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

[David McBride]

The reason is because 'type' is a type alias that changes nothing code
wise, it is merely a visual clue to the reader of code that these two
types are the same.  You could replace 'Repr AsTitle' with 'String'
and you would get the exact same error.

That said I do think you can do what you want with type families, but
I'm not having luck giving you a complete solution at this time.  This
would be a good question to ask on stackoverflow if no one here gives
a satisfactory answer.  Here is the code I had that doesn't quite
work, although I'm not sure why.  Maybe you can figure it out.  I
would love to know.

{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}

data AsShow
data AsTitle

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

instance Repr Int AsTitle where
  type ReprC Int AsTitle = String
  repr n = show n

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

On Wed, Sep 27, 2017 at 11:50 AM, Baa <aquagnu at gmail.com> wrote:
> 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
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