Re: [Haskell-cafe] Still stacking monad transformers
On Mon, 2008-10-13 at 18:28 +0100, Andrew Coppin wrote: Reid Barton wrote: It's not difficult: the operation is called mplus :: MyMonad a - MyMonad a - MyMonad a and already exists (assuming the author of ListT has not forgotten to write a MonadPlus instance). I see... I was under the impression that mplus is just any arbitrary binary operation over a given monad. How do you know what it does for a specific monad? Process of elimination. Sometimes, this doesn't narrow things down to a single operation, but it gives you a good idea of what you're supposed to expect. Firstly, mplus and mzero form a (natural) monoid, put together. That rules out a number of binary operations right there. Secondly, mzero has a null law with (=): mzero = f = mzero So, if you have a `mplus` b and a calls mzero at some point (not inside another call to mplus --- nice and informal, that description :), then you know b will be executed instead. (Maybe b will be executed *anyway*. I didn't say anything about that). So mplus and mzero are basically suitable for three kinds of things: * Exception handling * Back-tracking * Parallelism Usually, when you see a MonadPlus instance, you expect one or more of these. That's in the general case. ListT is a special case; the (somewhat idealized) specification of ListT (what people want to happen when they use ListT) is that ListT m in some sense `adds back-tracking' to m. Where back-tracking choice is implemented by mplus. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
Reid Barton wrote: It's not difficult: the operation is called mplus :: MyMonad a - MyMonad a - MyMonad a and already exists (assuming the author of ListT has not forgotten to write a MonadPlus instance). I see... I was under the impression that mplus is just any arbitrary binary operation over a given monad. How do you know what it does for a specific monad? Anyway, utilising this trick, I now have my function working quite well. Implementing negation is the only hard part; I need to unwind everything down to the list level, and check whether the list is empty, and do something different depending on whether it is or it isn't: foo = do ... let x = run_some_moadic_action if null x then ... else ... This obviously fails since x isn't a list, it's a StateT MyState (ListT (ErrorT MyError Ideneity)) x. I can't see a nice way to handle this. I found a way that works, but it's quite ugly... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
Jonathan Cast wrote: I see... I was under the impression that mplus is just any arbitrary binary operation over a given monad. How do you know what it does for a specific monad? Process of elimination. Sometimes, this doesn't narrow things down to a single operation, but it gives you a good idea of what you're supposed to expect. Firstly, mplus and mzero form a (natural) monoid, put together. That rules out a number of binary operations right there. Secondly, mzero has a null law with (=): mzero = f = mzero So, if you have a `mplus` b and a calls mzero at some point (not inside another call to mplus --- nice and informal, that description :), then you know b will be executed instead. (Maybe b will be executed *anyway*. I didn't say anything about that). So mplus and mzero are basically suitable for three kinds of things: * Exception handling * Back-tracking * Parallelism Usually, when you see a MonadPlus instance, you expect one or more of these. That's in the general case. ListT is a special case; the (somewhat idealized) specification of ListT (what people want to happen when they use ListT) is that ListT m in some sense `adds back-tracking' to m. Where back-tracking choice is implemented by mplus. Right. OK. So... isn't there a class somewhere called MonadChoice or similar, which defines (|)? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
On Mon, 2008-10-13 at 18:58 +0100, Andrew Coppin wrote: Jonathan Cast wrote: I see... I was under the impression that mplus is just any arbitrary binary operation over a given monad. How do you know what it does for a specific monad? Process of elimination. Sometimes, this doesn't narrow things down to a single operation, but it gives you a good idea of what you're supposed to expect. Firstly, mplus and mzero form a (natural) monoid, put together. That rules out a number of binary operations right there. Secondly, mzero has a null law with (=): mzero = f = mzero So, if you have a `mplus` b and a calls mzero at some point (not inside another call to mplus --- nice and informal, that description :), then you know