Presentation at the Sydney Clojure User Group

1. Continuation-passing style and
Macros with Clojure
Leonardo Borges
@leonardo_borges
http://www.leonardoborges.com
http://www.thoughtworks.com

2. CPS in a nutshell
• not new. It was coined in 1975 by Gerald Sussman and Guy Steele
• style of programming where control is passed explicitly in the form of a
continuation
• every function receives an extra argument k - the continuation
• makes implicit things explicit, such as order of evaluation, procedure
returns, intermediate values...
• used by functional language compilers as an intermediate representation
(e.g.: Scheme, ML, Haskell)

3. CPS - Pythagorean theorem
You know the drill...
a² + b² = c²
;;direct style
(defn pyth [a b]
(+ (* a a) (* b b)))

4. CPS - Pythagorean theorem
You know the drill...
a² + b² = c²
;;direct style
(defn pyth [a b]
(+ (* a a) (* b b)))
;;CPS
(defn pyth-cps [a b k]
(*-cps a a (fn [a2]
(*-cps b b (fn [b2]
(+-cps a2 b2 k))))))

5. WTF?!

6. Untangling pyth-cps
;;CPS
(defn *-cps [x y k]
(k (* x y)))
(defn +-cps [x y k]
(k (+ x y)))
(defn pyth-cps [a b k]
(*-cps a a (fn [a2]
(*-cps b b (fn [b2]
(+-cps a2 b2 k))))))

7. Untangling pyth-cps
;;CPS
(defn *-cps [x y k]
(k (* x y)))
(defn +-cps [x y k]
(k (+ x y)))
(defn pyth-cps [a b k]
(*-cps a a (fn [a2]
(*-cps b b (fn [b2]
(+-cps a2 b2 k))))))
(pyth-cps 5 6 identity) ;61

8. CPS - Fibonacci
;;direct style
(defn fib [n]
(if (<= n 1)
n
(+ (fib (- n 1)) (fib (- n 2)))))

9. CPS - Fibonacci
;;CPS
(defn fib-cps [n k]
(letfn [(cont [n1]
(fib-cps (- n 2) (fn [n2]
(k (+ n1 n2)))))]
(if (<= n 1)
(k n)
(recur (- n 1) cont))))
(fib-cps 20 identity);55

10. Another look at CPS
Think of it in terms of up to three functions:
• accept: decides when the computation should end
• return continuation: wraps the return value
• next continuation: provides the next step of the computation

11. CPS - Fibonacci
;;CPS
(defn fib-cps [n k]
(letfn [(cont [n1]
(fib-cps (- n 2) (fn [n2]
(k (+ n1 n2)))))]
(if (<= n 1)
(k n)
(recur (- n 1) cont)))) accept function
(fib-cps 20 identity);55

12. CPS - Fibonacci
;;CPS
(defn fib-cps [n k]
(letfn [(cont [n1]
(fib-cps (- n 2) (fn [n2]
(k (+ n1 n2)))))]
(if (<= n 1)
(k n) return continuation
(recur (- n 1) cont))))
(fib-cps 20 identity);55

13. CPS - Fibonacci
;;CPS
(defn fib-cps [n k] next continuation
(letfn [(cont [n1]
(fib-cps (- n 2) (fn [n2]
(k (+ n1 n2)))))]
(if (<= n 1)
(k n)
(recur (- n 1) cont))))
(fib-cps 20 identity);55

14. CPS - generic function
builders
(defn mk-cps [accept? end-value kend kont]
(fn [n]
((fn [n k]
(let [cont (fn [v] (k (kont v n)))]
(if (accept? n)
(k end-value)
(recur (dec n) cont))))
n kend)))

15. CPS - generic function
builders
;;Factorial
(def fac (mk-cps zero? 1 identity #(* %1 %2)))
(fac 10); 3628800
;;Triangular number
(def tri (mk-cps zero? 1 dec #(+ %1 %2)))
(tri 10); 55

