On Thu, 10 Jan 2019 at 19:34, Damien Mattei
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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