Janis Voigtlaender wrote:
Wei Hu wrote:
Thanks for pointing out the sharing part. My original question is still unanswered: for lazy monads, can we give such a general mfix definition?
... (Constructing an explicit counterexample is left as an exercise ;-)
Oh, I couldn't resist to do my own exercise ...
import Control.Monad.Fix import Control.Monad.State.Lazy
data Nat = S Nat | Z deriving Show
tick :: State Nat () tick = get >>= put . S
test1 = runState (mfix (const tick)) Z
test2 = runState (mfix (const tick)) Z where mfix f = (mfix f) >>= f
Now: *Main> test1 ((),S Z) *Main> test2 ((),S (S (S (S (S (S (S (S (S (S (S (S (S (S (S (S (S (S ........ I think that proves the point. Ciao, Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de