structures are transformed back into box structures for displaying in the Mathematica front end. We can reveal how Mathematica sees this input at a low level by using the Show Expression command under the Cell menu.

The above expression contains a subexpression TagBox[SuperscriptBox["x", "2"], foo]]. It is important to note that this box expression as normally viewed in Mathematica looks visually like x² even though it has an embedded TagBox. Information contained in the tag is visually hidden from the user. When an expression containing a TagBox is input into Mathematica the default interpretation of the subexpression surrounded by the TagBox is to wrap the TagBox name around the parsed subexpression, in this case to wrap foo around x^2.

However you can define your own rules for the way specific tag boxes are parsed. For instance by using the low level function MakeExpression we can change how Mathematica will parse expressions containing TagBoxes.

Knowing that special behaviors can be set up for specific tags we can now examine the tags defined by the Notation package. The Notation package defines three tags that have special behaviors: NotationTemplateTag, NotationPatternTag, and NotationMadeBoxesTag. These are all string tags for two reasons. One, by using string tags we can avoid any potential problems to do with package contexts and redefinition of the symbols. Secondly, in Mathematica if a TagBox has a string tag and there is a named-style on the stylesheet path with the same name as the string tag then the TagBox will be displayed in that style. This lets us omit the BaseStyle option from the TagBox and consequently our box structures are smaller and more readable.

NotationTemplateTag is a string tag used by the Notation package to grab box structures before they can be parsed by Mathematica. In fact NotationTemplateTag acts rather like the tag literalBoxes defined above. All Notation , Symbolize and InfixNotation templates on the notation palette contain tag boxes with an embedded string tag: NotationTemplateTag. The embedded TagBox ensures that the Notation package can obtain the correct parsing information and retain the proper styling and grouping information. This embedded tag is used to capture the box structure, and the captured structure is thus wrapped with a "ParsedBoxWrapper".

For normal purposes it is usually sufficient that the patterns present in Notation and Symbolize statements are simple patterns. However, it is sometimes necessary or desirable to use more complicated patterns in notations. For example, a notation might only be valid when a certain pattern is a number. To allow more complex patterns you can embed a NotationPatternTag tag box inside a notation statement. It is critical that any notation you define which uses a complex pattern has an embedded NotationPatternTag, otherwise the pattern will be treated as a verbatim expression and not function as a pattern. Like NotationTemplateTag this should be a string tag. The notation palette has a button labeled InsertPatternWrapper which will embed a NotationPatternTag around the selection, as well as tint the background of the selection to indicate that a complex pattern is present. (This tinting occurs as a result of the named style NotationPatternTag since this is a string tag.)

It must also be pointed out that the pattern matching on the external representation is performed on the box structures, so usually you will have to make small transformations to convert box structures into normal expressions. Pattern matching on the internal representation follows conventional pattern matching.

You should be careful to avoid unwanted evaluation through testing functions when parsing expressions (see parsing without evaluation.)

We can see that the patterns a_?StringNumericQ and a_?NumericQ do not appear literally since they were surrounded by a NotationPatternTag in the notation statement.

The tag NotationMadeBoxesTag is intended for advanced users. It is also a string tag. It is used to indicate that box processing and formatting has already been done and that the Notation package should not perform any processing. Typically you would use this tag for surrounding your own functions that return expressions that have already been turned into boxes or parsed into expressions. To illustrate the tag NotationMadeBoxesTag we can examine a notation statement that might be part of a number of statements used to create a notation for Tensors.

We can see from the internal definition returned that there is no further processing of the expression createGridBox[inds], i.e. it is not surrounded by a MakeBoxes[..., StandardForm].

Using the option SyntaxForm you can change the precedence of an expression containing a TagBox. A tag box containing a SyntaxForm option will look like TagBox[box structure, tag, SyntaxForm -> string], where string is a string indicating the operator on which the precedence of the tag box is modeled. The following examples illustrate the SyntaxForm option .

We can illustrate the underlying groupings of the expressions above in the following table.

The SyntaxForm option value can be any operator string valid in Mathematica, that is, any operator contained in the UnicodeCharacters.tr file. The SyntaxForm value can also include symbols before and after the operator to indicate whether the precedence is that of a prefix operator, an infix operator, or a postfix operator. Some typical values for the SyntaxForm option are given in the table below.