CLHS: Function MACROEXPAND, MACROEXPAND-1

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Function MACROEXPAND, MACROEXPAND-1

Syntax:

macroexpand form `&optional` env => expansion, expanded-p

macroexpand-1 form `&optional` env => expansion, expanded-p

Arguments and Values:

form---a form.

env---an environment object. The default is nil.

expansion---a form.

expanded-p---a generalized boolean.

Description:

macroexpand and macroexpand-1 expand macros.

If form is a macro form, then macroexpand-1 expands the macro form call once.

macroexpand repeatedly expands form until it is no longer a macro form. In effect, macroexpand calls macroexpand-1 repeatedly until the secondary value it returns is nil.

If form is a macro form, then the expansion is a macro expansion and expanded-p is true. Otherwise, the expansion is the given form and expanded-p is false.

Macro expansion is carried out as follows. Once macroexpand-1 has determined that the form is a macro form, it obtains an appropriate expansion function for the macro or symbol macro. The value of *macroexpand-hook* is coerced to a function and then called as a function of three arguments: the expansion function, the form, and the env. The value returned from this call is taken to be the expansion of the form.

In addition to macro definitions in the global environment, any local macro definitions established within env by macrolet or symbol-macrolet are considered. If only form is supplied as an argument, then the environment is effectively null, and only global macro definitions as established by defmacro are considered. Macro definitions are shadowed by local function definitions.

Examples:

     (defmacro alpha (x y) `(beta ,x ,y)) =>  ALPHA
     (defmacro beta (x y) `(gamma ,x ,y)) =>  BETA
     (defmacro delta (x y) `(gamma ,x ,y)) =>  EPSILON
     (defmacro expand (form &environment env)
       (multiple-value-bind (expansion expanded-p)
           (macroexpand form env)
         `(values ',expansion ',expanded-p))) =>  EXPAND
     (defmacro expand-1 (form &environment env)
       (multiple-value-bind (expansion expanded-p)
           (macroexpand-1 form env)
         `(values ',expansion ',expanded-p))) =>  EXPAND-1

    ;; Simple examples involving just the global environment
     (macroexpand-1 '(alpha a b)) =>  (BETA A B), true
     (expand-1 (alpha a b)) =>  (BETA A B), true
     (macroexpand '(alpha a b)) =>  (GAMMA A B), true
     (expand (alpha a b)) =>  (GAMMA A B), true
     (macroexpand-1 'not-a-macro) =>  NOT-A-MACRO, false
     (expand-1 not-a-macro) =>  NOT-A-MACRO, false
     (macroexpand '(not-a-macro a b)) =>  (NOT-A-MACRO A B), false
     (expand (not-a-macro a b)) =>  (NOT-A-MACRO A B), false

    ;; Examples involving lexical environments
     (macrolet ((alpha (x y) `(delta ,x ,y)))
       (macroexpand-1 '(alpha a b))) =>  (BETA A B), true
     (macrolet ((alpha (x y) `(delta ,x ,y)))
       (expand-1 (alpha a b))) =>  (DELTA A B), true
     (macrolet ((alpha (x y) `(delta ,x ,y)))
       (macroexpand '(alpha a b))) =>  (GAMMA A B), true
     (macrolet ((alpha (x y) `(delta ,x ,y)))
       (expand (alpha a b))) =>  (GAMMA A B), true
     (macrolet ((beta (x y) `(epsilon ,x ,y)))
       (expand (alpha a b))) =>  (EPSILON A B), true
     (let ((x (list 1 2 3)))
       (symbol-macrolet ((a (first x)))
         (expand a))) =>  (FIRST X), true
     (let ((x (list 1 2 3)))
       (symbol-macrolet ((a (first x)))
         (macroexpand 'a))) =>  A, false
     (symbol-macrolet ((b (alpha x y)))
       (expand-1 b)) =>  (ALPHA X Y), true
     (symbol-macrolet ((b (alpha x y)))
       (expand b)) =>  (GAMMA X Y), true
     (symbol-macrolet ((b (alpha x y))
                       (a b))
       (expand-1 a)) =>  B, true
     (symbol-macrolet ((b (alpha x y))
                       (a b))
       (expand a)) =>  (GAMMA X Y), true

    ;; Examples of shadowing behavior
     (flet ((beta (x y) (+ x y)))
       (expand (alpha a b))) =>  (BETA A B), true
     (macrolet ((alpha (x y) `(delta ,x ,y)))
       (flet ((alpha (x y) (+ x y)))
         (expand (alpha a b)))) =>  (ALPHA A B), false
     (let ((x (list 1 2 3)))
       (symbol-macrolet ((a (first x)))
         (let ((a x))
           (expand a)))) =>  A, false

Affected By:

defmacro, setf of macro-function, macrolet, symbol-macrolet

Exceptional Situations: None.

See Also:

*macroexpand-hook*, defmacro, setf of macro-function, macrolet, symbol-macrolet, Section 3.1 (Evaluation)

Notes:

Neither macroexpand nor macroexpand-1 makes any explicit attempt to expand macro forms that are either subforms of the form or subforms of the expansion. Such expansion might occur implicitly, however, due to the semantics or implementation of the macro function.

The following X3J13 cleanup issues, not part of the specification, apply to this section:

- FUNCTION-TYPE:X3J13-MARCH-88
- MACROEXPAND-RETURN-VALUE:TRUE

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