[Haskell-cafe] quickCheck problem

Bryan Richter b at chreekat.net
Wed Oct 7 16:15:58 UTC 2015

On Tue, Oct 06, 2015 at 10:24:03PM +0200, Roelof Wobben wrote:
> Hello,
> I have written a function to test if three numbers are the same.
> Now I want to test my functions by using quickCheck.
> So I thought to test first if all three are the same.
> but how can I tell quickcheck that all numbers are the same.

Easy, just generate a single number. :) Make it correct by

    prop_allSame x =
        yourFunc x x x == True

> and second question :  hwo do the test likeif two numbers are the same.

Now the hard part is not generating two numbers that are the same, but
generating two *different* numbers. To do that you may need to create
a custom generator.

unequalPair = do
    x <- arbitrary
    y <- arbitrary `suchThat` y /= x
    return (x,y)

Now you can use that generator:

prop_twoSame = forAll unequalPair $ \(x,y) ->
    yourFunc x x y == False
    -- But you might want to test all permutations!
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