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

[Ryan Ingram]

On 11/21/07, apfelmus <apfelmus at quantentunnel.de> 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.

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.
That said, the generalization to monad transformers and the metaprogramming
techniques demonstrated look pretty cool.

   -- ryan
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