10 Font-lock support for automatic font selection

Font-lock mode causes Emacs to select automatically the font in which text is displayed (“fontify” it) so as to indicate its logical status. See Font Lock mode in The Emacs Editor. The first version of font-lock support for REDUCE mode was contributed by Rainer Schöpf. The current version applies to both REDUCE mode and REDUCE Run mode (Running REDUCE in an Emacs window) and provides 4 strictly inclusive level: “Symbolic” includes “Algebraic”, which includes “Basic”, which includes “Minimal”. The level can be selected using the standard font-lock facilities, or interactively most easily via the REDUCE major mode Syntax Highlighting sub-menu. The levels and corresponding highlighting are as follows:

0. Minimal: strings are displayed using font-lock-string-face and “syntactic” comments (namely comments from % to the end of the line, and C-style comments between /* and */ delimiters) are displayed using font-lock-comment-face.

   In REDUCE Run mode (Running REDUCE in an Emacs window) REDUCE warning and error messages, and REDUCE (rtrace) and Lisp trace output, are displayed using font-lock-warning-face.

1. Basic: as minimal level and additionally comment statements are displayed using font-lock-comment-face; the main keywords including block delimiters and group delimiters are displayed using font-lock-keyword-face.
2. Algebraic: as basic level and additionally procedure names are displayed using font-lock-function-name-face and procedure parameters are displayed using font-lock-variable-face; general types such as “algebraic” are displayed using font-lock-type-face; local variable types such as “scalar” are displayed using font-lock-type-face and variable names declared local are displayed using font-lock-variable-name-face; goto and label names and named constants such as “pi” and “Catalan” are displayed using font-lock-constant-face; declared operator, operator type such as “linear”, vector, array and matrix declarations are displayed using font-lock-type-face; operator, vector, and array and matrix identifiers (with or without bounds) are displayed using font-lock-function-face.
3. Symbolic: as algebraic level and additionally preprocessor #-directives are displayed using font-lock-preprocessor-face; the key symbolic-mode “types” fluid and global are displayed using font-lock-type-face and the variable names declared fluid or global are displayed using font-lock-variable-name-face; all other quoted objects are displayed using font-lock-constant-face; asserted types are displayed using font-lock-type-face; “declare” and “struct” statements, as used in “redlog”, are displayed using font-lock-keyword-face, font-lock-function-name-face and font-lock-type-face; module, endmodule and other symbolic-mode keywords are displayed using font-lock-keyword-face; module names are displayed using font-lock-constant-face; key symbolic-mode functions such as “flag” and “get” are displayed using font-lock-builtin-face; other symbolic-mode “types” such as “switch” and “share” are displayed using font-lock-type-face; declared lambda parameters are displayed using font-lock-variable-face.

Using Emacs’ default settings gives symbolic-level highlighting, but if you find this too gaudy or too slow then you might prefer to select a lower level. Font-lock mode can be turned on interactively in the normal way that any minor mode is turned on, e.g. it can be toggled on and off by the command font-lock-mode. It can also be turned on and off via the REDUCE mode Syntax Highlighting sub-menu. To turn on font-lock mode automatically with REDUCE mode, put this in your .emacs file:

(add-hook 'reduce-mode-hook 'turn-on-font-lock)

To control the operation of font-lock mode, customize the appropriate options in the Font Lock group. The default level of fontification used by any mode can be specified by customizing the option font-lock-maximum-decoration, which REDUCE mode respects.

Emacs provides standard facilities to control the use of different display faces. See Using Multiple Typefaces in The Emacs Editor. See Faces in The GNU Emacs Lisp Reference Manual, for further technical detail. To alter the appearance of a Font Lock face, use the customization buffer for the Font Lock Highlighting Faces group. See Customizing Faces in The Emacs Editor.

REDUCE mode passes information to font-lock mode via the value of the buffer-local variable font-lock-defaults, which could be re-set or modified via the REDUCE mode hook, although this is not recommended.

For more information see the description of the command font-lock-mode and related commands and variables, and/or the ELisp source code file font-lock.el.

Font-locking of major syntactic elements, such as comments and strings, is normally controlled by the syntax table for the text being edited. This leads to a problem with a language such as REDUCE, because the character ! represents an escape character within an identifier but not within a string. This is different from the convention in the languages (C and Emacs Lisp) that Emacs was primarily designed to support, in which the significance of the escape character does not depend on the context. The solution I adopt in REDUCE mode is to reset the syntax of ! from escape to punctuation when it occurs immediately followed by a double quote, i.e. as !", but only within a string.