Emacs - Understand elisp syntax (mapcar '1+ '(2 4 6))

It's really a function, which means add 1. you can try below example:

ELISP> (1+ 3)
4 (#o4, #x4, ?\C-d)

The implementation is at data.c:

DEFUN ("1+", Fadd1, Sadd1, 1, 1, 0,
       doc: /* Return NUMBER plus one.  NUMBER may be a number or a marker.
Markers are converted to integers.  */)
  (register Lisp_Object number)
{
  CHECK_NUMBER_OR_FLOAT_COERCE_MARKER (number);

  if (FLOATP (number))
    return (make_float (1.0 + XFLOAT_DATA (number)));

  XSETINT (number, XINT (number) + 1);
  return number;
}

'1+ is a reference to the function stored in that symbol, in this case it returns number incremented by 1. mapcar applies it to every element of the list, and then returns the resulting list.

+ has an arity of 2. It needs both (left-hand side) LHS and RHS. RHS comes from the list. Where does LHS come from? That's where 1 comes in.

You can replace it with '9+ and each element is incremented by 9.

(mapcar '9+ '(2, 4, 6)) 

Or you can replace the whole '1+ with a function that takes only one argument, such as:

(mapcar 'exp '(2, 4, 6))  

(9+ 2) or (2+ 2) does't work, no such function defined at all. –beetlej

That is because 9+ is neither a function nor a symbol at that position in the syntax. The correct syntax for the addition function is:

(+ 9 2)

or

(+ 2 2)

But '1+ worked previously because mapcar is expecting a function for the first argument and a sequence for the second argument. Both are supplied as quoted symbols.

(mapcar '1+ '(2, 4, 6))

Symbols reference is here.