• nil
  1. symbol with name "nil"
  2. logical truth value false
  3. empty list
• t
  1. truth value t
  2. symbol "t"
• booleanp obj return non-=nil= if obj is t or nil, otherwise not

• emacs-version &optional here function
• emacs-build-time variable
• emacs-version variable
• emacs-major-version variable
• emacs-minor-version variable

the read syntax of basic char is ?<X>, <X> stands for the character to be represented, so ?A stands for 'A'

eq obj1 obj2

return t if obj1 and obj2 are the same object, otherwise return nil integers with same value are considered to be the same object, so (eq 1 1) will return t

exception: (eq "abc" "abc") will return nil, but (eq "" "") will return t because the empty string is only stored one copy as expected, (eq '(1 2 3) '(1 2 3)) will return nil

equal obj1 obj2

return t if obj1 and obj2 have equal components, otherwise return nil

unlike eq, equal will look into arguments to check if there contents are the same, if obj1 and obj2 are eq, they must be equal. so, as expected, (equal '(1 2 3) '(1 2 3)) and (equal "abc" "abc") will both return t

• integer
• float

• floatp object
• integerp object
• numberp object
• natnump object : returns t if object is a natural number, such as 0, 1, 2…
• zerop object

• max num1 &rest nums return the maximum value
• min num1 &rest nums return the minimun value

• float number convert the number to float type
• truncate number &optional divisor convert the number to integer by rounding towards zero
• floor number &optional divisor convert the number to integer by rounding towards negative infinity
• ceiling number &optional divisor convert the number to integer by rounding towards positive infinity
• round number &optional divisor convert the number to integer by rounding towards nearest integer

• random &optional limit returns a pseudo-random integer

  if limit is a positive integer, the value is chosen in [0, limit), and of course only choose integer

  if limit is t, it will choose a new seed, otherwise Emacs will always use the same seed, so the returned pseudo-random integer sequences are always the same

• stringp object
• string-or-null-p object
• char-or-string-p object

• substring : to get substring
• concat : to concatenate strings together
• split-string : split string into several strings
• string= / string-equal : judge the equality of two strings
• string-prefix-p : check if a string is a prefix of another
• downcase / upcase : change a string or a character to opposite case

a cons cell has two slots, the first is called CAR and the second is called CDR, and it may looks like the structure below:

struct cons_cell {
    void * CAR;
    void * CDR;
};

so the two slots can hold any values, and lists are built up from cons cell, which stores object pointer in CAR slot, and stores the nexe node pointer in CDR slot

• car / cdr
• pop
• nth / nthcdr
• last

• cons
• list
• make-list
• append
• reverse
• number-sequence

• push
• add-to-list
• setcar / setcdr
• nconc &rest lists : return a list containing all the elements of lists, last CDR of each given list is set to point to next list
• memq object list : test if object is in list, if is in, return the sublist from the position object occurs the first time
• delq object list : delete all occurrences of object in list, note that it uses eq to check if two objects are equal, same as memq
• remq object list : returns a list copy with all elements eq to object removed
• member object list : like memq, only difference is it uses equal to check the equality of two objects
• delete object sequence : like delq, but uses equal for comparison
• remove object sequence : like delete, but it always returns a copy of processed sequence, no matter sequence is a list, vector, or a string

alist for short, it is a list consists of cons cells, that is to say, every element of alist is a cons cell, the CAR of the cons cell stores key while the CDR stores value

• assoc key alist : returns the first occurrence of key in alist, uses equal for comparison, note that it returns the whole cons cell, not only the CDR
• rassoc value alist : like assoc, but uses CDR for comparison, assoc uses CAR
• assq key alist : like assoc, but uses eq for comparison
• rassq value alist : …
• assq-delete-all / rassq-delete-all : …

• sequencep
• length
• elt sequence index : returns the element of sequence at index

• arrayp
• aref array index : returns element of array at index
• aset array index object : set object to the index element of array
• fillarray array object : fills array with object

• vectorp
• vector &rest objects : creates a vector with objects
• make-vector length object : makes a vector has length and filled by object
• vconcat &test sequences : returns a new vector containing all elements in sequences

not so useful, so skip them

each symbol has four components:

• print name: the name of the symbol
• value: symbol's current value as a variable
• function: symbol's function definition, can also hold a symbol, a keymap, or a keyboard macro
• property list: symbol's property list

symbols are stored in a vector called obarray, symbol name and symbol is with a one-to-one mapping relationship interning a symbol means hash it and put it into obarray, or find it from obarray, so there will never be two symbols with same name in one obarray, or two names pointing to one symbol.

*intern* : find a symbol from obarray, or create a new symbol and put it into obarray. so the finding action is also a interning operation

of course there are uninterned symbols, it means they are not put into any obarray, they may exist in some other object or as variable values

• symbol-name
• make-symbol
• intern name &optional obarray : returns the interned symbol with name name, creates a new one and puts it into obarray if no symbol found
• intern-soft name &optional obarray : returns the already interned symbol with name name, returns nil if not found
• mapatoms function &optional obarray : calls function once with each symbol in obarray, then returns nil

property list (so called plist) is like association list (alist), it is also key-value pair data structure, but it often uses symbol as the key, so a plist may always have unique keys, while alist may not.

• symbol-plist symbol
• setplist symbol plist
• get symbol property
• put symbol property value
• plist-get plist property
• plist-put plist property value
• plist-member plist property

how Emacs evaluates a form depends on its data type: symbols, lists, and "all other data types"

• self-evaluating forms: "all other data types"

  the result of self-evaluating is the object itself, so 25 will be evaluated to 25, "abc" will be evaluated to "abc"

• symbol forms: will be evaluated as a variable, so its value field will be returned
• list forms: the evaluating way of a list is determined by its first element, if first element is:
  • a symbol: the symbol's function field will be obtained and replace the symbol with that function
  • a function: evaluates the remaining elements of the list first, and then call the function with these evaluated remaining elements as parameters
  • a macro: evaluates the macro with the remaining elements as parameters without evaluating the remaining elements
  • special forms: such as if, and, evaluation of elements in the list depends on the special forms' syntax requirements

• let bindings… forms…
• let* bindings… forms… : like let, difference is a local variable binding will take effect immediately before next local variable binding takes place
• makeunbound symbol : cancel the symbol's binding to a value
• boundp variable : to judge if the variable is bound, returns t if its value is not void

global variables can be defined by defvar and defconst, they do not have actually difference, defconst is intent to inform human readers that the variable may should not be changed, but actually its value can be changed

• mapcar function sequence : applies function to each element in sequence in turn, return a list of results
• mapc function sequence : like mapcar, but it is used for side effects, return value will be ignored
• mapconcat function sequence separator : like mapcar, but function's result must be string, and the strings will be concatenated as return value