CODEWITHSUNDEEP
haskellquickcheck
Jack
Jack

Reputation: 41

Haskell, QuickCheck, falsify a (wrong) property:

Is there a way to falsify this (wrong) property:

prop :: Eq a => [a] -> Bool
prop xs = reverse xs == xs

When i Use QuickCheck and later VerboseCheck it gives 100 different forms of:

[(),(),(),(),(),(),(),(),(),(),(),(),(),(),()]

Passed:
and the final result is:

+++ OK, passed 100 tests.

Upvotes: 4

Views: 326

Answers (1)

Ørjan Johansen
Ørjan Johansen

Reputation: 18199

It just so happens that

  1. If you try to evaluate that in GHCi, it has to choose a particular instance type of Eq a to use, and with the ExtendedDefaultRules extension normally enabled in GHCi, it chooses ().
  2. For the type (), because it has only one (non-bottom) value, the proposition is actually true.

The simplest fix is to choose (almost) any other type by providing a type annotation:

Prelude Test.QuickCheck> quickCheck (prop :: [Int] -> Bool)
*** Failed! Falsifiable (after 4 tests and 3 shrinks): 
[0,1]

Upvotes: 10

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