16. Seaside - a more practical use
of CPS
• continuation-based web application framework for Smalltalk
• UI is built as a tree of independent, stateful components
• uses continuations to model multiple independent flows between
different components

17. Seaside - a more practical use
of CPS
• continuation-based web application framework for Smalltalk
• UI is built as a tree of independent, stateful components
• uses continuations to model multiple independent flows between
different components
• memory intensive
• not RESTful by default

18. Seaside - Task example [1]
go
" [ self chooseCheese.
" self confirmCheese ] whileFalse.
" self informCheese
[1] Try it yourself (http://bit.ly/seaside-task)

19. Seaside - Task example
chooseCheese
" cheese := self
" " chooseFrom: #( 'Greyerzer' 'Tilsiter' 'Sbrinz' )
" " caption: 'What''s your favorite Cheese?'.
" cheese isNil ifTrue: [ self chooseCheese ]
confirmCheese
" ^ self confirm: 'Is ' , cheese , 'your favorite Cheese?'
informCheese
" self inform: 'Your favorite is ' , cheese , '.'

20. CPS - Other real world
usages
• web interactions ~ continuation invocation [2]
• event machine + fibers in the Ruby world [3]
• functional language compilers
• ajax requests in javascript - callbacks anyone?
• node.js - traditionally blocking functions take a callback instead
• ...
[2] Automatically RESTful Web Applications (http://bit.ly/ydltH6)
[3] Untangling Evented Code with Ruby Fibers (http://bit.ly/xm0t51)

21. Macros
If you give someone Fortran, he has Fortran.
If you give someone Lisp, he has any language he pleases.
- Guy Steele

22. Macros
• Data is code is data
• Programs that write programs
• Magic happens at compile time
• Most control structures in Clojure are built out of macros

23. e.g.: avoiding nesting levels
(def guitar
{:model "EC-401FM"
:brand "ESP"
:specs {
:pickups {:neck {:brand "EMG"
:model "EMG 60"}
:bridge {:brand "EMG"
:model "EMG 81"}}
:body "Mahoganny"
:neck "Mahoganny"}})

24. e.g.: avoiding nesting levels
(def guitar
{:model "EC-401FM"
:brand "ESP"
:specs {
:pickups {:neck {:brand "EMG"
:model "EMG 60"}
:bridge {:brand "EMG"
:model "EMG 81"}}
:body "Mahoganny"
:neck "Mahoganny"}})
(:model (:neck (:pickups (:specs guitar))))

25. e.g.: avoiding nesting levels
what if we could achieve the same like this instead?
(t guitar :specs :pickups :neck :model)

26. Macros to the rescue!
(defmacro t
([v form] (if (seq? form)
`(~(first form) ~v ~@(rest form))
(list form v)))
([v form & rest] `(t (t ~v ~form) ~@rest)))

27. Macros to the rescue!
(defmacro t
([v form] (if (seq? form)
`(~(first form) ~v ~@(rest form))
(list form v)))
([v form & rest] `(t (t ~v ~form) ~@rest)))
(t guitar :specs :pickups :neck :model)

28. What’s with all that `~@ ?

29. Quoting
Prevents evaluation

30. Quoting
Prevents evaluation
(def my-list (1 2 3))

31. Quoting
Prevents evaluation
(def my-list (1 2 3))
;java.lang.Integer cannot be cast to clojure.lang.IFn

32. 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!

33. 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

34. 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)

35. 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)
Syntax-quote: automatically qualifies all unqualified symbols

36. 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)
Syntax-quote: automatically qualifies all unqualified symbols
`my-list ;user/my-list

37. Unquote
Evaluates some forms in a quoted expression

38. Unquote
Evaluates some forms in a quoted expression
Before unquoting...

39. Unquote
Evaluates some forms in a quoted expression
Before unquoting...
`(map even? my-list)

40. Unquote
Evaluates some forms in a quoted expression
Before unquoting...
`(map even? my-list)
;;(clojure.core/map clojure.core/even? user/my-list)

41. Unquote
Evaluates some forms in a quoted expression
Before unquoting...
`(map even? my-list)
;;(clojure.core/map clojure.core/even? user/my-list)
After...

