open-input-text-editor in Editors

• (open-input-text-editor text-editor start-position end-position snip-filter) -> input-port
     text-editor : text% object
     start-position = 0 : exact non-negative integer
     end-position = 'end : exact non-negative integer or 'end
   snip-filter = (lambda (s) (values s 1)) : a procedure of one argument: a snip% object

  Creates an input port that draws its content from text-editor. The editor content between positions start-position and end-position is the content of the port. If end-position is 'end, the content runs until the end of the editor. If a snip that is not a string-snip% object spans start-position or end-position, the entire snip contributes to the port. If a string-snip% instance spans start-position, only the part of the snip after start-position contributes, and if a string-snip% object spans end-position, only the part before end-position contributes.

  An instance of string-snip% in text-editor generates a character sequence in the resulting port. All other kinds of snips are passed to snip-filter to obtain a ``special'' value for the port, plus the special value's span. If a snip is returned as the first result from snip-filter, and if the snip is an instance of readable-snip<%>, the snip generates a sequence of special values (and spans) for the port through the read-one-special method. If snip-filter returns any other kind of snip, it is copied for the special result, and the second result of snip-filter is used as the span. Finally, a non-snip first result from snip-filter is used directly as the special result, and the second value from snip-filter is the span.