yes for use with the DB (Database.MySQL.Simple.QueryResults) it is necessary. It should be confusing about String for the compiler... Le 10/01/2019 13:00, Ivan Lazar Miljenovic a écrit :

On Thu, 10 Jan 2019 at 19:34, Damien Mattei mailto:mattei@oca.eu> wrote:

Hi,

i have this definition:

{-# LANGUAGE FlexibleInstances #-}

class ConcatenateMaybeString a where cms :: Maybe String -> a -> Maybe String

instance ConcatenateMaybeString (Maybe String) where cms mf ms = mf >>= (\f -> ms >>= (\s -> return (f ++ s)))

instance ConcatenateMaybeString String where cms mf s = mf >>= (\f -> return (f ++ s))

when i use it on : f `cms` ("." ::String) `cms` s it works

but not on this: f `cms` "." `cms` s

"." is too ambigious to compile:

*Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted )

UpdateSidonie.hs:373:43: error: • Ambiguous type variable ‘a0’ arising from a use of ‘cms’ prevents the constraint ‘(ConcatenateMaybeString a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance ConcatenateMaybeString (Maybe String) -- Defined at UpdateSidonie.hs:169:11 instance ConcatenateMaybeString String -- Defined at UpdateSidonie.hs:177:11 • In the first argument of ‘cms’, namely ‘f `cms` "."’ In the expression: f `cms` "." `cms` s In the expression: let f = fmap head resBDwords s = fmap (head . tail) resBDwords mp = Just "." :: Maybe String .... in f `cms` "." `cms` s | 373 | in f `cms` "." `cms` s) :: Maybe String | ^^^^^^^^^^^

UpdateSidonie.hs:373:51: error: • Ambiguous type variable ‘a0’ arising from the literal ‘"."’ prevents the constraint ‘(Data.String.IsString a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance Data.String.IsString Query -- Defined in ‘Database.MySQL.Simple.Types’ instance Data.String.IsString Tx.Text -- Defined in ‘Data.Text’ instance (a ~ Char) => Data.String.IsString [a] -- Defined in ‘Data.String’ ...plus six instances involving out-of-scope types (use -fprint-potential-instances to see them all) • In the second argument of ‘cms’, namely ‘"."’ In the first argument of ‘cms’, namely ‘f `cms` "."’ In the expression: f `cms` "." `cms` s | 373 | in f `cms` "." `cms` s) :: Maybe String | ^^^ Failed, no modules loaded. Prelude>

any idea?

Do you have the OverloadedStrings or OverloadedLists LANGUAGE pragmas enabled?

Damien

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-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com mailto:Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com

Le 10/01/2019 15:27, Tom Ellis a écrit :

On Thu, Jan 10, 2019 at 12:34:04PM +0100, Damien Mattei wrote:

...

i have this definition:

{-# LANGUAGE FlexibleInstances #-}

class ConcatenateMaybeString a where cms :: Maybe String -> a -> Maybe String

instance ConcatenateMaybeString (Maybe String) where cms mf ms = mf >>= (\f -> ms >>= (\s -> return (f ++ s)))

instance ConcatenateMaybeString String where cms mf s = mf >>= (\f -> return (f ++ s))

Trying to simulate overloading like this is ultimately going to lead to more frustration than benefit. I strongly suggest you just define two different functions.

Hello Tom, those functions could be seen as a "style exercise" , for me,coming from untyped languages such as Scheme or LisP it's Haskell which is a frustration :-) it's not 2 function but more that are necessary because arguments and their corresponding types could be in any order, if i define different functions i will have 3 functions with different names: cms1::String -> Maybe String -> Maybe String cms2::Maybe String -> String -> Maybe String cms3::Maybe String -> Maybe String -> Maybe String i prefer to have a single overloaded operator like this : class ConcatenateMaybeString a b where (+%+) :: a -> b -> Maybe String instance ConcatenateMaybeString (Maybe String) (Maybe String) where (+%+) mf ms = mf >>= (\f -> ms >>= (\s -> return (f ++ s))) instance ConcatenateMaybeString (Maybe String) String where (+%+) mf s = mf >>= (\f -> return (f ++ s)) instance ConcatenateMaybeString String (Maybe String) where (+%+) f ms = ms >>= (\s -> return (f ++ s)) usable like this: f +%+ ("." :: String) +%+ s if i did not need OverloadedStrings i could even simply wrote: f +%+ "." +%+ s

i'm not so pessimist,i'm beginning to have fun with haskell...really! about teaching ressource i will be happy to know where i can find them... i only post in haskell cafe when i have not find the answer online (tutorials,stackoverflow,real haskell book http://book.realworldhaskell.org/etc,etc... in the hunded pages i search i even get a look at Categories for the working Mathematician https://books.google.fr/books?id=6KPSBwAAQBAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false) it is only when i have exhausted all the online ressource that i post to the cafe... On Thu, Jan 10, 2019 at 6:52 PM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:

On Thu, Jan 10, 2019 at 04:23:49PM +0100, Damien Mattei wrote:

...

Le 10/01/2019 15:27, Tom Ellis a écrit :

...

On Thu, Jan 10, 2019 at 12:34:04PM +0100, Damien Mattei wrote:

...

i have this definition:

{-# LANGUAGE FlexibleInstances #-}

class ConcatenateMaybeString a where cms :: Maybe String -> a -> Maybe String

instance ConcatenateMaybeString (Maybe String) where cms mf ms = mf >>= (\f -> ms >>= (\s -> return (f ++ s)))

instance ConcatenateMaybeString String where cms mf s = mf >>= (\f -> return (f ++ s))

Trying to simulate overloading like this is ultimately going to lead to more frustration than benefit. I strongly suggest you just define two different functions.

those functions could be seen as a "style exercise" , for me,coming from untyped languages such as Scheme or LisP it's Haskell which is a frustration :-)

I think you're going to get significantly more frustrated with Haskell if you try to learn it like this by yourself rather than by working through some widely approved teaching resource. Of course, how you spend your time is up to you, but if you're frustrated with Haskell then trying to make it up as you go along is only going to worsen the feeling! _______________________________________________ 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.