Elisp

Lisp stands for LISt Processing. Elisp is the dialect of Lisp used in emacs, a text editor.

In Lisp, both data and programs are represented the same way: they are both lists of words, numbers, or other lists, sparated by whitespace and surrounded by parentheses.

Parts of lisp

Expression

Lisp programs are made up of expressions: lists or single atoms

Atom

Atoms are multi-character symbols, like forward-paragraph, single character symbols like +, strings of characters between double quotation marks, or numbers. Arrays are also considered to be an atom. Any object that is not a Cons Cell is an atom.

List

Made up of zero or more atoms or inner lists, separated by whitespace and surrounded by parentheses. A list can be empty.

String

Between quotation marks

Symbol

Looks like a word. A name that can have a value attached to it just as it can have a function definition attached to it. It can have both at the same time. Or it can have just one or the other.

Number

Evaluates to itself.

Function

A set of instructions to the computer that tell the computer to do something

Variable

A symbol that is not quoted and that doesn't have parentheses around it. We bind a variable to a value.

Special forms

Some functions are unusual and do not work the usual manner. They are called special forms. Used for special jobs like defining a function, or 'if'.

Quote

A single-quote ‘'’ tells the Lisp interpreter that it should return the following expression as written, and not evaluate it as it would if the quote were not there.

Macro

A construct which differs from a function in that it translates a lisp expression into another expression that is to be evaluated in place of the original expression, e.g. 'when'.

Argument (of a function)

Information passed to the function. Atoms or lists that follow the function.

Predicate

A function to determine whether some preperty is true or false.

Comment

It starts with ;

Interactive function

A function we evaluate by typing M-x or through a key or keychord.

Buffer

Place where we do things. It's not a file, but it can be visiting a file.

car (first)

Reports what the first item in the list is.

cdr (rest)

The part of the list that follows the first item.

Cons cell

An object that consists of two slots, the car slot and the cdr slot. Each slot can hold any lisp object.

List

A series of cons cells, linked together so that the CDR slot of each cons cell holds either the next cons cell or the empty list.

Dotted pair notation

General syntax for cons cells that represents the CAR and CDR explicitly. (a . b) stands for a cons cell whose car is the object a and whose CDR is the object b.

Association list

Alist records a mapping from keys to values. It is a list of cons cells called associations. The CAR of each cons cell is the key, and the CDR is the associated value.

Collection of one or more files, formatted and bundled in such a way that users can easily download, install, uninstall and upgrade it.

Evaluating parts of lisp

• A ' in front of a list is called a quote. It tells lisp to do nothing with the list other than take it as it is written.
• When there's no quote preceeding a list, it is a command for the computer to obey.
• When you evaluate a symbol by itself, its value is returned.
• When you evaluate a quoted variable (e.g. 'flower), what we will see in the echo area is the symbol itself, flower.
• The interpreter works on the innermost list first and the works on the outside list.
• Evaluating a symbolic expression most commonly causes the interpreter to return a value and perhaps carry out a side effect; or produce an error.

Useful functions

• (message "this message appears in the echo area") ; we can use %d for integers, %s for strings
• (set 'flowers '(rose violet daisy buttercup)) ; Set the value of the symbol flower to the list. It returns the value (list) and binds the symbol flower to the list as a side-effect.
• (setq flowers '(rose violet daisy buttercup))special form that quotes the first argument automatically. Can set multiple values directly.
• (defun function-name (arguments...) "optional doc" (interactive) body...) macro to define a function definition
• (let varlist body...) special form to bind a symbol to a value. It creates a local variable.
• (if true-or-false-test then-part else-part) special form for conditional.
• cons, car, cdr, nthcdr, nth, setcar, setcdr...
• (eval '(+ 2 2)) to evaluate a quoted expression.

Other things

• C-u allows to pass numeric arguments to interactive functions
• False is nil, anything else is true.
• Nil has two meanings: false and empty list.
• Quote returns data which you should not modify, and which may share structure between themselves. It's data part of the program. Do not change it!
• Lists and quotes are different. quote “distributes” over its arguments, so '(+ 1 2) is like (list '+ '1 '2)

Emacs specific functions

• load-file loads an elisp file.
• load-library loads with a library name (installed in load-path)
• load similar to load-library but lower-level. Same file can be loaded repeatedly.
• require ensure the library is loaded once only.
• autoload allows you to pre-load a function without installing it by specifing its name, its library file, documentation and if it should be interactive.