The document discusses the importance and functionality of macros in Clojure, highlighting their ability to create domain-specific languages and manipulate code as data. It covers the concept of macro-expansion, quoting, syntax-quote, unquote, and unquote-splicing, providing practical examples for better understanding. The author, Leonardo Borges, invites readers to engage with the content through coding exercises and further reading resources.

1. Bending Clojure to your Will:
Macros and Domain Specific
Languages
CUFP - Boston, 2013
Leonardo Borges
@leonardo_borges
www.leonardoborges.com
www.thoughtworks.com
Thursday, 19 September 13

2. Leonardo Borges
@leonardo_borges
www.leonardoborges.com
www.thoughtworks.com
• Thoughtworker
• Functional Programming enthusiast
• Clojure Evangelist
• Founder & Organiser of the Sydney
Clojure User Group (clj-syd)
• World traveller
• Fan of Murray’s Beers :)
about:me
Thursday, 19 September 13

3. Why macros?
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4. It’s fun
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5. Thursday, 19 September 13

6. It’s powerful
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7. Thursday, 19 September 13

8. It’s mind bending
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9. Thursday, 19 September 13

10. If you give someone Fortran, he has Fortran.
If you give someone Lisp, he has any language he pleases.
- Guy Steele
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11. But what are macros?
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12. •Data is code is data
•Programs that write programs
•Magic happens at macro-expansion time
•Most control structures in Clojure are built out of macros
But what are macros?
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13. Macro-expansion time
(defmacro arg-logger [& args]
(prn "Called with: " args)
`(do ~@args))
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14. Macro-expansion time
(arg-logger
(+ 2 3))
Every usage of a macro
(do (+ 2 3))
Gets replaced with its expansion
prior to compilation
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15. Runtime
(arg-logger
(+ 2 3))
;;"Called with: " ((+ 2 3))
Arguments are handed into macros unevaluated
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16. More on that later
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17. Debugging macros
(macroexpand '(cond true
"true"
:else "false"))
;;(if true "true" (clojure.core/cond :else "false"))
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18. Debugging macros
(require '[clojure.walk :as w])
(w/macroexpand-all '(cond true
"true"
:else "false"))
;;(if true "true" (if :else "false" nil))
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19. Quoting
Prevents evaluation
(def my-list (1 2 3))
;java.lang.Integer cannot be cast to clojure.lang.IFn
(def my-list '(1 2 3)) ;Success!
'my-list ;my-list
'(1 2 3) ;(1 2 3)
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20. Syntax-quote
Automatically qualifies all unqualified symbols
`my-list ; user/my-list
`prn ; clojure.core/prn
Note the backtick!
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21. Unquote
Evaluates some forms in a quoted expression
Before unquoting...
`(map even? my-list)
;;(clojure.core/map clojure.core/even? user/my-list)
After...
`(map even? '~my-list)
;;(clojure.core/map clojure.core/even? (quote (1 2 3)))
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22. Unquote-splicing
“Unpacks” a sequence
Before unquote-splicing...
`(+ ~my-list) ;;(clojure.core/+ (1 2 3))
(eval `(+ ~my-list)) ;;java.lang.Integer cannot be cast
to clojure.lang.IFn
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23. Unquote-splicing
After...
`(+ ~@my-list) ;;(clojure.core/+ 1 2 3)
(eval `(+ ~@my-list)) ;;6
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24. All of these are useful inside
macros!
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25. Jam time!
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26. Jam time!
•Get the code - http://bit.ly/cufp-2013-macros
•Make sure you have leiningen 2.x installed
•Run $lein midje or lein midje :autotest from the project root
•Watch the tests fail!
•Fix them :)
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27. Summary
(defmacro macro-name [& args] ...)Defining macros
(def my-list '(1 2 3))Quoting
`(+ my-list) ; (clojure.core/+ user/my-list)Syntax-quote
`(+ ~my-list) ; (clojure.core/+ (1 2 3))Unquote
(macroexpand '...)
(require '[clojure.walk :as w])
(w/macroexpand-all '...)
Debugging
Unquote-splicing `(+ ~@my-list) ; (clojure.core/+ 1 2 3)
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28. Questions?
Leonardo Borges
@leonardo_borges
www.leonardoborges.com
www.thoughtworks.com
Thursday, 19 September 13

29. Go deeper
Let Over Lambda - http://bit.ly/let-over-lambda
On Lisp - http://bit.ly/on-lisp-book
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