Property Based Testing

5min lightning talk for the SoCraTes Belgium meetup and CukeUp London.

Transcript

1. Property Based Testing
   @mathiasverraes
   

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3. function inc(x) { return x + 1; }
   

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4. inc x = x + 1
   

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5. -- Tests
   double 1 `should_be` 2
   double 2 `should_be` 4
   -- Implementation
   double x
   | x == 1 = 2
   | x == 2 = 4
   

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6. A property of double
   double_is_always_even :: Int -> Bool
   double_is_always_even x = even (double x)
   

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7. > quickCheck double_is_always_even
   Failed: 0
   (after 1 test)
   Exception:
   Non-exhaustive patterns in function double
   

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8. double x = x * 2
   

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9. quickCheck double_is_always_even
   Passed: 0
   Passed: 1
   Passed: -3
   Passed: -1
   (...)
   Passed: -58
   Passed: 89
   +++ OK, passed 100 tests.
   

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10. More properties of double
    double_compare_to_input x
    | x > 0 = double x > x
    | x < 0 = double x < x
    | x == 0 = True
    double_minus_input x = double x - x == x
    

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11. Distributivity Law
    reverse_is_distributive xs ys =
    reverse (xs++ys) == reverse xs ++ reverse ys
    

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12. > quickCheck reverse_is_distributive
    Passed: [] []
    Passed: [] [1]
    Passed: [1] []
    Failed: [3,-3] [0,2,0]
    Passed: [] [0,2,0]
    Failed: [3] [0,2,0]
    Failed: [-3] [0,2,0]
    Failed: [0] [0,2,0]
    Failed: [0] [2,0]
    (...)
    Falsifiable (after 4 tests and 6 shrinks): [0] [1]
    

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13. Oops...
    reverse_is_distributive xs ys =
    reverse (xs++ys) == reverse ys ++ reverse xs
    

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14. Passed: [] []
    Passed: [1,-2] [0]
    Passed: [-2] []
    (...)
    Passed:
    [-34,44,-58,-41,-17,-53,-14,27,54,46,-10,-46,-20,46]
    [-9,-32,-47,50,43,-47,-43,-61,37,4,-59,48,34]
    +++ OK, passed 100 tests.
    

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15. Split 1
    -- define
    split :: Char -> String -> [String]
    -- so that
    split '@' "foo@example.com" == ["foo","example.com"]
    split '/' "/usr/include" == ["", "usr", "include"]
    1 https://www.schoolofhaskell.com/user/pbv/an-introduction-to-quickcheck-testing
    

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16. -- splitting an empty list results in an empty list
    split char [] = []
    split char str
    | null after = before : []
    | otherwise = before : split char (tail after)
    where
    before = takeWhile (/=char) str
    after = dropWhile (/=char) str
    

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17. Property:
    Splitting and unsplitting
    -- test
    unsplit '@' ["foo","example.com"] == "foo@example.com"
    unsplit '/' ["", "usr", "include"] == "/usr/include"
    --implementation
    unsplit :: Char -> [String] -> String
    unsplit char = concat . intersperse [char]
    

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18. -- property
    unsplit_inverses_split str =
    forAll (elements str)
    (\char -> unsplit char (split char str) == str)
    

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19. > quickCheck unsplit_inverses_split
    Passed: ""
    Passed: "\252\210" '\252'
    Passed: "\163^\EOT" '\163'
    Passed: "v\RSs" 'v'
    Failed: "y" 'y'
    Failed: "a" 'a'
    Falsifiable (after 6 tests and 1 shrink): "a" 'a'
    

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20. split 'a' "a" == [""]
    unsplit 'a' [""] == ""
    -- should be:
    split 'a' "a" == ["", ""]
    unsplit 'a' ["", ""] == "a"
    

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21. Modify the definition of split
    split char [] = []
    ...
    -- becomes
    split char [] = [""]
    ...
    > quickCheck unsplit_inverses_split
    +++ OK, passed 100 tests.
    

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22. Property Based Testing
    → test lots of random cases cheaply
    → encourage thinking about general properties
    Thanks :-)
    @mathiasverraes
    

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