Re: [Haskell-beginners] ListT + Writer

If you fill in Identity for m, ListT m Int ends up becoming Identity [Int], which is just a list. Doing effects over a list is a classic FoldM. You can use the foldl library on hackage to do most things, I think. To do it over a list of lists, that actually sounds like something you would use ListT for, but I don't use it much in my code, so I can't say for sure how well it will work for you. For what it is worth I've also heard that the version of ListT in base is not quite correct, although I don't know why. If you are feeling adventurous, check out the pipes tutorial http://hackage.haskell.org/package/pipes-4.3.4/docs/Pipes-Tutorial.html, there is a ListT done right section toward the end where it shows how to construct and use IO code from his version of a ListT. That would require you to understand and buy into the pipes ecosystem. On Thu, May 25, 2017 at 2:37 PM, aquagnu wrote:

David, many thanks for such a detailed answer and explanation!!

So, if I know that I'll iterate over list (vs abstract "m"'s bind), can I replace "m" with Identity, to get list instead of abstract "ListT m"?

And last question: what is more canonical (Haskelish) way to iterate over list with state? Fold*/State/Writer? Actually my ultimate goal was to process list items with some state and possible IO (REST calls, something else)... Is the usage of ListT + Writer (or ListT + State) a good solution or better is to make all logic in one function and to pass it to fold* (so state will be accumulating value)? I will iterate over items of items of this list too: some of them are also lists, so ListT looks more naturally IMHO, but I'm not sure...

David, in all cases - many thanks!!

В Thu, 25 May 2017 12:55:00 -0400 David McBride пишет:

...

To start all these types with T at the end are transformers. They are a type that is wrapped around some inner m. StateT s m, ErrorT e m a, and so on.

In order to use do notation, you must be in a type which is an instance of Monad.

newtype ListT (m :: * -> *) a = ListT {runListT :: m [a]} instance [safe] Monad m => Monad (ListT m)

newtype WriterT w (m :: * -> *) a = WriterT {runWriterT :: m (a, w)} instance [safe] (Monoid w, Monad m) => MonadWriter w (WriterT w m)

These types and their instances say the following: ListT m a is a Monad if m is a Monad. WriterT w m a is a Monad if m is a Monad and w is a Monoid.

So in order to use do notation in a WriterT String (ListT m) a, I must add the Monad m contstraint to proc, and also ensure that the writer's w is a monoid (it is because it is a string).

Now to pass in a ListT as an argument, I must construct one. Remember that in order to use the return function, m must be in a monad, so I must add the Monad constraint.

foo :: Monad m => ListT m Int foo = ListT (return [1,2,3])

test = (runListT $ runWriterT (proc3 foo)) >>= print

proc3 :: Monad m => ListT m Int -> WriterT String (ListT m) Int proc3 foo = do tell ("started: " :: String) x <- lift foo y <- lift $ ListT (return [3,4,5]) lift $ guard (y /= 5) tell ("x:" ++ show x) tell ("y:" ++ show y) return (x * y)

As you saw in the other comment in this thread, most people use a type alias to make it more palatable.

type MyApp m a = WriterT String (ListT m) Int -- or type MyApp a = WriterT String (ListT IO) Int

proc3 :: Monad m =>ListT m a -> MyApp m Int -- or proc3 :: ListT m a -> MyApp Int

On Thu, May 25, 2017 at 12:11 PM, Baa wrote:

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В Thu, 25 May 2017 11:52:01 -0400 David McBride пишет:

Hello, David! Am I right that "WriterT ... ListT" is "list of writers"? As I understand, internal representation is "m (a, w)" where m is a-la List? So, this is list of "writers"? I am confused only of this "m" in your "proc1" function, because I suppose this must be Identity and type becomes "WriterT String [Int]" ? Or?

Can this function "proc1" be modified in the way to get input list and to "iterate" over its elements with "do el <- ..." but to can call Writer's tell in the same time? This is the problem for my mind - I can not understand how to pass input list and to have writer inside :) You call ListT's bind but over internal hardcoded list values...

...

ListT is a bit weird in that it affects whatever monad is underneath it, so the order of your types in your Transformer stack matters. Both ways have different meanings and each have legitimate uses. In any case you must use the lift function to get to the monad below the one you are at.

import Control.Monad.List import Control.Monad.Writer

test :: IO () test = do (runListT $ runWriterT proc1) >>= print (runWriterT $ runListT proc2) >>= print return ()

proc1 :: Monad m => WriterT String (ListT m) Int proc1 = do tell ("started: " :: String) x <- lift $ ListT (return [1,2]) y <- lift $ ListT (return [3,4,5]) lift $ guard (y /= 5) tell ("x:" ++ show x) tell ("y:" ++ show y) return (x * y)

proc2 :: Monad m => ListT (WriterT String m) Int proc2 = do lift $ tell ("started: " :: String) x <- ListT (return [1,2]) y <- ListT (return [3,4,5]) guard (y /= 5) lift $ tell (" x:" ++ show x) lift $ tell (" y:" ++ show y)

return (x * y)

On Thu, May 25, 2017 at 11:10 AM, Baa wrote:

...

Hello, everybody!

I can process list in monad style with "do" syntax and to use "guard" function in the body. Something like:

fn :: [a] -> [a] fn lst = do el <- lst guard $ condition el ... return $ change el

How can I do the same but with possibility to call "tell" of "Write" monad in the fn's body? As I understand it should be:

ListT (Writer w) Int

for this example?

- but how to write it? - how to call (run) it? - and how is it safe ("transformers" package has bug in ListT, so "mtl" must be used?)? - is there other canonical way to do it without to use fold*, recursive calls/fix, State/RWS ?

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-- Best regards, Paul a.k.a. 6apcyk _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners