State Monad, sequences of random numbers and monadic code

Question

I\'m trying to grasp the State Monad and with this purpose I wanted to write a monadic code that would generate a sequence of random numbers using a Linear Congruential Gene

Answer 1

First of all, your example is overly complicated because it doesn't need to store the val in the state monad; only the seed is the persistent state. Second, I think you will have better luck if instead of using the standard state monad, you re-implement all of the state monad and its operations yourself, with their types. I think you will learn more this way. Here are a couple of declarations to get you started:

data MyState s a = MyState (s -> (s, b))

get :: Mystate s s
put :: s -> Mystate s ()

Then you can write your own connectives:

unit :: a -> Mystate s a
bind :: Mystate s a -> (a -> Mystate s b) -> Mystate s b

Finally

data Seed = Seed Int
nextVal :: Mystate Seed Bool

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As for your trouble desugaring, the do notation you are using is pretty sophisticated. But desugaring is a line-at-a-time mechanical procedure. As near as I can make out, your code should desugar like this (going back to your original types and code, which I disagree with):

 nextVal = get >>= \ Random seed val ->
                      let seed' = updateSeed seed
                          val'  = even seed'
                      in  put (Random seed' val') >>= \ _ -> return val'

In order to make the nesting structure a bit clearer, I've taken major liberties with the indentation.