Macros Exercises

Note: This is a class activity, nothing needs to be delivered.

Write a Clojure macro called debug, that provides a simple debugging facility for Clojure code. When (debug original-expression) is evaluated, a debugging line should be printed to the standard output with the following format:

debug: original-expression => evaluated-expression

Additionally, the debug macro should return the result of the evaluated expression. For example, the following code
```
(inc (debug (* 4 5)))
```
should return 21 and print this to the standard output:
```
debug: (* 4 5) => 20
```
Test your function with the following code:
```
(defn fact
  [n]
  (if (zero? n)
      1
      (debug (* n (fact (dec n))))))

(println (fact 5))
```
The expected output is:
```
debug: (* n (fact (dec n))) => 1
debug: (* n (fact (dec n))) => 2
debug: (* n (fact (dec n))) => 6
debug: (* n (fact (dec n))) => 24
debug: (* n (fact (dec n))) => 120
120
```

Write a Clojure macro called IF (note the uppercase letters). Its purpose is to provide a conditional statement that is syntactically a bit more similar to those found in languages like Pascal or Fortran. It has the following form:

(IF condition :THEN exp₁ exp₂ … :ELSE exp₃ exp₄ …)

It should basically expand to the following conventional Clojure if form:

(if condition (do exp₁ exp₂ …) (do exp₃ exp₄ …))

Almost everything is optional in the IF macro, except condition. Also, the :ELSE keyword may come before the :THEN keyword. The following examples show some legal forms and their results:

user=> (IF (> 3 1) :THEN 'ok :ELSE 'oops)
ok
user=> (IF (> 3 1) :THEN 'ok)
ok
user=> (IF (< 3 1) :ELSE 'ok)
ok
user=> (IF (> 3 1) :ELSE 'oops)
nil
user=> (IF (> 3 1) :THEN)
nil
user=> (IF (> 3 1))
nil
user=> (macroexpand-1 '(IF (> 3 1) 
                           :ELSE (println "Else section.") 'oops 
                           :THEN (println "Then section.") 'ok))
(if (> 3 1) (do (println "Then section.") (quote ok)) 
            (do (println "Else section.") (quote oops)))            
user=> (macroexpand-1 '(IF (< 3 1) :ELSE 'ok))
(if (< 3 1) (do) (do (quote ok)))

Any expression in-between condition and the first occurrence of the :THEN or :ELSE keywords should just be ignored:

user=> (macroexpand-1 '(IF (< 3 1) 1 2 3 :THEN 4 5 6 :ELSE 7 8 9))            
(if (< 3 1) (do 4 5 6) (do 7 8 9))

Programming Languages

Macros Exercises