What do you want the type of `omap` to be for `UserName`? There are two
reasonable definitions:
omap :: (Text -> Text) -> (UserName -> UserName)
omap :: (Char -> Char) -> (UserName -> UserName)
The first one is saying that a UserName is a container of a single Text
value. The second is that a UserName is a container of a sequence of Char
values. Once you figure out what the answer to this question is, you'll
need to use an associated type called Element to specify what you intend,
e.g.:
type Element UserName = Char
On Thu, Aug 10, 2017 at 11:35 AM, Baa
Hello, Dear List!
I want to make functor of my type like
newtype UserName = UserName { unName :: Text }
sure, it's impossible, so I will make MonoFunctor instead of (from library mono-traversable). So, I try:
instance MonoFunctor UserName where omap fn (UserName n) = UserName $ fn $ n
but I get error
• Couldn't match expected type ‘Element UserName’ with actual type ‘Text’ • In the second argument of ‘($)’, namely ‘unName an’ In the second argument of ‘($)’, namely ‘fn $ unName an’ In the expression: UserName $ fn $ unName an
interesting is that Text "type Element Text" of "type family Element mono" (instance?). So, how to make mono-functor for such `UserName` structure?
-- Best regards, Paul _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners