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

[Sylvain Henry]

Hi,

The issue is that (Repr a ~ Repr b) doesn't imply (a ~ b). Indeed: 
forall a b. Repr a ~ String ~ Repr b

So given the type of `repr n`:
repr n :: forall b. ReprC Int b => Repr b

When you write:
repr n :: Repr AsTitle

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

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
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