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
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