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I'm trying to write a macro in Lisp that re-implements let using itself. This is a trivial exercise which has no practical purpose; however after giving a response to a related question, I realized I should probably learn more about macros. They're touted as one of the great things about Lisp, but I rarely use them.

Anyway, here's what I tried first:

(defmacro mylet (args &rest exp) `(let ,args (dolist (x ,exp) x)))

but when I try something like:

 (mylet ((a 5) (b 2)) (print (+ a b)))

this throws up an error:

  #1=(PRINT (+ A B)) is not a symbol or lambda expression in the form (#1#) .

args (a and b) are set properly, but the print statement doesn't work. I think it's because I'm using two levels of indirection-- referring to a variable that I've created within the macro. But I can't seem to figure out how to fix it! Any ideas?

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Jeff
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1 Answers1

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Your macro expands to:

(LET ((A 5) (B 2))
  (DOLIST (X ((PRINT (+ A B)))) X))

which is invalid because ((PRINT (+ A B))) is not a valid expression. There is also an issue that using an interned symbol in macro expansion can lead to variable capture, but that is not directly relevant (read more in PCL).

Using DOLIST here is unnecessary, and compilcated to get right (you would have to convert all subforms to anonymous function in order to stick them in a list, funcall them in sequence and then store the final result in order to conform to PROGN behaviour). You can just use PROGN, or, since LET includes an implicit PROGN, just splice the body using the ,@ feature of backquote mechanism:

(defmacro mylet (args &body exp) `(let ,args ,(cons 'progn exp)))

(defmacro mylet (args &body exp) `(let ,args ,@exp))
Ramarren
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  • Thanks, this is a great answer-- Never thought of using cons+progn and I didn't know about the splice. However, I don't see where you got ((print (+ a b)))... Shouldn't it be trying to evaluate (print (+ a b))? What am I missing? Even without the dolist, something simple like (first ,exp) does not work either. – Jeff Jun 01 '11 at 18:37
  • I got `((print (+ a b))` from the [macroexpander](http://www.lispworks.com/documentation/HyperSpec/Body/f_mexp_.htm) which is the best way to test if macros expand to what you think they do. The &rest/&body (those are equivalent and the difference is only meaningful to the human reader) argument is always a list of all remaining arguments, therefore the value of variable `exp` at macroexpansion time is the object with printed representation `((print (+ a b))`, which is inserted as is into resulting code. Then the code is executed in the same way as if it was typed in. – Ramarren Jun 01 '11 at 19:09
  • Whoops, there's supposed to be a ' in front of that so that it's treated as a list... but even if there is, it still doesn't work. – Jeff Jun 01 '11 at 19:32
  • The system will not evaluate random lists just because they happen to look like code. A list is a list, and if it is treated as a list then it is treated as a list, which in this case means it is discarded by element, because you don't do anything with them in the DOLIST. – Ramarren Jun 01 '11 at 19:48