[Haskell-cafe] Re: Is my code too complicated?

[Ertugrul Soeylemez]

Bulat Ziganshin <bulat.ziganshin at gmail.com> wrote:

> Saturday, July 3, 2010, 4:25:22 PM, you wrote:
>
> > This has proven very useful for me.  My usual way is writing monad
> > transformers and sticking them together, often together with
> > concurrent programming.
>
> > ... /what/ my code is
> > doing, because it's written in natural language as much as possible
>
> can we see such code? i always thought that monad transformers are
> hard to use since you need to lift operations from inner monads on
> every use

This may appear like a disadvantage, but as your monad gets more
complicated, this becomes a virtue, because it gives you great type
safety.  In general, you can write specialized lifting functions for
specialized monads.  Here is an example monad:

  type CounterT = StateT

  runCounterT :: (Functor m, Num c) => CounterT c m a -> m a
  runCounterT c = fst <$> runStateT 0 c

  increment :: (Monad m, Num c) => CounterT c m ()
  increment = sets_ (+1)

  decrement :: (Monad m, Num c) => CounterT c m ()
  decrement = sets_ (subtract 1)

  printCounter :: (BaseM m IO, Show c) => CounterT c m ()
  printCounter = get >>= inBase . print

Let's build a custom monad using CounterT somewhere in the middle:

  type MyMonad = IdT (IdT (CounterT Integer (IdT IO)))

Now let's write the specialized lifting functions:

  myMonadIO :: IO a -> MyMonad a
  myMonadIO = inBase

  myMonadInnerId :: IdT IO a -> MyMonad a
  myMonadInnerId = lift . lift . lift

  myMonadCtr :: CounterT Integer (IdT IO) a -> MyMonad a
  myMonadCtr = lift . lift

  myMonadOuterId :: IdT (CounterT Integer (IdT IO)) a -> MyMonad a
  myMonadOuterId = lift

As you can see this can get quite ugly and tiresome.  There is a much
easier approach, inspired by monadLib's 'inBase' function:

  class (Monad m, Monad n) => CounterM m n | m -> n where
    inCtr :: n a -> m a

  instance Monad m => CounterM (CounterT c m) (CounterT c m) where
    inCtr = id

  instance CounterM m n => CounterM (IdT m) n where
    inCtr = lift . inCtr

This requires a bunch of type system extensions, though, most notably
the UndecidableInstances extension.  But it's safe to use here.  Now you
can get along without custom lifting functions entirely:

  testComp ::  MyMonad ()
  testComp = do
    x <- inCtr $ increment >> increment >> get
    inBase $ print x
    y <- inCtr $ decrement >> decrement >> get
    inBase $ print y

The type system calculates the proper number of lifts for you here and
provides them through the 'inCtr' function.


Greets,
Ertugrul


-- 
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