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(emacs-lisp-intro.info)Parts of let Expression

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The Parts of a `let' Expression
-------------------------------

   A `let' expression is a list of three parts.  The first part is the
symbol `let'.  The second part is a list, called a "varlist", each
element of which is either a symbol by itself or a two-element list,
the first element of which is a symbol.  The third part of the `let'
expression is the body of the `let'.  The body usually consists of one
or more lists.

   A template for a `let' expression looks like this:

     (let VARLIST BODY...)

The symbols in the varlist are the variables that are given initial
values by the `let' special form.  Symbols by themselves are given the
initial value of `nil'; and each symbol that is the first element of a
two-element list is bound to the value that is returned when the Lisp
interpreter evaluates the second element.

   Thus, a varlist might look like this: `(thread (needles 3))'.  In
this case, in a `let' expression, Emacs binds the symbol `thread' to an
initial value of `nil', and binds the symbol `needles' to an initial
value of 3.

   When you write a `let' expression, what you do is put the
appropriate expressions in the slots of the `let' expression template.

   If the varlist is composed of two-element lists, as is often the
case, the template for the `let' expression looks like this:

     (let ((VARIABLE VALUE)
           (VARIABLE VALUE)
           ...)
       BODY...)