The problem is with

  Left err -> throwIO err

Because of the type of 'runCRand', we know err is an instance of ContainsGenError e0, but which one?  We need a concrete error type before we can run this code.  Looking at the docs there seems to be only one instance of ContainsGenError, GenError, so a quick an dirty solution would be to change it to

  Left err -> throwIO (err :: GenError) -- should work

But keep in mind, if there were any other ContainsGenError instances, like from an external library that is adding a new type of random generator to this library that fails in a new way, you would not be catching that.

On Mon, Dec 12, 2016 at 12:52 PM, Ovidiu Deac <ovidiudeac@gmail.com> wrote:

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-Monad-CryptoRandom.html#v:getCRandomR

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!

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