[Haskell-cafe] overloading functions

[Damien Mattei]

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 <mattei at oca.eu
> <mailto:mattei at 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 at gmail.com <mailto:Ivan.Miljenovic at gmail.com>
> http://IvanMiljenovic.wordpress.com