Video and slides synchronized, mp3 and slide download available at URL http://bit.ly/1piTS6D. Jessica Kerr talks about Clojure and explores the potential of contracts as the best-yet compromise between types and tests. Filmed at qconsf.com. Jessica Kerr is a functional developer on the JVM. She enjoys Scala, Clojure and Elm. She recently joined Stripe.

1. Contracts in Clojure:
Settling Types vs Tests
@jessitron

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Watch the video with slide
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http://www.infoq.com/presentations
/clojure-contracts

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Strategy
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Highlights
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Presented at QCon San Francisco
www.qconsf.com

4. What do we know?
How do we know it?

5. Informal
Reasoning
Formal
Proofs
Experimental
Evidence

6. def formatReport(data: ReportData): ExcelSheet
// Scala

7. types

9. (defn format-report [report-data]
…)

10. (function-name arg1 arg2)
function application

11. (function-name arg1 arg2)
(defn function-name [param1 param2]
(println "Hello QCon")
(do-something-with (and param1 param2)))
function definition
last expression is the result

12. (defn function-name [param1 param2]
(println "Hello QCon")
(do-something-with (and param1 param2)))
square braces
make a vector

13. (defn format-report [report-data]
…)

14. (defn ad-performance-report [params]
(-> (fetch-events params)
(analyze-ad-performance params)
format-report))

15. (defn ad-performance-report [params]
(-> (fetch-events params)
(analyze-ad-performance params)
format-report))

16. ch-events [params]
…)
(defn fet

17. {:when 12:34:56 7/8/90
:what "show"
:who "abc123"}
curly braces make a map
keyword

18. {:when org.joda.time.DateTime
:what java.lang.String
:who java.lang.String}
(def Event
)
give a thing a name

19. (def Event
)
(:require [schema.core :as s])
dependency
{:when DateTime
:what s/Str
:who s/Str})

20. Event
(def Incident s/Str)
(def Customer s/Str)
(def
)
{:when DateTime
:what Incident
:who Customer})

21. [Event]
(def Event
)
{:when DateTime
:what Incident
:who Customer})

22. …)
(defn fetch-events [params]
[Event]

23. [Event]
(:require [schema.core :as s])
…)
(s/defn fetch-events
[params]
:-
[Event]

24. (deftest fetch-events-test
…
(= (expected (fetch-events input))))

25. (deftest fetch-events-test
…
(= (expected (fetch-events input))))
(use-fixtures schema.test/validate-schemas)

26. (defn ad-performance-report [params]
(-> (fetch-events params)
(analyze-ad-performance params)
format-report))
[Event]

27. (defn analyze-ad-performance [events params]
(-> events
(group-up params)
summarize
add-total-row
(add-headers params)))

28. [Event]
(defn analyze-ad-performance [events params]
(-> events
(group-up params)
summarize
add-total-row
(add-headers params)))

29. [[Event]][Event]
(defn analyze-ad-performance [events params]
(-> events
(group-up params)
summarize
add-total-row
(add-headers params)))

30. [[Event]][Event]
(defn analyze-ad-performance [events params]
(-> events
(group-up params)
summarize
add-total-row
(add-headers params)))

31. [[Event] ]

32. [[Event] Summation]

33. [( one [Event] )
( one Summation )]

34. [(s/one [Event] "event list")
(s/one Summation "group sum")]

35. {:groups [[(s/one [Event] "event list")
(s/one Summation "group sum")]]}

36. {:groups [[(s/one [Event] "event list")
(s/one Summation "group sum")]]
:total Totals}

37. {:header Headers
:groups [[(s/one [Event] "event list")
(s/one Summation "group sum")]]
:total Totals})

38. (def ReportData
{:header Headers
:groups [[(s/one [Event] "event list")
(s/one Summation "group sum")]]
:total Totals})

39. (s/defn analyze-ad-performance :- ReportData
[events :- [Event]
params :- Params]
(-> events
(group-up params)
summarize
add-total-row
(add-headers params)))

40. (s/defn analyze-ad-performance :- ReportData
…)

41. (s/defn analyze-ad-performance
:- (at-least ReportData)
…)

42. (s/defn analyze-ad-performance
:- (at-least ReportData)
…)
(defn at-least [map-schema]
(merge map-schema
{s/Any s/Any}))

