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Many of the actions performed by an editor is described in terms of the syntactically unit(s) they affect. The syntax module is responsible for actually dividing the buffer into syntactical units, but the unit protocol is the basic interface for acting on these units. A unit is some single syntactical construct - for example a word, a sentence, an expression or a definition. The unit protocol defines a number of generic functions for the various unit types that permit a uniform interface to moving a mark a given number of units, deleting a unit, killing a unit, transposing two units and so forth. A number of macros are also provided for automatically generating all these functions, given the definition of two simple movement functions. All generic functions of the unit protocol dispatch on a syntax, so that every syntax can implement its own idea of what exactly, for example, an “expression” is. Defaults are provided for some units - if nothing else has been specified by the syntax, a word is considered any sequence of alphanumeric characters delimited by non-alphanumeric characters. The type of unit that a protocol function affects is represented directly in the name of the function - this means that a new set of functions must be generated for every new unit you want the protocol to support. In most cases, the code for these functions is very repetitive and similar across the unit types, which is why the motion protocol offers a set of macros that can generate function definitions for you. These generator macros define their generated functions in terms of basic motion functions.
A basic motion function is a function named FORWARD-ONE-unit
or backward-one-unit of the signature (mark
syntax) that returns true if any motion happened or false if a
limit was reached.
There isn't really a single all-encompassing unit protocol, instead, it
is divided into two major parts - a motion protocol defining functions
for moving point in terms of units, and an editing protocol for changing
the buffer in terms of units. Both use a similar interface and a general
mechanism for specifying the action to take if the intended operation
cannot be carried out.
Note that forward-object and backward-object, by virtue of
their low-level status and placement in the buffer protocol (see
buffer.lisp) do not obey this protocol, in that they have no syntax
argument. Therefore, all frob-object functions and commands
(see Editing Protocol) lack this argument as well. There are no
forward-one-object or backward-one-object functions.