I have to produce a crypto random UUID. I haven't found simple examples. and I used the one from hre (see type CRand) http://hackage.haskell.org/package/monadcryptorandom-0.7.0/docs/Control-Mona... My attempt is the following: cryptoRandomUUID :: IO UUID.UUID cryptoRandomUUID = do g <- newGenIO:: IO SystemRandom case runCRand impl g of Left err -> throwIO err Right (v, g') -> return v where impl = do w1 <- getCRandom w2 <- getCRandom w3 <- getCRandom w4 <- getCRandom return $ UUID.fromWords w1 w2 w3 w4 ...but the compilation fails miserably with: • Ambiguous type variable ‘e0’ arising from a use of ‘runCRand’ prevents the constraint ‘(ContainsGenError e0)’ from being solved. Relevant bindings include impl :: CRandT SystemRandom e0 Data.Functor.Identity.Identity UUID.UUID (bound at src/Party.hs:75:9) Probable fix: use a type annotation to specify what ‘e0’ should be. These potential instance exist: instance ContainsGenError GenError -- Defined in ‘Control.Monad.CryptoRandom’ • In the expression: runCRand impl g In a stmt of a 'do' block: case runCRand impl g of { Left err -> throwIO err Right (v, g') -> return v } In the expression: do { g <- newGenIO :: IO SystemRandom; case runCRand impl g of { Left err -> throwIO err Right (v, g') -> return v } } ... What's the problem here? Are there some good examples for generating crypto-randoms? Thanks!