How clojure macro parses special symbol?

1
votes

When I re-implement a macro written in Scheme with Clojure, I get into a trouble. The macro tries to load pairs of testing data into a all-tests var for later use.

Because the arguments for the macro is variable-length and contains special undefined symbol, i.e. =>, I simply don't know how to parse it like what Scheme syntax-rules does.

Scheme Version:

(define all-tests '())

;;; load tests into all-tests
(define-syntax add-tests-with-string-output
  (syntax-rules (=>)
    [(_ test-name [expr => output-string] ...)
     (set! all-tests
        (cons 
           '(test-name [expr string  output-string] ...)
            all-tests))]))

(add-tests-with-string-output "integers"      
  [0  => "0\n"]                    
  [1  => "1\n"]                    
  [-1 => "-1\n"]                   
  [10  => "10\n"]                    
  [-10 => "-10\n"]                   
  [2736 => "2736\n"]               
  [-2736 => "-2736\n"]             
  [536870911 => "536870911\n"]     
  [-536870912 => "-536870912\n"]   
)

My current unsuccessful Clojure Version:

(def all-tests (atom '()))

(defmacro add-tests-with-string-output
  [test-name & body]
  `(loop [bds# (list body)]
    (when-not (empty? bds#)
      (println (first bds#))
      (recur (rest bds#)))))

Ps: I am using println to test my code right now. When it works, I will try to do the parsing and loading work.

2
clojuremacrossyntax-rules

2 Answers

1
votes

The first macro forms a loop and the second one a doseq (so is simpler). Both should behave the same. Also I find it a good idea to extract as much logic out of macros into auxiliary functions. Functions are easier to debug, test and write. If the macro were a bit more complicated I might have left even less logic in it.

(def all-tests (atom '()))

(defn add-test [test-name expr output-string]
  (swap! all-tests #(cons (list test-name [expr output-string]) %)))

(defmacro add-tests-with-string-output
  [test-name & body]
  ;`(loop [bds# '(~@body)]
  `(loop [bds# '~body] ; edit
    (when-not (empty? bds#)
      (let [bd# (first bds#)
            expr# (first bd#)
            output-string# (last bd#)]
        (add-test ~test-name expr# output-string#)
        (recur (rest bds#))
        ))))

(defmacro add-tests-with-string-output2
  [test-name & body]
  ;`(doseq [bd# '(~@body)]
  `(doseq [bd# '~body] ; edit
    (let [expr# (first bd#)
          output-string# (last bd#)]
      (add-test ~test-name expr# output-string#))))

user=> (add-tests-with-string-output "test1" [0  => "0\n"] [1  => "1\n"])
nil
user=> (add-tests-with-string-output2 "test2" [0  => "0\n"] [1  => "1\n"])
nil
user=> @all-tests
(("test2" [1 "1\n"]) ("test2" [0 "0\n"]) ("test1" [1 "1\n"]) ("test1" [0 "0\n"]))
0
votes

After trials and errors, finally I figure out how to solve it.

First use Destructuring to tackle the arguments of variable-length; later do not use Syntax-Quoting, i.e. backquote `, inside the macro, because if so, once you need to unquote ~ the argument, i.e. body, you will get error msg like this due to the special symbol =>:

CompilerException java.lang.RuntimeException: Unable to resolve symbol: => in this context

Below is my solution. If you get better one, or you know the reason why Syntax-Quote and Unquote go wrong, please let me know.

;;; load tests into all-tests
(def all-tests (atom '()))
(defmacro add-tests-with-string-output
  [test-name & body]
  (loop [bds body, tests '()]
    (if (empty? bds)
      (do 
        (swap! all-tests #(cons (cons test-name tests) %))
        nil)
      (let [pair (first bds),
            input (first pair)
            output (last pair)]
        (recur (rest bds) (cons (list input ''string output) tests))))))