[Haskell-beginners] crypto random UUID generation

[David McBride]

I was hasty with my words.  Exceptions in haskell have always been my
cryptonite.  If you are handling the only known exception properly then you
are doing everything right.

On Tue, Dec 13, 2016 at 6:41 AM, Ovidiu Deac <ovidiudeac at gmail.com> wrote:

> 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 <toad3k at gmail.com> 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 <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/d
>>> ocs/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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>>>
>>>
>>
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>
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