Re: [Haskell-beginners] Overriding >>= for a Monad?

On Thu, Feb 26, 2009 at 5:46 PM, Patrick LeBoutillier wrote:

Hi all,

I'm using Control.Monad.StateT as such:

 data TapState = TapState {   planSet :: Bool,   noPlan :: Bool,   skipAll :: Bool,   testDied :: Bool,   expectedTests :: Int,   executedTests :: Int,   failedTests :: Int  } deriving (Show)

 type TAP a = StateT TapState IO a

but I'd like to provide my own >>= function. Is there a way to "derive" a new type from StateT in order to implement my own >>=? Is thin done using "instance"?

Thanks,

Patrick

-- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

The most direct way would probably to copy StateT's source code and tinker with the >>= definition (also change the name to MyStateT or whatever). You could also define

newtype MyStateT s a = MyStateT (StateT s IO a)

instance Monad (MyStateT s) where return = MyStateT . return (>>=) = ...

Alex