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

28 Sep 2017

      David, hello again!

Interesting is that class with one parameter is fine:

  class Repr a where
    type ReprAs a
    reprx :: a -> ReprAs a

  instance Repr Int where
    type ReprAs Int = Int
    reprx n = n

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

but when I added 2nd param, I get ambogouse error again:

  class Repr a b where
    type ReprAs a b
    repr :: a -> ReprAs a b

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

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

which looks the same as with type-alias: GHC says:

    30  12 error           error:
     • Couldn't match type ‘ReprAs Int b0’ with ‘String’
       Expected type: ReprAs Int AsTitle
         Actual type: ReprAs Int b0
       The type variable ‘b0’ is ambiguous
     • In the second argument of ‘($)’, namely
         ‘(repr n :: ReprAs Int AsTitle)’
       In a stmt of a 'do' block: print $ (repr n :: ReprAs Int AsTitle)
       In the expression:
         do { let n = ...;
              print $ (repr n :: ReprAs Int AsTitle) } (intero)

looks that `type ReprAs Int AsTitle` is treating type-alias and GHC can
not match resulting `String` with `ReprAs Int AsTitle`.

As I understand, the root of the problem is that class parameters are
not the same as in results' parameters (contraposition types) - they
turn out to be free/unbound (execuse my English).

For example, to avoid this problem in the class I added extension:

  {-# LANGUAGE AllowAmbiguousTypes #-}

I don't know is a way in Haskell to say to reuse already bound class
parameters in "methods" bodies... I found this:

https://stackoverflow.com/questions/4174187/reading-and-representing-input-w...

Another variant which is compiling (w/ func-deps):

  {-# LANGUAGE FunctionalDependencies #-}
  {-# LANGUAGE MultiParamTypeClasses #-}

  data AsShow
  data AsTitle

  type ReprAs a = String

  class Repr a b | a -> b where
    repr :: a -> ReprAs b

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

  -- instance Repr Int AsShow where
  --   repr n = "a number '" ++ show n ++ "'"

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

but due to `a -> b` it's impossible to instantiate another `Repr Int`!
So, I think soultion is in:

  - func deps
  - or type families/associative types

But unfortunately my knowledge of Haskell is limited to find it. I see
only that I need to add another param to func dep of class - to
"extend" dependency which allows to determine result type not only on
one input argument's type...

Thanks David!

===
Best regards, Paul
...
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  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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