CODEWITHSUNDEEP
compilationmacroscommon-lisp
Mooncrater
Mooncrater

Reputation: 4861

Compilation and Macros

This is a quotation on the topic "compilation and macro expansion", from the book Common Lisp: A gentle introduction to symbolic computation.

(defmacro bad-announce-macro ()
(format t "~%Hi mom!"))
(defun say-hi ()
(bad-announce-macro))
> (compile 'say-hi)
Hi, mom!
SAY-HI
> (say-hi)
NIL

In the above example the macro was expanded as part of the process of compiling SAY-HI. So the compiler said ‘‘Hi, mom!’’ The result of the macro was NIL, so that’s what got compiled into the body of SAY-HI. When we call the compiled SAY-HI function, it says nothing because the macro has been replaced with its expansion.

In this quotation, the author says that

the macro has been replaced with its expansion

Okay, so shouldn't it atleast show the printed "Hi mom!" ? Because though the macro doesn't return anything, but it still replaced by something (it's expansion). Based on the code, I hypothesized that
when a function is called after it is compiled, all the macros, that are called in it's body, are expanded into the result they return , not into whatever they have in their own bodies.

I'm not sure if this is right. And the reason for doing this is also not clear.

Upvotes: 0

Views: 78

Answers (1)

Rainer Joswig
Rainer Joswig

Reputation: 139241

How often a macro is expanded is unspecified.

Interpreted Lisp in LispWorks:

CL-USER 49 > (say-hi)

Hi mom!
Hi mom!
NIL

The macro expansion is done twice at runtime here.

In compiled code we expect that no macro expansion is necessary at runtime. Thus in your example nothing will be printed, since your generated code does nothing:

CL-USER 50 > (compile 'say-hi)

Hi mom!
SAY-HI
NIL
NIL

CL-USER 51 > (say-hi)
NIL

The macro expansion is NIL.

CL-USER 52 > (macroexpand '(bad-announce-macro))

Hi mom!
NIL        ; <- the macro expansion
T

Upvotes: 2

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