42. 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)

43. 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)))

44. Unquote-splicing
Unpacks the sequence at hand

45. Unquote-splicing
Unpacks the sequence at hand
Before unquote-splicing...

46. Unquote-splicing
Unpacks the sequence at hand
Before unquote-splicing...
`(+ ~my-list)

47. Unquote-splicing
Unpacks the sequence at hand
Before unquote-splicing...
`(+ ~my-list)
;;(clojure.core/+ (1 2 3))

48. Unquote-splicing
Unpacks the sequence at hand
Before unquote-splicing...
`(+ ~my-list)
;;(clojure.core/+ (1 2 3))
(eval `(+ ~my-list))

49. Unquote-splicing
Unpacks the sequence at hand
Before unquote-splicing...
`(+ ~my-list)
;;(clojure.core/+ (1 2 3))
(eval `(+ ~my-list))
;;java.lang.Integer cannot be cast to clojure.lang.IFn

50. Unquote-splicing
Unpacks the sequence at hand
Before unquote-splicing...
`(+ ~my-list)
;;(clojure.core/+ (1 2 3))
(eval `(+ ~my-list))
;;java.lang.Integer cannot be cast to clojure.lang.IFn
After...

51. Unquote-splicing
Unpacks the sequence at hand
Before unquote-splicing...
`(+ ~my-list)
;;(clojure.core/+ (1 2 3))
(eval `(+ ~my-list))
;;java.lang.Integer cannot be cast to clojure.lang.IFn
After...
`(+ ~@my-list)

52. Unquote-splicing
Unpacks the sequence at hand
Before unquote-splicing...
`(+ ~my-list)
;;(clojure.core/+ (1 2 3))
(eval `(+ ~my-list))
;;java.lang.Integer cannot be cast to clojure.lang.IFn
After...
`(+ ~@my-list)
;;(clojure.core/+ 1 2 3)

53. Unquote-splicing
Unpacks the sequence at hand
Before unquote-splicing...
`(+ ~my-list)
;;(clojure.core/+ (1 2 3))
(eval `(+ ~my-list))
;;java.lang.Integer cannot be cast to clojure.lang.IFn
After...
`(+ ~@my-list)
;;(clojure.core/+ 1 2 3)
(eval `(+ ~@my-list)) ;6

54. back to our macro...
(defmacro t
([v form] (if (seq? form)
`(~(first form) ~v ~@(rest form))
(list form v)))
([v form & rest] `(t (t ~v ~form) ~@rest)))

55. back to our macro...
(defmacro t
([v form] (if (seq? form)
`(~(first form) ~v ~@(rest form))
(list form v)))
([v form & rest] `(t (t ~v ~form) ~@rest)))
better now?

56. Macro expansion

57. Macro expansion
(macroexpand '(t guitar :specs :pickups :neck :model))

58. Macro expansion
(macroexpand '(t guitar :specs :pickups :neck :model))
;;expands to

59. Macro expansion
(macroexpand '(t guitar :specs :pickups :neck :model))
;;expands to
(:model (user/t (user/t (user/t guitar :specs) :pickups) :neck))

60. Macro expansion
(macroexpand '(t guitar :specs :pickups :neck :model))
;;expands to
(:model (user/t (user/t (user/t guitar :specs) :pickups) :neck))
(require '[clojure.walk :as walk])

61. Macro expansion
(macroexpand '(t guitar :specs :pickups :neck :model))
;;expands to
(:model (user/t (user/t (user/t guitar :specs) :pickups) :neck))
(require '[clojure.walk :as walk])
(walk/macroexpand-all '(t guitar :specs :pickups :neck :model))

62. Macro expansion
(macroexpand '(t guitar :specs :pickups :neck :model))
;;expands to
(:model (user/t (user/t (user/t guitar :specs) :pickups) :neck))
(require '[clojure.walk :as walk])
(walk/macroexpand-all '(t guitar :specs :pickups :neck :model))
;;expands to

