Basic Lisp Macro error

Question

Little help here please . I am trying to create this lisp macro which takes a list (of numbers) as input and returns the sum of those numbers. The code

(setf g (list 1 2 3 4))

(defmacro add-test(var)
    `(+ ,@var))

(add-test g) gives this error

The value G is not of type LIST.
[Condition of type TYPE-ERROR]

At the same time (add-test (1 2 3 4)) gives the correct result which is 10.

Can you please explain , why is it not working when variable is passed in to the function?

Other details -

Lispbox - SBCL

Ubuntu Linux

Thanks in advance

Rainer Joswig · Accepted Answer · 2015-02-04 20:19:28Z

10

That's easy and one of the most common macro questions.

(add-test g)

Now on macro expansion, the macro ADD-TEST is called with the parameter VAR getting the value G, a symbol.

Then you try a list operation. The backquote expression

`(+ ,@var)

The value of VAR is G, and you try to splice that into the list (+ ... ). Now the returned expression is (+ . G).

CL-USER 12 > (macroexpand '(add-test g))
(+ . G)
T

(+ . G) is not a valid Lisp form. It's not valid source code.

Remember, the parameters to a Macro are the unevaluated source expressions.

Compare that with the following:

CL-USER 13 > (macroexpand '(add-test (1 2 3 4)))
(+ 1 2 3 4)
T

You said: 'Can you please explain, why is it not working when variable is passed in to the function?'

Remember, ADD-TEST is NOT a function, it is a macro. A macro gets the source code passed and returns a new form - that form is then later evaluated.

edited Feb 4, 2015 at 20:19

answered Sep 20, 2010 at 21:00

Rainer Joswig

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7 Comments

bdev Over a year ago

Thanks Rainer for the detailed answer. That helped a lot.

Paul Nathan Over a year ago

Out of curiosity, let's say we take bdev's idea further. Suppose I have some sort of optimizable expression (a list of integers!), and I want to optimize the expression prior to "run-time". How would I do that? (although I suppose (eval G) might work...).

Rainer Joswig Over a year ago

@Paul Nathan: you can EVAL it, COMPILE it, FUNCALL it, APPLY it - there are almost no limits. See also DEFINE-COMPILER-MACRO, which allows you to write optimizations.

bdev Over a year ago

@Rainer - I finally found the solution for this - (defmacro add-test(x) `(+,@(evalx))) Thanks again for explaining this.

bdev Over a year ago

@Paul - Thanks for the 'eval' tip.

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