I think it has to do with the closure in the *fn *used when generating the
form in the macro.
If you change this function:
*(defn make-greeter []*
* (let [message "hello"]*
* (fn [user-name]*
* (str message ", " user-name))))*
To this one:
*(defn make-greeter []
*
* (fn [user-name]*
* (let [message "**hello**"]*
* (str message ", " user-name))))*
There's no closure on the *message* local var, so you don't get any errors
when the form the macro produces is compiled.
*constantly** *also creates a closure on its argument:
*(defn constantly*
* "Returns a function that takes any number of arguments and returns x."*
* {:added "1.0"*
* :static true}*
* [x] (fn [& args] x))*
So the source of the problem seems to be the same.
Although I'm not completely sure why this doesn't work, I think it kind of
makes sense since the *fn* returned by *make-greeter* is actually produced
in the *defmacro* body and not in the form generated, somehow the closure
is lost and the error is generated.
Someone with a better understanding of macros please jump in.
Juan
On Sunday, December 16, 2012 11:24:35 AM UTC-3, jarppe wrote:
>
> Hi,
>
> I have this macro (complete file https://www.refheap.com/paste/7633):
>
> *(*defmacro defgreeter [greeter-name]
>
> *(*let [greeter *(*make-greeter*)*]
>
> `*(*do
>
> *(*defn ~greeter-name [user-name#]
>
> *(*~greeter user-name#*)**)**)**)**)*
>
>
> It works as expected when make-greeter is defined like this:
>
> *(*defn make-greeter []
>
> *(*fn [user-name]
>
> *(*str "Hello, " user-name*)**)**)*
>
>
> I can use it like this:
>
> *(*defgreeter hello*)*
>
> *(*hello "jarppe"*)*
>
> How ever, if I change make-greeter to this I get IllegalArgumentException:
>
> *(*defn make-greeter []
>
> *(*let [message "hello"]
>
> *(*fn [user-name]
>
> *(*str message ", " user-name*)**)**)**)*
>
>
> Interestingly, this does not work either:
>
> *(*defn make-greeter []
>
> *(*constantly "what erver"*)**)*
>
> What am I missing?
>
> --
> -jarppe
>
>
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