Aha, so I missed: type instance Element UserName = Text and now this is right: instance MonoFunctor UserName where omap fn n = UserName $ fn $ unName n Yes, this will be omap :: (Text -> Text) -> (UserName -> UserName). Many thanks!!

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 wrote:

...

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?

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