[Haskell-cafe] Re: An interesting monad: "Prompt"

apfelmus apfelmus at quantentunnel.de
Thu Nov 22 14:43:29 EST 2007

Ryan Ingram wrote:
> apfelmus wrote:
>> A slightly different point of view is that you use a term implementation
>> for your monad, at least for the interesting primitive effects
> That's a really interesting point of view, which had struck me slightly, but
> putting it quite clearly like that definitely helps me understand what is
> going on.
> In fact, it seems like I can implement the original "list" and "state"
> examples from the Unimo paper in terms of Prompt as well, using a
> specialized observation function.  For example:
>  data StateP s a where
>    Get :: StateP s s
>    Put :: s -> StateP s ()
> runStateP :: Prompt (StateP s) a -> s -> (a,s)
> runStateP (PromptDone a)     s = (a,s)
> runStateP (Prompt Get k)     s = runStateP (k s) s
> runStateP (Prompt (Put s) k) _ = runStateP (k ()) s
> instance MonadState s (Prompt (StatePrompt s)) where
>    get = prompt Get
>    put = prompt . Put
> Strangely, this makes me less happy about Prompt rather than more; if it's
> capable of representing any reasonable computation, it means it's not really
> the "targeted silver bullet" I was hoping for.  On the other hand, it still
> seems useful for what I am doing.

It appears that your prompt data type is basically Unimo with  Bind  and 
  Effect  fused, i.e.

   data Prompt p a where
      Return     :: a -> Prompt p a
      BindEffect :: p b -> (b -> Prompt p a) -> Prompt p a

I think that an explicit  Bind  isn't needed at all, the whole Unimo 
"framework" can be recast in terms of this type. This simplifies it 
considerably: the helper function  observe_monad  can be dropped and 
observation functions like  run_list  or run_state  can be implemented 
by directly pattern matching on Prompt. ( unit_op  and  bind_op  are 
nothing else than the two cases of this match)

(The other minor difference is that effects  p  does not explicitly 
contain monadic actions, but it's easy to introduce the recursion 

   data EffectPlus a where
      Mplus :: Prompt EffectPlus a -> Prompt EffectPlus a -> EffectPlus a
      Mzero :: EffectPlus a

In Unimo, the effect can be parametrized on the monad, whereas it's 
fixed here. But this is straightforward to rectify.)

> I definitely feel like the full term implementation (like the Unimo paper
> describes) is overkill; unless I'm misunderstanding what's going on there,
> you're basically destroying any ability for the compiler to reason about
> your computations by reifying them into data.  As long as (>>=) and return
> are functions that get inlined, lots of extraneous computation can be
> eliminated as the compiler "discovers" the monad laws through compile-time
> beta-reduction; once you turn them into data constructors that ability
> basically goes away.

I don't know what the compiler does, so I wouldn't recommend unlimited 
enthusiasm :)

But there's an efficiency issue with your implementation that the full 
term implementation doesn't have (contrary to what I believed in a 
previous post about the state moand): just like with lists and ++, using 
 >>= left-recursively runs in quadratic instead of linear time. Here's a 

   data Effect a = Foo a        -- example effect
   Bef m := BindEffect Foo      -- shorthand for the lengthy constructor

   x   = BindEffect Foo Return  -- just the example effect
       = z Return

   m >> n = m >>= \_ -> n       -- we use >> for simplicity

Now, consider evaluation to WHNF:

   (x >> x)
   => Bef f >> x                     -- reduce x to WHNF
   => Bef f (\b -> f b >> x)         -- definition of >>

   (x >> x) >> x
   => ..
   => (Bef f (\b -> f b >> x)) >> x  -- reduce (x >> x)
   => Bef f (\b -> (\b2 -> f b2 >> x) b >> x)
    = Bef f (\b -> (f b >> x) >> x)  -- shorthand

We see that in the general case, the evaluation of

   (..(((x >> x) >> x) >> x) ... ) = foldl1 (>>) (replicate n x)

will produce the expression

   Bef f (\b -> (..(((f b >> x) >> x) >> x)..))

in O(n) time. The problem is that this contains another left-recursive 
chain of (>>) which will take O(n-1) time to evaluate and produce 
another such chain when the monad is executed. Thus, the whole thing 
will run in O(n^2).

A context passing implementation (yielding the ContT monad transformer) 
will remedy this. See also

   John Hughes. The Design of a Pretty-printing Library.


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