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Chapter 6. Hemlock Programming

6.7. Commands

6.7.1. Introduction

The way that the user tells Hemlock to do something is by invoking a command. Commands have three attributes:


A command's name provides a way to refer to it. Command names are usually capitalized words separated by spaces, such as Forward Word.


The documentation for a command is used by on-line help facilities.


A command is implemented by a Lisp function, which is callable from Lisp. Defining Commands




Holds a string-table associating command names to command objects. Whenever a new command is defined it is entered in this table.


defcommand {command-name | (command-name function-name &key)} lambda-list command-doc {function-doc} {form}*


Defines a command named name. defcommand creates a function to implement the command from the lambda-list and forms supplied. The lambda-list must specify one required argument, see below, which by convention is typically named p. If the caller does not specify function-name, defcommand creates the command name by replacing all spaces with hyphens and appending "-command". Any keyword arguments are as for make-command. Command-doc becomes the command documentation for the command. Function-doc, if present, becomes the documentation for the function and should primarily describe issues involved in calling the command as a function, such as what any additional arguments are.


make-command name documentation function &key :transparent-p


Defines a new command named name, with command documentation documentation and function function. If :transparent-p is true, the command becomes transparent. The command in entered in the string-table *command-names*, with the command object as its value. Normally command implementors will use the defcommand macro, but this permits access to the command definition mechanism at a lower level, which is occasionally useful.


commandp command


Returns t if command is a command object, otherwise nil.


command-documentation command



command-function command



command-name command


Returns the documentation, function, or name for command. These may be set with setf. Command Documentation

Command documentation is a description of what the command does when it is invoked as an extended command or from a key. Command documentation may be either a string or a function. If the documentation is a string then the first line should briefly summarize the command, with remaining lines filling the details. Example:

(defcommand "Forward Character" (p)
  "Move the point forward one character.
   With prefix argument move that many characters, with negative
   argument go backwards."
. . .)

Command documentation may also be a function of one argument. The function is called with either :short or :full, indicating that the function should return a short documentation string or do something to document the command fully.

6.7.2. The Command Interpreter

The command interpreter is the functionality invoked by the event handler to process key-events from the keyboard and dispatch to different commands on the basis of what the user types. When the command interpreter executes a command, we say it invokes the command. The command interpreter also provides facilities for communication between contiguously running commands, such as a last command type register. It also takes care of resetting communication mechanisms, clearing the echo area, displaying partial keys typed slowly by the user, etc. Controlling The Command Interpreter

[Hemlock Variable]

Command Abort Hook


The command interpreter invokes the function in this variable whenever someone aborts a command (for example, if someone called editor-error). Editor Input

The canonical representation of editor input is a key-event structure. Users can bind commands to keys, which are non-empty sequences of key-events. A key-event consists of an identifying token known as a keysym and a field of bits representing modifiers. Users define keysym names by supplying names that reflect the legends on their keyboard's keys. Users define modifier names similarly, but the system chooses the bit and mask for recognizing the modifier. You can use keysym and modifier names to textually specify key-events and Hemlock keys in a #k syntax. The following are some examples:

#k"control-x meta-d"

This is convenient for use within code and in init files containing bind-key calls.

The #k syntax is delimited by double quotes. Within the double quotes, spaces separate multiple key-events. A single key-event optionally starts with modifier names terminated by hyphens. Modifier names are alphabetic sequences of characters which the system uses case-insensitively. Following modifiers is a keysym name, which is case-insensitive if it consists of multiple characters, but if the name consists of only a single character, then it is case-sensitive.

You can escape special characters---hyphen, double quote, open angle bracket, close angle bracket, and space---with a backslash, and you can specify a backslash by using two contiguously. You can use angle brackets to enclose a keysym name with many special characters in it. Between angle brackets appearing in a keysym name position, there are only two special characters, the closing angle bracket and backslash.

For more information on key-events see the Key-events section. Binding Commands to Keys

The command interpreter determines which command to invoke on the basis of key bindings. A key binding is an association between a command and a sequence of key-events. A sequence of key-events is called a key and is represented by a single key-event or a sequence (list or vector) of key-events.

Since key bindings may be local to a mode or buffer, the current environment determines the set of key bindings in effect at any given time. When the command interpreter tries to find the binding for a key, it first checks if there is a local binding in the current buffer, then if there is a binding in each of the minor modes and the major mode for the current buffer, and finally checks to see if there is a global binding. If no binding is found, then the command interpreter beeps or flashes the screen to indicate this.


bind-key name key &optional kind where


This function associates command name and key in some environment. Key is either a key-event or a sequence of key-events. There are three possible values of kind:

:global--- The default, make a global key binding.

:mode--- Make a mode specific key binding in the mode whose name is where.

:buffer--- Make a binding which is local to buffer where.

