[Haskell-cafe] quickCheck problem
Richard A. O'Keefe
ok at cs.otago.ac.nz
Wed Oct 7 23:16:29 UTC 2015
You can generate two unequal integers by doing
prop_twoDifferent :: Int -> Int -> Bool
prop_twoDifferent x y = yourFn2 x y'
where y' = if y >= x then y+1 else y
Letting the number of distinct Int values be N,
there are N*N pairs of Ints but only N*(N-1) pairs of
*different* Ints, so this technique will generate some
x y' pairs more often than others. In fact you will
get (x,minBound::Int) with probability 2/N and
(x,y>minBound) with probability 1/N.
The advantage of using a guard is that the probabilities
come out right (it's the "rejection method", in
You can use that method to generate
x x y
x y x
y x x
You can iterate the same idea to generate triples.
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