63. Macro expansion
(macroexpand '(t guitar :specs :pickups :neck :model))
;;expands to
(:model (user/t (user/t (user/t guitar :specs) :pickups) :neck))
(require '[clojure.walk :as walk])
(walk/macroexpand-all '(t guitar :specs :pickups :neck :model))
;;expands to
(:model (:neck (:pickups (:specs guitar))))

64. Macro expansion
(macroexpand '(t guitar :specs :pickups :neck :model))
;;expands to
(:model (user/t (user/t (user/t guitar :specs) :pickups) :neck))
(require '[clojure.walk :as walk])
(walk/macroexpand-all '(t guitar :specs :pickups :neck :model))
;;expands to
(:model (:neck (:pickups (:specs guitar))))
However our macro is worthless. Clojure implements this
for us, in the form of the -> [4] macro
[4] The -> macro on ClojureDocs (http://bit.ly/yCyrHL)

65. Implementing unless
We want...
(unless (zero? 2)
(print "Not zero!"))

66. 1st try - function

67. 1st try - function
(defn unless [predicate body]
(when (not predicate)
body))

68. 1st try - function
(defn unless [predicate body]
(when (not predicate)
body))
(unless (zero? 2)
(print "Not zero!"))
;;Not zero!

69. 1st try - function
(defn unless [predicate body]
(when (not predicate)
body))
(unless (zero? 2)
(print "Not zero!"))
;;Not zero!
(unless (zero? 0)
(print "Not zero!"))
;;Not zero!

70. 1st try - function
(defn unless [predicate body]
(when (not predicate)
body))
(unless (zero? 2)
(print "Not zero!"))
;;Not zero!
(unless (zero? 0)
(print "Not zero!"))
;;Not zero!
Oh noes!

71. 1st try - function
Function arguments are eagerly evaluated!

72. Unless - our second macro
(defmacro unless [predicate body]
`(when (not ~predicate)
~@body))

73. Unless - our second macro
(defmacro unless [predicate body]
`(when (not ~predicate)
~@body))
(macroexpand '(unless (zero? 2)

74. Unless - our second macro
(defmacro unless [predicate body]
`(when (not ~predicate)
~@body))
(macroexpand '(unless (zero? 2)
(print "Not zero!")))

75. Unless - our second macro
(defmacro unless [predicate body]
`(when (not ~predicate)
~@body))
(macroexpand '(unless (zero? 2)
(print "Not zero!")))
;;expands to

76. Unless - our second macro
(defmacro unless [predicate body]
`(when (not ~predicate)
~@body))
(macroexpand '(unless (zero? 2)
(print "Not zero!")))
;;expands to
(if (clojure.core/not (zero? 2))

77. Unless - our second macro
(defmacro unless [predicate body]
`(when (not ~predicate)
~@body))
(macroexpand '(unless (zero? 2)
(print "Not zero!")))
;;expands to
(if (clojure.core/not (zero? 2))
(do (print "Not zero!")))

78. Unless - our second macro
(defmacro unless [predicate body]
`(when (not ~predicate)
~@body))
(macroexpand '(unless (zero? 2)
(print "Not zero!")))
;;expands to
(if (clojure.core/not (zero? 2))
(do (print "Not zero!")))
You could of course use the if-not [5] macro to the
same effect
[5] The if-not macro on ClojureDocs (http://bit.ly/yOIk3W)

79. Thanks!
Questions?!
Leonardo Borges
@leonardo_borges
http://www.leonardoborges.com
http://www.thoughtworks.com

80. References
• The Joy of Clojure (http://bit.ly/AAj760)
• Automatically RESTful Web Applications (http://bit.ly/ydltH6)
• Seaside (http://www.seaside.st)
• http://en.wikipedia.org/wiki/Continuation-passing_style
• http://matt.might.net/articles/by-example-continuation-passing-style/
• http://en.wikipedia.org/wiki/Static_single_assignment_form
Leonardo Borges
@leonardo_borges
http://www.leonardoborges.com
http://www.thoughtworks.com