This processes key for key translations before establishing the binding.

If the key is some prefix of a key binding which already exists in the specified place, then the new one will override the old one, effectively deleting it.

do-alpha-key-events is useful for setting up bindings in certain new modes.


command-bindings command


This function returns a list of the places where command is bound. A place is specified as a list of the key (always a vector), the kind of binding, and where (either the mode or buffer to which the binding is local, or nil if it is a global).


delete-key-binding key &optional kind where


This function removes the binding of key in some place. Key is either a key-event or a sequence of key-events. kind is the kind of binding to delete, one of :global(the default), :mode or :buffer. If kind is :mode, where is the mode name, and if kind is :buffer, then where is the buffer.

This function signals an error if key is unbound.

This processes key for key translations before deleting the binding.


get-command key &optional kind where


This function returns the command bound to key, returning nil if it is unbound. Key is either a key-event or a sequence of key-events. If key is an initial subsequence of some keys, then this returns the keyword :prefix. There are four cases of kind:

:current--- Return the current binding of key using the current buffer's search list. If there are any transparent key bindings for key, then they are returned in a list as a second value.

:global--- Return the global binding of key. This is the default.

:mode--- Return the binding of key in the mode named where.

:buffer--- Return the binding of key local to the buffer where.

This processes key for key translations before looking for any binding.


map-bindings function kind &optional where


This function maps over the key bindings in some place. For each binding, this passes function the key and the command bound to it. Kind and where are the same as in bind-key. The key is not guaranteed to remain valid after a given iteration. Key Translation

Key translation is a process that the command interpreter applies to keys before doing anything else. There are two kinds of key translations: substitution and bit-prefix. In either case, the command interpreter translates a key when a specified key-event sequence appears in a key.

In a substitution translation, the system replaces the matched subsequence with another key-event sequence. Key translation is not recursively applied to the substituted key-events.

In a bit-prefix translation, the system removes the matched subsequence and effectively sets the specified bits in the next key-event in the key.

While translating a key, if the system encounters an incomplete final subsequence of key-events, it aborts the translation process. This happens when those last key-events form a prefix of some translation. It also happens when they translate to a bit-prefix, but there is no following key-event to which the system can apply the indicated modifier. If there is a binding for this partially untranslated key, then the command interpreter will invoke that command; otherwise, it will wait for the user to type more key-events.


key-translation key


This form is setf-able and allows users to register key translations that the command interpreter will use as users type key-events.

This function returns the key translation for key, returning nil if there is none. Key is either a key-event or a sequence of key-events. If key is a prefix of a translation, then this returns :prefix.

A key translation is either a key or modifier specification. The bits translations have a list form: (:bits {bit-name}*).

Whenever key appears as a subsequence of a key argument to the binding manipulation functions, that portion will be replaced with the translation. Transparent Key Bindings

Key bindings local to a mode may be transparent. A transparent key binding does not shadow less local key bindings, but rather indicates that the bound command should be invoked before the first normal key binding. Transparent key bindings are primarily useful for implementing minor modes such as auto fill and word abbreviation. There may be several transparent key bindings for a given key, in which case all of the transparent commands are invoked in the order they were found. If there no normal key binding for a key typed, then the command interpreter acts as though the key is unbound even if there are transparent key bindings.

The :transparent-p argument to defmode determines whether all the key bindings in a mode are transparent or not. In addition a particular command may be declared to be transparent by the :transparent-p argument to defcommand and make-command.

6.7.3. Command Types

In many editors the behavior of a command depends on the kind of command invoked before it. Hemlock provides a mechanism to support this known as command type.




This returns the command type of the last command invoked. If this is set with setf, the supplied value becomes the value of last-command-type until the next command completes. If the previous command did not set last-command-type, then its value is nil. Normally a command type is a keyword. The command type is not cleared after a command is invoked due to a transparent key binding.

6.7.4. Command Arguments

There are three ways in which a command may be invoked: It may be bound to a key which has been typed, it may be invoked as an extended command, or it may be called as a Lisp function. Ideally commands should be written in such a way that they will behave sensibly no matter which way they are invoked. The functions which implement commands must obey certain conventions about argument passing if the command is to function properly. The Prefix Argument

Whenever a command is invoked it is passed as its first argument what is known as the prefix argument. The prefix argument is always either an integer or nil. When a command uses this value it is usually as a repeat count, or some conceptually similar function.




This function returns the current value of the prefix argument. When set with setf, the new value becomes the prefix argument for the next command. If the prefix argument is not set by the previous command then the prefix argument for a command is nil. The prefix argument is not cleared after a command is invoked due to a transparent key binding. Lisp Arguments

It is often desirable to call commands from Lisp code, in which case arguments which would otherwise be prompted for are passed as optional arguments following the prefix argument. A command should prompt for any arguments not supplied.

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