Re: [Haskell-beginners] crypto random UUID generation

13 Dec 2016

      Thanks! It works.

Why is this a "quick and dirty" fix and what would be the "clean" fix?

On Mon, Dec 12, 2016 at 8:15 PM, David McBride  wrote:
...
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 
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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