[Haskell-beginners] Ambiguous type error: multiparam class + type alias

[Baa]

Hello, List!

The further - the more interesting... For example, I want own `Show` analogue,
which will be parameterized with type meaning "context"/"reason". So, to represent
title of the value I will call:

  repr x::TitleRepr

or simple `Show`:

  repr x::ShowRepr

or

  repr x::Repr AsShow

etc. I try:

	{-# LANGUAGE AllowAmbiguousTypes #-}
	{-# LANGUAGE MultiParamTypeClasses #-}

	data AsShow
	data AsTitle

	type Repr a = String

	class ReprC a b where
	  repr :: a -> Repr b

	instance ReprC Int AsTitle where
	  repr n = "a number " ++ show n

	main = do
	  let n = 5 :: Int
	  print $ (repr n :: Repr AsTitle)

and sure I get error:

     • Ambiguous type variable ‘b0’ arising from a use of ‘repr’
       prevents the constraint ‘(ReprC Int b0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘b0’ should be.
       These potential instance exist:
         instance ReprC Int AsTitle
           -- Defined at .../.stack-work/intero/intero31144sPV.hs:12:10
     • In the second argument of ‘($)’, namely
         ‘(repr n :: Repr AsTitle)’
       In a stmt of a 'do' block: print $ (repr n :: Repr AsTitle)
       In the expression:
         do { let n = ...;
              print $ (repr n :: Repr AsTitle) } (intero)

Sure, I can use as `Repr` not type-alias but `newtype`. But in this
case I will need additional call (show/runRepr/coerce/etc.):

  coerce $ (repr x::AsTitle)

So, what is the reason that GHCI is see that `repr n` is `::Repr AsTitle`
(AsTitle!!) and says me that `b0` is ambigous?! It should know
what concreate `repr` I mean! :) If I use `newtype` - no problem, so I
suppose problem is in the type alias. It's not sterling type for such
goal, right?

Another question is: how to accomplish such goal, i.e. without to
make additional call (coerce/show/runRepr/etc) when `repr` will return
`String`, wrapped in newtype?


PS. Execuse such silly and training example. Actually, I planned to made
such class and to use it instead of `Show`. Sure, it can be splitted to
several classed (my current state) but.. example is to learn Haskell
and to understand my errors...


===
Best regards, Paul