[Haskell-cafe] random-fu confusion

[Alex Rozenshteyn]

Okay, I figured the immutability bit out and I got the IORef example
working, but I can't get it to work with state.

> put (pureMT 0) >>= runRVar flipCoin

gives me two type errors: "No instance for (MonadState PureMT m)" and "No
instance for (RandomSource m ())"

> runState $ put (pureMT 0) >>= runRVar flipCoin
> runState $ put (pureMT 0) >> get >>= runRVar flipCoin
> put (pureMT 0) >> get >>= runRVar flipCoin

and other desperate attempts, some of which in hindsight are too
embarrassing to list give me similar errors.  I'm trying to do figure out
how to do this without going to the IO monad (so I can run it with the same
seed to replicate results).

On Tue, Sep 7, 2010 at 3:14 PM, James Andrew Cook <mokus at deepbondi.net>wrote:

> A PureMT generator is immutable, so must be threaded through the monad in
> which you are sampling.  There are RandomSource instances provided for a few
> special cases, including "IORef PureMT" in the IO monad.  For example:
>
> main = do
>    mt <- newPureMT
>    src <- newIORef mt
>    flips <- runRVar (replicateM 20 flipCoin) src
>    print flips
>
> Alternatively, the functions in the module you mentioned can be used to
> define additional instances, such as:
>
> instance MonadRandom (State PureMT) where
>    supportedPrims _ _ = True
>    getSupportedRandomPrim = getRandomPrimFromPureMTState
>
> And RandomSource instances look almost the same.  See the
> Data.Random.Source.PureMT source for examples.  (I thought I had included
> this particular instance in the distribution but I apparently missed it.
>  The next release will probably include this as well as corresponding
> instances for the 'transformers' package, possibly separated out into
> 'random-fu-mtl' and 'random-fu-transformers' packages).
>
> The "StdRandom" type is a convenient "RandomSource" designating this
> instance in the State PureMT monad.  Personally, I prefer to use the
> "sample" function for this purpose, as well as the "sampleFrom" function in
> place of runRVar/runRVarT.  GHCi does not display the "sample" functions'
> types properly - they are defined for RVarT as well as for all Distribution
> instances.
>
> Sorry it took so long responding.
>
> -- James
>
> On Sep 2, 2010, at 10:01 AM, Alex Rozenshteyn wrote:
>
> > I seem to be having confusion at the runRVar level of random-fu.
> >
> > I can't figure out how to use the Data.Random.Source.PureMT module to get
> a meaningful random source (I can't get my code to type-check).
> >
> > I wrote a [trivial] flipCoin function
> > > flipCoin = uniform False True
> > and am trying to fill in the final place of runRVar
> > > :t runRVar (replicateM 20 flipCoin)
> > runRVar (replicateM 20 flipCoin)
> >   :: (RandomSource m s) => s -> m [Bool]
> >
> >
> > --
> >           Alex R
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>


-- 
          Alex R
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