Mukesh, You need to write a generator function for the type (IntMap Index) which keeps track of the n parameter that you're using in arbIndex. You can't rely on the Arbitrary instance for (IntMap a) because it doesn't have access to the n parameter that you are so controlling in arbIndex. (Actually, you could use the "size" number that QuickCheck keeps behind the scenes, using the functions "sized" and "resize", but that would have the consequence of affecting all other arbitrary values generated as part of that recursion--including the floats and ints and so on, which may or may not be what you want.) Also, since your trees are unranked, rather than binary as in the paper example, take care with the factor of 2 that you're using to resize the generated subtrees. You might find that the resulting trees are still much bigger than you want, or have different statistical properties. Hope that helps-- Ezra On Friday, September 23, 2011 4:22 AM, "mukesh tiwari" wrote: 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 [1]QuickCheck: A Lightweight Tool for Random Testing of Haskell Programs 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe References Visible links 1. http://www.eecs.northwestern.edu/%7Erobby/courses/395-495-2009-fall/quick.pd... Hidden links: 2. http://www.eecs.northwestern.edu/%7Erobby/courses/395-495-2009-fall/quick.pd...