Parenthesis Matching

MrEd provides general-purpose ``parenthesis''-matching utilities that work on buffers. The utilities are paramerized with respect to:

• parens -- Pairs (cons cells) of opening and closing bracket strings. These brackets can be nested and must be balanced. The opening and closing string do not have to be different.
• quotes -- Pairs of opening and closing quote bracket strings. Within a pair of quotes, all other bracket, quote, and comment strings are ignored. Pairs of quote strings do not have to correspond to quotes in the language. For example, C comments using ``/*'' and ``*/'' are considered quotes for parenthesis-matching purposes.
• comments -- A list of comment-starting strings. Comments of this form run until the end of the line.

A backslash (\) is assumed to be the (only) method for escaping parenthesis, quote, and comment markers. The parenthesis-balancing utilities are only guaranteed to act meaningfully when the starting position is outside quoted and commented expressions. When a pair of opening and closing brackets are not distinct, the actions on these brackets are only meaningful for searches starting outside the brackets.

• (mred:forward-match edit start end parens quotes comments) -- Returns the position in edit that ``closes'' the text at start, or #f if no closing position is found (either because a parenthesis mis-match is discovered or the end boundary was reached). The match must occur before end (inclusive).

  Spaces immediately following start are skipped. If the text at start is an open parenthesis or open quote, then the matching position is the closing parenthesis or quote. If a comment immediately follows start, it is skipped over as whitespace. If a closing parenthesis immediately follows start (after skipping whitespace), then #f is returned. Otherwise, the matching position is the position before the first whitespace, parenthesis, quote, or comment character after start.

  If cache is not #f, it must be an instance of mred:match-cache%. A cache object can be used to speed up successive calls to mred:forwardward-match. However, a buffer using a cache must call the cache's forward-invalidate method when the buffer is modified. Different caches should be used for forward and backward matching. See the match cache section for more information.

  The cache parameter is optional. It defaults to #f.

• (mred:backward-match edit start end parens quotes comments containing? cache) -- Returns the position in edit that ``opens'' the text ending at start, or #f if no opening position is found (either because a parenthesis mis-match is discovered or the end boundary was reached). The match must occur before end (inclusive). Note that start > end, since start specifies the starting position of the search, not the earliest buffer position to be considered.

  Spaces immediately preceding start are skipped. If the text at start is an close parenthesis or close quote, then the matching position is the openng parenthesis or quote. If a comment immediately precedes start, then the comment is skipped as whitespace. If an opening parenthesis immediately precedes start, then the matching position is start - 1. Otherwise, the matching position is the first whitespace or parenthesis character before start.

  If containing? is not #f, then the matching procedure is modified as follows:

  • Searching iterates backwards until some search fails. Then, the location of the last sucessful search is returned.
  • If a mis-match is detected, then #f is returned.
  • If there are no matches (and no mis-matches) before start, start itself is returned.

  If cache is not #f, it must be an instance of mred:match-cache%. A cache object can be used to speed up successive calls to mred:backward-match. However, a buffer using a cache must call the cache's invalidate method when the buffer is modified. Different caches should be used for forward and backward matching. See the match cache section for more information.

  The containg? and cache parameters are optional. They both default to #f.

• (mred:balanced? edit start end parens quotes comments) -- Returns #t if the text edit between positions start and end is balanced. The text is balanced if there are no unclosed parenthses or quotes, there are no closing parentheses that do not match an open parenthesis, and there are no mis-matched parentheses.

The standard parenthesis, quote, and comment parameters for Scheme are defined in mred:scheme-paren-pairs, mred:scheme-quote-pairs, and mred:scheme-paren-comments. These lists are used to define Scheme-specific matching function:

• (mred:scheme-forward-match edit start end cache)

• (mred:scheme-backward-match edit start end cache)

• (mred:scheme-backward-containing-sexp edit start end [cache #f]) -- Returns the beginning of the interior of the (non-atomic) S-expression containing start.

• (mred:scheme-balanced? edit start end)