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

[Baa]

Hello, Sylvain. 

Your solution assumes that I need to pass `@_ @AsTitle` anywhere where I
call `repr`? So, instead of `repr n::AsTitle` or `repr n::Something AsTitle`
(depends on implementation) I'll write `repr @_ @AsTitle n`, right?

> You only add a constraint as follows:
> repr n :: forall b. (Repr b ~ Repr AsTitle, ReprC Int b) => Repr b

Yes... So, `b` is unbound/free type param. But I can bind it with func.
deps, yes?


===
Best regards, Paul


> 
> But as we have seen, it doesn't imply (b ~ AsTitle) and b remains
> ambiguous.
> 
> A solution is to fix b explicitly with a type application:
> 
> {-# LANGUAGE TypeApplications #-}
> ...
> repr @_ @AsTitle n
> 
> 
> Best regards,
> Sylvain
> 
> 
> On 27/09/2017 17:50, Baa 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
> > _______________________________________________
> > Beginners mailing list
> > Beginners at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners  
> 
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