Can try and catch be in different (but nested) macros?

Question

The try is in one macro, the catch in a second one that is called by the first. How to get the following to work?

(defmacro catch-me []
  `(catch ~'Exception ~'ex
     true))

(defmacro try-me []
  `(try (+ 4 3)
        (catch-me)))

Expanding try-me looks good:

(clojure.walk/macroexpand-all '(try-me))

yields

(try (clojure.core/+ 4 3) (catch Exception ex true))

but calling (try-me) yields:

"Unable to resolve symbol: catch in this context",

which, BTW, is also the message you would get in the REPL when using catch when not in a try.

UPDATE:

This is how I can get it to work (thanks, @Barmar), here you can see the actual context of my code:

(defmacro try-me [& body]
  `(try
     ~@body
     ~@(for [[e msg] [[com.mongodb.MongoException$Network "Database unreachable."]
                      [com.mongodb.MongoException "Database problem."]
                      [Exception "Unknown error."]]]
         `(catch ~e ~'ex
            (common/site-layout
             [:div {:id "errormessage"}
              [:p ~msg]
              [:p "Error is: " ~e]
              [:p "Message is " ~'ex]])))))

but this is what I was hoping for (using a separate macro catch-me):

(defmacro try-me [& body]
  `(try
     ~@body
     (catch-me com.mongodb.MongoException$Network "Database unreachable.")
     (catch-me com.mongodb.MongoException "Database problem.")
     (catch-me Exception "Unknown error.")))

I think this would be easier to write / maintain.

Any ideas? I need syntax-quoting because I am passing parameters, that is why unfortunately Arthur's answer cannot be applied (or can it somehow?), but I didn't post my actual context until just now.

Answer 1

The reason you get that error is because the syntax for try is:

(try expr* catch-clause* finally-clause?)

This means that there can be any number of expr forms before the catch and finally clauses. try scans the exprs until it finds one that begins with catch or finally. It does this before expanding any macros, since it's just trying to figure out where the exprs and and the catch/finally clauses begin. It collects all the catch and finally clauses and establishes the appropriate error handling environment for them.

Once it does this, it executes all the expr forms normally. So it expands their macros, and then executes them. But catch is not a function or special form, it's just something that try looks for in the earlier step. So when it's executed normally, you get the same error as when you type it into the REPL.

What you should probably do is write a macro that you wrape around your entire code that expands into the try/catch expression that you want. Without an example of what you're trying to accomplish, it's hard to give a specific answer.

Answer 2

The short answer is YES, though nesting macros with special forms can lead to some double-quoting headaches like this one. It is necessarily to prevent the symbols from being evaluated at both levels of expansion:

user> (defmacro catch-me []                                   
          '(list 'catch 'Exception 'ex  
                       'true))

user> (defmacro try-me []
    `(try (+ 4 3)              
                  ~(catch-me)))
#'user/try-me

user> (try-me)
7

and to see that it catches the exception as well:

user> (defmacro try-me []
    `(try (/ 4 0)
              ~(catch-me)))
#'user/try-me
user> (try-me)
true