Re: [Haskell-cafe] Help with triple stack monad

21 Jun 2016


      This doesn't seem to work for me, but I'm using 7.6.3 from Debian
Jessie. I'm curious what the difference is. What extensions and modules
do you import? I get error

ReaderStateRandom.hs:19:73:
    No instance for (MonadRandom (ReaderT r (StateT s (Rand g))))
      arising from the 'deriving' clause of a data type declaration
    Possible fix:
      add an instance declaration for
      (MonadRandom (ReaderT r (StateT s (Rand g))))
      or use a standalone 'deriving instance' declaration,
           so you can specify the instance context yourself
    When deriving the instance for (MonadRandom
                                      (ReaderStateRandom r s g))

On 06/21/2016 09:33 AM, Daniel Bergey wrote:
...
On 2016-06-21 at 02:12, Christopher Howard  wrote:
...
However, it seems that I must implement MonadRandom myself, as there is
no instance for this sort of arrangement already.
Deriving the class also works, at least with GHC-7.10.3:
newtype ReaderStateRandom r s g a = RSR {
  rSR :: ReaderT r (StateT s (Rand g)) a
  } deriving (Functor, Applicative, Monad, MonadReader r, MonadState s,
  MonadRandom)
bergey
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