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(librep.info)Backquoting

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Backquoting
-----------

   As seen in the previous sections, macros are a very powerful
mechanism of defining new control structures. However due to the need
to create the expansion, i.e. the form that will be actually evaluated,
they can often be complex to write and understand.

   We have already seen that constants may be produced through the use
of the quote-mark (Note: Quoting), here another form of quoting is
described, where only some of the quoted object is actually constant.
This is known as "backquoting", since it is introduced by the backquote
character ``', a shortcut for the `backquote' macro.

 - Macro: backquote arg
     Constructs a new version of ARG (a list). All parts of LIST are
     preserved except for expressions introduced by comma (`,')
     characters, which are evaluated and spliced into the list. For
     example:

          `(1 2 ,(+ 1 2))
              => (1 2 3)

     Also, the `,@' prefix will splice the following _list_ into the
     output list, at the same level:

          `(1 2 ,@(list 3))
              => (1 2 3)

   Backquoting allows macros expansions to be created from static
templates. For example the `when' macro shown in the previous sections
can be rewritten as:

     (defmacro when (condition #!rest body)
       `(cond (,condition ,@body)))

which is easier to read, since it is a lot closer to the actual
expansion.