On Nov 3, 9:57 am, David Nolen <[EMAIL PROTECTED]> wrote:
> (defmacro foobar []
> `'(+ a b))
>
> (let
> [a 5
> b 6]
> (eval (foobar)))
>
> I know that the above is completely useless but I'm just trying to get
> an intuitive understanding of how macros work under Clojure. At the
> REPL when I try to evaluate the second form, I get an null pointer
> exception. Why can't the result of the foobar macro access the values
> of a and b?
>
If you macroexpand,
>(macroexpand '(foobar))
you'll see that the a and b symbols in your macro are resolved to be
vars in the namespace in which the macro is defined (I defined the
macro in the user namespace):
(quote (clojure/+ user/a user/b))
Note that symbol + was resolved to be the + function from boot.clj,
since my user namespace refers to the clojure namespace.
So when the foobar macro is run, the resulting code doesn't look at
the a and b in your let, but instead tries to find vars named a and b.
If you:
> (def a 1)
> (def b 2)
then execute
> (let
> [a 5
> b 6]
> (eval (foobar)))
you'll get 3.
Hope that helps.
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