[Haskell-beginners] crypto random UUID generation

[David McBride]

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 at 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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