b will be executed instead. (Maybe b will be executed *anyway*. I didn't say anything about that). So mplus and mzero are basically suitable for three kinds of things: * Exception handling * Back-tracking * Parallelism Usually, when you see a MonadPlus instance, you expect one or more of these. That's in the general case. ListT is a special case; the (somewhat idealized) specification of ListT (what people want to happen when they use ListT) is that ListT m in some sense `adds back-tracking' to m. Where back-tracking choice is implemented by mplus. Right. OK. So... isn't there a class somewhere called MonadChoice or similar, which defines (|)? It's called Alternative: class Applicative f = Alternative f where empty :: f a (|) :: f a - f a - fa It's basically MonadPlus, weakened to just applicative functions. (I think that the name (|) was probably found after mplus, which is why MonadPlus doesn't use it.) So you can expect, for an arbitrary monad, that the good defintion(s) for mplus and the good definition(s) for (| ) will coincide. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
On Mon, 13 Oct 2008, Andrew Coppin wrote: Reid Barton wrote: It's not difficult: the operation is called mplus :: MyMonad a - MyMonad a - MyMonad a and already exists (assuming the author of ListT has not forgotten to write a MonadPlus instance). I see... I was under the impression that mplus is just any arbitrary binary operation over a given monad. How do you know what it does for a specific monad? I had imagined the definition of mplus to be similar in spirit to what bind is for a specific monad. i.e. it's part of that monad's strategy for achieving what it does. As for knowing what it does, trial and error, reading API docs and source. :) Something similar to this discussion had come up recently for me, list monad's MonadPlus implementation. We found ourselves doing something like this to model 'use default value if empty' for strings: let foo = case str of [] - default s - s Right away I was wishing I could do this: let foo = str `or-if-empty` default If it was a Maybe, this works with mplus: (Just foo) `mplus` (Just bar) == Just foo Nothing `mplus` (Just bar) == Just bar But not so much for list, mplus just ain't defined that way, instead doing concatination: foo `mplus` bar == foobar `mplus` bar == bar I ended up writing a special mplus' for this (thanks to #haskell!): mplus' :: (MonadPlus m, Eq (m a)) = m a - m a - m a mplus' x y | x == mzero = y | otherwise = x foo `mplus'` bar == foo `mplus'` bar == bar -- Dino Morelli email: [EMAIL PROTECTED] web: http://ui3.info/d/ irc: dino- pubkey: http://ui3.info/d/dino-4AA4F02D-pub.gpg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
Hi Andrew, On Mon, Oct 13, 2008 at 19:58, Andrew Coppin [EMAIL PROTECTED] wrote: Right. OK. So... isn't there a class somewhere called MonadChoice or similar, which defines (|)? Just to pitch in a helpful tip, Hoogle is excellent for these kind of questions (which come up very often): http://www.haskell.org/hoogle/?q=%3C|%3E cheers, Arnar ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
Dino Morelli wrote: I was wishing I could do this: let foo = str `or-if-empty` default If it was a Maybe, this works with mplus: (Just foo) `mplus` (Just bar) == Just foo Nothing `mplus` (Just bar) == Just bar But not so much for list, mplus just ain't defined that way, instead doing concatination: foo `mplus` bar == foobar `mplus` bar == bar The difference between Maybe-style MonadPlus and List-style MonadPlus is discussed on http://www.haskell.org/haskellwiki/MonadPlus_reform_proposal. With that proposal, you could use morelse with both Maybe and lists. Personally, I would like to call these beasts | (for Maybe-style mplus) and + (for List-style mplus). One could even have a | b = a | pure b a + b = a + pure b a + b = pure a + b in the style of * and *. Note that I assume Applicative as a superclass of Monad, while I'm talking about a better world anyway. That said, I would consider using something like the + sketched here for Strings as a hack, because I do not see Strings as Chars under the List functor, even if they technically may be exactly that. Since abc does not represent multiple results, I don't think should represent failure. Instead, I would wrap optional strings in Maybe, and then use fromMaybe to handle the default values: fromMaybe bar (Just foo) = foo fromMaybe bar Nothing = bar Now, with the operators sketched above, fromMaybe = flip (|), so one could use Just foo | bar = foo Nothing| bar = bar which seems to closely correspond to what you want. Tillmann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Still stacking monad transformers
