Hi, I have import Control.Monad.Random import Control.Monad.State.Lazy import Control.Monad.Random.Class Though, I think the last one is superfluous. On 06/17/2016 08:31 PM, Ivan Lazar Miljenovic wrote:

On 18 June 2016 at 14:22, Christopher Howard wrote:

...

Hi. I'm working through "Haskell Design Patterns" and got inspired to try to create my first monad stack. What I really wanted though (not shown in the book) was to combine State and Rand. I daresay I got something to compile:

walk :: RandomGen g => StateT (Float, Float) (Rand g) (Float, Float) walk = do (x, y) <- get put (x + 1, y + 1) get >>= return

Can you provide more details? At the minimum, what imports/packages did you use (strict or lazy State? Which MonadRand did you use?).

Note also that "get >>= return" is equivalent to just "get"; you could even combine the first two lines using the "modify" function.

...

However, the moment I try to insert a getRandomR or something in it, I get an error

Could not deduce (MonadRandom (StateT (Float, Float) (Rand g))) arising from a use of `getRandomR' <...snip...> add an instance declaration for (MonadRandom (StateT (Float, Float) (Rand g)))

I see there are instances

MonadRandom m => MonadRandom (StateT s m) RandomGen g => MonadRandom (Rand g)

in Control.Monad.Random.Class, so I am not quite sure what is expected of me.

Is this the MonadRandom package? If so, I don't see the StateT instance there.

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Thanks, that was it! On 06/17/2016 09:27 PM, Erik Rantapaa wrote:

You are likely missing a `lift` when you call the random functions.

Here is an example:

import Control.Monad.State import Control.Monad.Random

walk :: RandomGen g => StateT (Float, Float) (Rand g) (Float, Float) walk = do (x, y) <- get put (x + 1, y + 1) get >>= return

foo :: RandomGen g => StateT (Float,Float) (Rand g) () foo = do a <- lift $ getRandomR (1,6) b <- lift $ getRandomR (4,10) (x,y) <- get put (x+a, y+b)

test1 = do g <- getStdGen print $ runRand (runStateT foo (0.0, 3.14)) g

Because the State monad is encapsulating (transforming) the random monad, you have to `lift` operations in the random monad so that they become operations in the transformed monad.

On Friday, June 17, 2016 at 11:22:57 PM UTC-5, Christopher Howard wrote:

Hi. I'm working through "Haskell Design Patterns" and got inspired to try to create my first monad stack. What I really wanted though (not shown in the book) was to combine State and Rand. I daresay I got something to compile:

walk :: RandomGen g => StateT (Float, Float) (Rand g) (Float, Float) walk = do (x, y) <- get put (x + 1, y + 1) get >>= return

However, the moment I try to insert a getRandomR or something in it, I get an error

Could not deduce (MonadRandom (StateT (Float, Float) (Rand g))) arising from a use of `getRandomR' <...snip...> add an instance declaration for (MonadRandom (StateT (Float, Float) (Rand g)))

I see there are instances

MonadRandom m => MonadRandom (StateT s m) RandomGen g => MonadRandom (Rand g)

in Control.Monad.Random.Class, so I am not quite sure what is expected of me.

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