[Haskell-cafe] overloading functions

10 Jan 2019

      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?

Damien

[Haskell-cafe] overloading functions

Damien Mattei