43. What do we know?

44. What do we know?
data shape

45. What do we know?
data shape
value boundaries

46. :title
- string
- not empty
- capitalized
Headers

47. (def Headers
{:title
(s/constrained
s/Str
(s/pred (complement empty?) "nonempty")
(s/pred capitalized? "Title Caps"))
…})

48. What do we know?
data shape
data value boundaries
relationships within values

49. What do we know?
data shape
data value boundaries
relationships within values
What could we know?

50. What do we know?
data shape
relationships within values
What could we know?
produced types
data value boundaries

51. (s/defn fetch-events :- [Event]
[params]
…)
Seq[Event]

52. (s/defn fetch-events :- [Event]
[params]
…)

53. (st/defn fetch-events :+ (st/LazySeq Event)
[params]
…)
what
if?
check this later

54. (st/defn fetch-events :+ (st/LazySeq Event)
[params]
…)
(:require [schematron.core :as st])

55. (s/defn a-higher-order-function
[predicate :- (s/=> Bool Event)
…]
…)

56. (st/defn a-higher-order-function
[predicate :+ (st/=> Bool Event)
…]
…)
what
if?
check this later

57. (s/defn a-higher-order-function :- Event
[predicate :- (st/=> Bool Event)
…]
…)

58. (st/defn [A] a-higher-order-function :- A
[predicate :+ (st/=> Bool A)
…]
…)
what
if?
(a-higher-order-function Event is-happy …)

59. What do we know?
data shape
data value boundaries
relationships within values
What could we know?
produced types
relationships between types

60. What do we know?
What could we know?
produced types
relationships between types
relationships between values
data shape
relationships within values
data value boundaries

61. (defn group-up [events params]
{:post [(as-lazy-as events %)]
…))
postcondition

62. produced types
relationships between types
relationships between values
data shape
relationships within values
data value boundaries

63. How do we know it?

64. (deftest analyze-ad-performance-test
(testing "grouping of rows"
(let […
result (analyze-ad-performance
events
{})
(is (= expected (:groups result))))))

65. (use-fixtures schema.test/validate-schemas)
(deftest analyze-ad-performance-test
(testing "grouping of rows"
(let […
result (analyze-ad-performance
events
{})
(is (= expected (:groups result))))))

66. (let […
result (analyze-ad-performance
events
{})
(is (= expected (:groups result))))))
Input does not match schema Params
Missing required key :title
Missing required key :start
Missing required key :end

67. (let […
result (analyze-ad-performance
events
(sc/complete {} Params)
(is (= expected (:groups result))))))
"Fill in everything else with something random until it
meets this schema"

68. {:title "YhKEzII",
:start "-217863493-11-21T00:54:39.872Z"],
:end "-256417656-09-30T01:08:11.904Z"]}
any string
any date
before now
any date before
the start

69. (deftest analyze-ad-performance-test
(testing "grouping of rows"
(let […
result (analyze-ad-performance
[events]
(sample-one param-gen))
(is (= expected (:groups result))))))
(use-fixtures schema.test/validate-schemas)

70. 1 test is an anecdote
generative tests are evidence

71. ?

72. ?

73. ?
Experimental
Evidence

74. (defspec analyze-ad-performance-spec 100
(for-all [events events-gen
params param-gen]
(analyze-ad-performance events params))))
(use-fixtures schema.test/validate-schemas)
(s/defn analyze-ad-performance :- ReportData
…)

75. What do we know?
How do we know it?
Schemas
Generative Tests

78. B
A Contract

80. B
A

81. AA

82. A
B

83. A
B

84. implementation
schemas
generators
tests

85. implementation
schemas
tests
testkit
client libs
gen

86. B
A

87. Science!
test.check
prismatic/schema
Clojure

88. Science!
generative tests
types and contracts
… your language …

89. Science!
testkits
client libraries
… your services …

90. Informal
Reasoning
Formal
Proofs
Experimental
Evidence

91. @jessitron
blog.jessitron.com
https://github.com/jessitron/contracts-as-types-examples
examples
resources
https://github.com/Prismatic/schema
http://hintjens.com/blog:85 The End of Software Versions
http://david-mcneil.com/post/114783282473/extending-
prismatic-schema-to-higher-order
https://github.com/jessitron/schematron
http://dl.acm.org/citation.cfm?id=2661156
Static typing and productivity: Stefik & Hanenberg 2014

92. Watch the video with slide synchronization on
InfoQ.com!
http://www.infoq.com/presentations/clojure-
contracts