[Haskell-beginners] How to make instance of MonoFunctor

[Baa]

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 <aquagnu at gmail.com> 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?
> >
> >
> > --
> > Best regards, Paul
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> >