Macros differ from functions in two ways:
1. They're executed at compile-time, rather than run-time.
2. Their arguments are unevaluated.
Your if-zero function probably doesn't act the way you expect it to,
because you're expecting test-expr to be evaluated.
If all of the arguments to your macro are literal, it will work as you
expect:
(if-zero 0 :true :false)
=> (if (= 0 0) :true :false)
=> :true
But if any argument is a variable or expression, it won't work:
(let [t 0] (if-zero t :true :false))
=> (if (= 't 0) :true :false)
=> :false
In the above case, the symbol t isn't evaluated, and you're testing whether
the symbol t is equal to 0, not the value assigned to t.
Your macro should look more like:
(defmacro if-zero [test-expr true-expr false-expr]
`(if (= ~test-expr 0) ~true-expr ~false-expr))
The output of this macro is an expression. After the compiler expands the
macro, it then evaluates the resulting expression.
- James
On 16 June 2014 22:14, Christopher Howard <[email protected]> wrote:
> Disclosure: I'm rather fascinated with macros (the things they are
> supposed to allow us to do), but I'm still rather mystified about how
> they actually work. It seems like every time I try to write one, it
> rarely behaves anything like I expect.
>
> Question: Is there any obvious problems with this seemingly innocent
> macro?:
>
> (defmacro if-zero [test-expr true-expr false-expr]
> (if (= test-expr 0) true-expr false-expr))
>
> It doesn't seem to work the way I would expect when used inside other
> functions.
>
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