On Wed, May 20, 2009 at 9:59 AM, Michael Reid <kid.me...@gmail.com> wrote:
>
> On Wed, May 20, 2009 at 8:44 AM, pmf <phil.fr...@gmx.de> wrote:
>>
>> On May 20, 4:47 am, Per <nondual...@gmail.com> wrote:
>>> ;; The macro
>>> (defmacro def-fields [name tgs]
>>> `(defstruct ~name ~@(map #(symbol (str ":" %)) tgs))
>>> )
>>
>> If you replace the call to 'symbol' with a call to 'keyword', it works
>> (I think this is what you intended).
>
> I don't think its that simple. I still get the error if I change it to
> a call to 'keyword'. The problem I believe is that because def-fields
> is a macro, the arguments are not evaluated, so in the macro, tgs is
> bound to the symbol 'tags _not_ the value of tags as defined above the
> macro.
Yes. And attempts to resolve or evaluate a macro's args at
compile time can get squirrelly, will never work with
locals, etc.
Generally, it's best to see if you can find a non-macro that
will build what you want, so that you can let the evaluation
happen at runtime, where it will behave more often as
expected. In this case you're in luck, because defstruct (a
macro) is built on create-struct (a function):
(defmacro def-fields [name tgs]
`(def ~name (apply create-struct (map keyword ~tgs))))
user=> (def tags ["name" "age"])
#'user/tags
user=> (def-fields fs tags)
#'user/fs
user=> (def testfs (struct fs "Peter" "5"))
#'user/testfs
user=> testfs
{:name "Peter", :age "5"}
--Chouser
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