On Thursday 03 February 2011 20:18:43, michael rice wrote:
Given the first program, it seems that the unchanging first element of the tuple could be handled by a Reader monad, leading to the second program, where b becomes the state, but how do I get the constant a from the Reader monad?
You need a monad-transformer to use both, Reader and State. You can use either ReaderT Double (State Double) or StateT Double (Reader Double) (they're isomorphic). Then you can query the modifiable state with get (from the MonadState class) and the immutable with ask (from the MonadReader class) type Heron = StateT Double (Reader Double) sqrtH :: Heron Double sqrtH = do a <- ask b <- get let c = 0.5*(b + a/b) if (good enough) then return c else put c >> sqrtH mySqrt a = runReader (evalStateT sqrtH (a*0.5)) a