With the following module: module Main where import Random data Foo = Foo StdGen main :: IO() main = do let rs = randoms (Foo (mkStdGen 39)) :: [Int] rRs = randomRs (0,9) (Foo (mkStdGen 39)) :: [Int] putStrLn $ show $ take 100 rs putStrLn $ show $ take 100 rRs instance RandomGen Foo where genRange _ = (0, 1) next (Foo g) = (val `mod` 2, Foo g') where (val, g') = random g split _ = error "Not implemented" ghc gives me [-2147476078,7482,-2147476078,-2147476078,-2147476079,-2147476078,7483,7482,7482,-2147476079,-2147476078,-2147476079,-2147476079,7482,-2147476078,-2147476078,-2147476078,-2147476079,7482,7483,7482,7483,7482,-2147476078,-2147476078,-2147476078,7483,-2147476079,7482,-2147476078,-2147476079,-2147476078,7483,-2147476079,7483,7482,-2147476079,7483,7482,7483,-2147476078,-2147476079,-2147476079,7482,-2147476078,7482,-2147476079,-2147476079,7482,-2147476078,7483,7483,-2147476079,-2147476078,7483,7483,-2147476078,-2147476079,-2147476078,-2147476079,-2147476078,7483,7483,7482,7482,7483,-2147476078,-2147476079,-2147476079,7482,7483,-2147476078,-2147476079,-2147476079,-2147476078,7482,7483,-2147476079,7482,7482,7482,7483,-2147476079,-2147476079,-2147476078,7482,7482,7482,7482,-2147476079,7482,7482,-2147476079,7483,-2147476078,7482,7483,-2147476079,-2147476079,7482] [1,2,2,1,1,2,1,2,1,1,1,2,2,2,2,1,2,1,1,1,1,2,1,1,1,1,2,1,1,2,1,2,1,2,1,1,2,1,2,2,2,1,2,2,2,1,1,2,1,2,1,2,2,2,1,1,2,1,1,2,1,1,1,2,2,2,1,1,2,2,2,1,1,1,2,2,2,1,2,2,1,2,1,1,1,1,2,1,1,2,2,1,1,1,1,1,2,1,1,2] The first list doesn't seem to cover the whole spectrum and I was expecting the second list to be composed over 0s through 9s. Is my understanding wrong? nhc complains Context for Random.Random needed in left hand pattern at 17:11. and hugs doesn't seem to know genRange exists. With it commented out it returns the same as ghc. Thanks Ian