Hello all I have a data type data Index = Index {indexSize::Float, indexIds::[Int], indexDown::(IntMap.IntMap Index)} | IndexLeaf {indexSize::Float, indexIds::[Int]} | IndexEmpty {indexSize::Float} deriving ( Show , Eq ) I derived some thing like this for QuickCheck testing instance Arbitrary a => Arbitrary ( IntMap a ) where arbitrary = liftM IntMap.fromList arbitrary instance Arbitrary Index where arbitrary = oneof [ liftM3 Index arbitrary arbitrary arbitrary , liftM2 IndexLeaf arbitrary arbitrary , liftM IndexEmpty arbitrary ] but its generating infinite list of data. After reading the paper QuickCheck: A Lightweight Tool for Random Testing of Haskell Programshttp://www.eecs.northwestern.edu/%7Erobby/courses/395-495-2009-fall/quick.pd... http://www.eecs.northwestern.edu/%7Erobby/courses/395-495-2009-fall/quick.pd... i tried to write above one using sized but i am not sure how to write this . instance Arbitrary a => Arbitrary ( IntMap a ) where arbitrary = liftM IntMap.fromList arbitrary instance Arbitrary Index where arbitrary = sized arbIndex arbIndex 0 = liftM IndexEmpty arbitrary arbIndex 1 = liftM2 IndexLeaf arbitrary arbitrary arbIndex n = frequency [ ( 1 , liftM IndexEmpty arbitrary ) , ( 1 , liftM2 IndexLeaf arbitrary arbitrary ) , ( 2 , liftM2 Index arbitrary arbitrary ( arbIndex $ div n 2 ) ) ] but from this type i can see arbIndex $ div n 2 will return Gen Index not Gen ( IntMap Index ) and i am getting compiler error . Could some one please tell me , how to write arbIndex n in terms of smaller n like in the paper arbTree 0 = liftM Leaf arbitrary arbTree n = frequency[ ( 1 , liftM Leaf arbitrary ) , ( 4 , liftM2 Branch ( arbTree $ div n 2 ) ( arbTree $ div n 2 ) ) ] Thank You Mukesh Tiwari