[Haskell-cafe] Question about the Monad instance for Iteratee (from the enumerator package)

19 Apr 2011


      Hello,

for reference, said instance is:
...
instance Monad m => Monad (Iteratee a m) where
  return x = yield x (Chunks [])
m0 >>= f = ($ m0) $ fix $
      \bind m -> Iteratee $ runIteratee m >>= \r1 ->
      	case r1 of
      		Continue k -> return (Continue (bind . k))
      		Error err -> return (Error err)
      		Yield x (Chunks []) -> runIteratee (f x)
      		Yield x extra -> runIteratee (f x) >>= \r2 ->
      			case r2 of
      				Continue k -> runIteratee (k extra)
      				Error err -> return (Error err)
      				Yield x' _ -> return (Yield x' extra)
The thing I don't understand yet is the last line: Why is it OK to discard the 
leftover input from the (f x) Iteratee and yield just the leftover input from 
the first one (m0)?

Cheers,
Daniel

[Haskell-cafe] Question about the Monad instance for Iteratee (from the enumerator package)

Daniel Schüssler