I am becoming extremely frustrated now. The task I want to perform is simple, yet I simply cannot make Haskell do what I want. I've given up hope of ever getting my program to handle infinite result sets. That means I can make do with just ListT. So I have the following monad: type MyMonad x = StateT MyState (ListT Identity) x Now I'm trying to run two computations, starting from _the same state_, and combine the two resulting lists. The trouble is, I am literally losing the will to live trying to comprehend the whinings of the type checker. The operation I'm trying to perform is perfectly simple; I don't understand why this has to be so damned *difficult*! _ Any suggestions? I found that by using the brief and easily memorable construction runIdentity $ runListT $ runStateT foo state I can get at the result set for each action, and combine them. But nothing in hell seems to transform this from [((), MyState)] back into MyMonad (). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
On Sun, 2008-10-12 at 18:08 +0100, Andrew Coppin wrote: I am becoming extremely frustrated now. The task I want to perform is simple, yet I simply cannot make Haskell do what I want. I've given up hope of ever getting my program to handle infinite result sets. That means I can make do with just ListT. So I have the following monad: type MyMonad x = StateT MyState (ListT Identity) x Now I'm trying to run two computations, starting from _the same state_, and combine the two resulting lists. The trouble is, I am literally losing the will to live trying to comprehend the whinings of the type checker. The operation I'm trying to perform is perfectly simple; I don't understand why this has to be so damned *difficult*! _ Any suggestions? Have you tried pure lazy functional programming without stacked monads? I've never been convinced that stacked monads is a good way to write ordinary code. Monad transformers are great for building your own custom monads but they should be wrapped in a newtype and made abstract. One shouldn't have to see the multiple layers. If ordinary code is full of 'lift' then it would seem to me that one is doing something wrong. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
On 12 Oct 2008, at 21:08, Andrew Coppin wrote: I found that by using the brief and easily memorable construction runIdentity $ runListT $ runStateT foo state I can get at the result set for each action, and combine them. But nothing in hell seems to transform this from [((), MyState)] back into MyMonad (). Well, State monad (and StateT transformer) doesn't work with STATE, they work with STATE CHANGES. So, instead of [((), MyState], you should have something like (MyState - [((), MyState)]). And that can be transformed to MyMonad () quite easily: Prelude Control.Monad.State Control.Monad.List Control.Monad.Identity :t \f - StateT $ ListT . Identity . f \f - StateT $ ListT . Identity . f :: (s - [(a, s)]) - StateT s (ListT Identity) a ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
On Sun, Oct 12, 2008 at 1:08 PM, Andrew Coppin [EMAIL PROTECTED] wrote: I am becoming extremely frustrated now. The task I want to perform is simple, yet I simply cannot make Haskell do what I want. I've given up hope of ever getting my program to handle infinite result sets. Did you miss this message? http://article.gmane.org/gmane.comp.lang.haskell.cafe/45952/ -- Dave Menendez [EMAIL PROTECTED] http://www.eyrie.org/~zednenem/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Still stacking monad transformers
David Menendez wrote: On Sun, Oct 12, 2008 at 1:08 PM, Andrew Coppin [EMAIL PROTECTED] wrote: I am becoming extremely frustrated now. The task I want to perform is simple, yet I simply cannot make Haskell do what I want. I've given up hope of ever getting my program to handle infinite result sets. Did you miss this message? http://article.gmane.org/gmane.comp.lang.haskell.cafe/45952/ And if you don't like that one, there's also LogicT http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logict. The function you're looking for is called Control.Monad.Logic.interleave. I know LogicT and fair disjunction were brought up earlier, though I seem to have mislaid the post. In case you don't like the efficient Logic or LogicT implementations of MonadLogic, defining your own only requires that you can define msplit :: m a - m (Maybe (a, m a)) which pulls the first content out of your monad without forcing the rest of it. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
