haskell - Defining bind for home-made monad transformer -

i have defined own version of writert along function unwrap it:

newtype wt w m = wt (m a, w)  unwt :: (monoid w, monad m) => wt w m -> (m a, w) unwt (wt cmaw) = cmaw 

now trying define monad (wt w m), without success:

instance (monoid w, monad m) => monad (wt w m)   return x = wt (return x, mempty)   wtwma >>= fawtwmb = wt $ let (ma, w1) = unwt wtwma                                (mb, w2) = unwt $ <- ma                                                     fawtwmb                            in (mb, mappend w1 w2) 

error located in do-expression, @ point try extract a ma:

expected type: wt w m a, actual type: m 

i have tried few variations, similar outcome. unable define bind monad.

my main question is: if monad inside couple, how extract value?

imagine following computation:

tellline :: wt string io () tellline =     input <- wt (getline, "")     wt (return (), input)  impossible :: string impossible = snd (unwt tellline) 

if works "expected", should impossible string user entered. however, cannot outside of io; such impossible not pure string here. not possible yield w recieved result of computation in monad.

another possibility return w occurred before first bind, don't have depend on monadic actions. alas, 1 of identity laws thwarts us.

return x >>= f = f x 

here can see "before first bind" cannot meaningful concept, since there bind on left not on right. returning w occurs before first bind surely violate monad law.

the remaining possibility w mempty. not work either, because of other identity law.

m >>= return = m 

so if m has non-mempty value of w, bind annihilate , violate law.

this wt cannote monad transformer. there writert transfomer, defined m (a,w) avoids these problems.