That is most likely, because ap is not in Prelude. You need to import Control.Monad. Von: Damien Mattei Gesendet: Mittwoch, 27. Februar 2019 11:54 An: Yuji Yamamoto Cc: haskell-cafe Betreff: Re: [Haskell-cafe] example of monad fromhttp://learnyouahaskell.com not working can you give me a complete solution please? i suppose i can set pure = return but have some diffculties with <*> , ap does not works On Wed, Feb 27, 2019 at 11:10 AM Yuji Yamamoto wrote: It's the one of the biggest changes of Haskell since LYHG was released. As you guess, now any instance of Monad must be an instance of Applicative. So you have to declare Prob as an instance of Applicative: instance Applicative Prob where    pure = ...    f <*> x = ... 2019年2月27日(水) 18:56 Damien Mattei : i'm trying this example (see code below) from : http://learnyouahaskell.com/for-a-few-monads-more#making-monads when trying to compile this: import Data.Ratio newtype Prob a = Prob { getProb :: [(a,Rational)] } deriving Show instance Functor Prob where     fmap f (Prob xs) = Prob $ map (\(x,p) -> (f x,p)) xs     thisSituation :: Prob (Prob Char) thisSituation = Prob     [( Prob [('a',1%2),('b',1%2)] , 1%4 )     ,( Prob [('c',1%2),('d',1%2)] , 3%4)     ] flatten :: Prob (Prob a) -> Prob a flatten (Prob xs) = Prob $ concat $ map multAll xs     where multAll (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs   instance Monad Prob where   return x = Prob [(x,1%1)]   m >>= f = flatten (fmap f m)   fail _ = Prob [] l1 = Prob [('a',2%3),('b',1%3)] multAllExt :: (Prob a, Rational) -> [(a, Rational)] multAllExt (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs --Main> :type multAllExt --multAllExt :: (Prob a, Rational) -> [(a, Rational)] --Main> multAllExt (l1,1 % 4) --[('a',1 % 6),('b',1 % 12)] i get this error: GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help Prelude> :load monade.hs [1 of 1] Compiling Main             ( monade.hs, interpreted ) monade.hs:21:10: error:     • No instance for (Applicative Prob)         arising from the superclasses of an instance declaration     • In the instance declaration for ‘Monad Prob’    | 21 | instance Monad Prob where    |          ^^^^^^^^^^ Failed, no modules loaded. it fails when i add the last part of the example: instance Monad Prob where   return x = Prob [(x,1%1)]   m >>= f = flatten (fmap f m)   fail _ = Prob [] seems the Monad needs an instance of the Applicative to be instanciated... what is wrong? regards, Damien _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. -- 山本悠滋 twitter: https://twitter.com/igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep

i mean if y had: a <- coin b <- coin c <- loadedCoin but only compute return (all (==Tails) [a] what i really does ot understand is how the probability is normalised , yes monad again keeps an air of mystery for me.... On Thu, Feb 28, 2019 at 12:05 AM Damien Mattei wrote:

thanks for your code, pretty example, i understand well flatten and all the function but at last i cannot figure out how the result come.... in the example: flipThree :: Prob Bool flipThree = do a <- coin b <- coin c <- loadedCoin return (all (==Tails) [a,b,c])

the strange thing is if i add more variables as : d <- coin it oes not change the probability but the output yes! example: Prob {getProb = [(True,1 % 80),(True,1 % 80),(True,9 % 80),(True,9 % 80),(True,1 % 80),(True,1 % 80),(True,9 % 80),(True,9 % 80),(True,1 % 80),(True,1 % 80),(True,9 % 80),(True,9 % 80),(True,1 % 80),(True,1 % 80),(True,9 % 80),(True,9 % 80)]}

On Wed, Feb 27, 2019 at 12:01 PM Jos Kusiek wrote:

...

That is most likely, because ap is not in Prelude. You need to import Control.Monad.

*Von: *Damien Mattei *Gesendet: *Mittwoch, 27. Februar 2019 11:54 *An: *Yuji Yamamoto *Cc: *haskell-cafe *Betreff: *Re: [Haskell-cafe] example of monad fromhttp:// learnyouahaskell.com not working

can you give me a complete solution please?

i suppose i can set pure = return

but have some diffculties with <*> , ap does not works

On Wed, Feb 27, 2019 at 11:10 AM Yuji Yamamoto < whosekiteneverfly@gmail.com> wrote:

It's the one of the biggest changes of Haskell since LYHG was released.

As you guess, now any instance of Monad must be an instance of Applicative.

So you have to declare Prob as an instance of Applicative:

instance Applicative Prob where

pure = ...

f <*> x = ...

2019年2月27日(水) 18:56 Damien Mattei :

i'm trying this example (see code below) from :

http://learnyouahaskell.com/for-a-few-monads-more#making-monads

when trying to compile this:

import Data.Ratio

newtype Prob a = Prob { getProb :: [(a,Rational)] } deriving Show

instance Functor Prob where fmap f (Prob xs) = Prob $ map (\(x,p) -> (f x,p)) xs

thisSituation :: Prob (Prob Char) thisSituation = Prob [( Prob [('a',1%2),('b',1%2)] , 1%4 ) ,( Prob [('c',1%2),('d',1%2)] , 3%4) ]

flatten :: Prob (Prob a) -> Prob a flatten (Prob xs) = Prob $ concat $ map multAll xs where multAll (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs

instance Monad Prob where return x = Prob [(x,1%1)] m >>= f = flatten (fmap f m) fail _ = Prob []

l1 = Prob [('a',2%3),('b',1%3)]

multAllExt :: (Prob a, Rational) -> [(a, Rational)] multAllExt (Prob innerxs,p) = map (\(x,r) -> (x,p*r)) innerxs

--Main> :type multAllExt --multAllExt :: (Prob a, Rational) -> [(a, Rational)]

--Main> multAllExt (l1,1 % 4) --[('a',1 % 6),('b',1 % 12)]

i get this error:

GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> :load monade.hs [1 of 1] Compiling Main ( monade.hs, interpreted )

monade.hs:21:10: error: • No instance for (Applicative Prob) arising from the superclasses of an instance declaration • In the instance declaration for ‘Monad Prob’ | 21 | instance Monad Prob where | ^^^^^^^^^^ Failed, no modules loaded.

it fails when i add the last part of the example:

instance Monad Prob where return x = Prob [(x,1%1)] m >>= f = flatten (fmap f m) fail _ = Prob []

seems the Monad needs an instance of the Applicative to be instanciated...

what is wrong?

regards,

Damien

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