Feb 17, 2015

1. Clojure 4 Feb 17, 2015

2. Macros Code is data We have heard this before. It is what makes Lisp so amenable to the use of macros. Examples from Mastering Clojure Macros. . .

3. Transforming code • (read-string “(+ 1 2 3 4 5)”) • (class (read-string “(+ 1 2 3 4 5)”)) • (eval (read-string “(+ 1 2 3 4 5)”)) • (class (eval (read-string “(+ 1 2 3 4 5)”))) • (let [expression] (read-string “(+ 1 2 3 4 5)”)] (cons (read-string “*”) (rest expression))) • (let [expression] (quote (+ 1 2 3 4 5))] (cons (quote *) (rest expression))) • ‘(+ 1 2 3 4 5)

4. Our first macro • (defmacro when “Evaluates test. If logical true, evaluates body.” [test & body] (list ‘if test (cons ‘do body))) • (when (= 2 (+ 1 1)) (print “You got”) (print “ the touch!”) (println))

5. First macro continued • (list ‘if ‘(= 2 (+ 1 1)) (cons ‘do ‘((print “You got”) (print “ the touch!”) (println)))) (if (= 2 (+ 1 1)) (do (print “You got”) (print “ the touch!”) (println)))

6. Another example • (defmacrocond “Long comment here.” [& clauses] (when clauses (list ‘if (first clauses) (if (next clauses) (second clauses) (throw (IllegalArgumentException. “cond requires an even number of forms”))) (cons ‘clojure.core/cond (next (next clauses))))))

7. macroexpand-1 • (macroexpand-1 ‘(when (= 1 2) (println “math is broken”))) • (macroexpand-1 nil) • (defmacro broken-when [test & body] (list test (cons ‘do body))) • Use macroexpand-1 to figure out why this doesn’t work. • Note that macroexpand-1 expands one level.

8. macroexpand • (defmacro when-falsy [test & body] (list ‘when (list ‘not test) (cons ‘do body))) • (macroexpand-1 ‘(when-falsy (= 1 2) (println “hi”))) • (macroexpand ‘(when-falsy (= 1 2) (println “hi”)))

9. assert • (defmacro assert [x] (when *assert* ;; make sure enabled (list ‘when-not x (list ‘throw (list ‘new AssertionError (list ‘str “Assert failed: “ (list ‘pr-str (list ‘quote x)))))))) • (assert (= 1 2)) • Real assert is even more complicated.

10. Assert with syntax quote • (defmacro assert [x] (when *assert* `(when-not ~x (throw (new AssertionError (str “Assert failed: “ (pr-str ‘~x))))))) • Before when-not above is a back-quote (aka a syntax quote). It selectively quotes all but things with ~ or ~@.

11. Syntax quote • (def a 4) • ‘(1 2 3 a 5) • (list 1 2 3 a 5) • `(1 2 3 ~a 5) • The book says syntax quote (`)is a little cockeyed and ready to party. • Unquote (~) is the thing that evaluates inside of syntax quoted code/data. • Unquote-splice (~@) is like unquote, but it splices in the result.

12. Unquote-splice • (def other-numbers ‘(4 5 6 7 8)) • `(1 2 3 ~other-numbers 9 10) • (concat ‘(1 2 3) other-numbers ‘( 9 10)) • `(1 2 3 ~@other-numbers 9 10)

13. Symbol capture • (def y 100) • (defmacro make-adder [x] `(fn [~’y] (+ ~x ~’y))) • ((make-adder (+ y 3)) 5) • Fix the problem with gensym • (gensym) generates a unique new symbol

14. Symbol capture cont. • (defmacro make-adder [x] (let [y (gensym)] `(fn [~y] (+ ~x ~y)))) • Auto-gensym makes this more concise. • (defmacro make-adder [x] `(fn [y#] (+ ~x y#)))

15. Another example • (defmacro and ([] true) ([x] x) ([x & next] `(let [and# ~x] (if and# (and ~@next) and#))))

16. More detail about and • Version that isn’t quite right. . . • (defmacro our-and [x] ([] true) ([x] x) ([x & next] `(if ~x (our-and ~@next) ~x))) • (our-and (do (println “hi there”) (= 1 2)) (= 3 4)) • Prints “hi there” twice.