[Haskell-cafe] ambiguous type variable question

[Zoran Bošnjak]

Hi all,
I want to convert a type level structure to the value level structure of the same shape.
This is the simplified example:

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE UndecidableInstances #-}

module Test where

import GHC.TypeLits
import Data.Kind
import Data.Proxy

-- This data structure serves both for 'type level' and 'value level'.
data Content n s b
    = ContentRaw
    | ContentTable [(n, s)]
    | ContentBool b

deriving instance (Show n, Show s, Show b) => Show (Content n s b)

-- Convert from types to values.
class IsSchema s where
    type SchemaVal s :: Type
    schema :: SchemaVal s

instance IsSchema 'ContentRaw where
    type (SchemaVal 'ContentRaw) = Content Integer String Bool
    schema = ContentRaw

instance IsSchema ('ContentTable '[]) where
    type (SchemaVal ('ContentTable '[])) = Content Integer String Bool
    schema = ContentTable []

instance
    ( IsSchema ('ContentTable ts)
    , t ~ '(n, s), KnownNat n, KnownSymbol s
    , SchemaVal ('ContentTable ts) ~ Content Integer String Bool
    ) => IsSchema ('ContentTable (t ': ts)) where
    type SchemaVal ('ContentTable (t ': ts)) = Content Integer String Bool
    schema = case schema @('ContentTable ts) of
        ContentTable lst ->
            let n = natVal (Proxy @n)
                s = symbolVal (Proxy @s)
            in ContentTable ((n,s) : lst)
        _ -> error "internal error"

If I remove the type parameter 'b', 'ContentBool b' and coresponding 'Bool' from the sample, the conversion works as expected.

>>> schema @'ContentRaw
ContentRaw

>>> schema @('ContentTable ( '(1,"test1") ': '(2,"test2") ': '[]))
ContentTable [(1,"test1"),(2,"test2")]

But for some reason, as soon as I extend the example with 'b' and 'Bool' as shown above, it results in the following error:

    * Could not deduce (SchemaVal ('ContentTable ts)
                        ~ Content Integer String b0)
      from the context: (IsSchema ('ContentTable ts), t ~ '(n, s),
                         KnownNat n, KnownSymbol s,
                         SchemaVal ('ContentTable ts) ~ Content Integer String Bool)
        bound by the instance declaration at test2.hs:(34,5)-(37,43)
      The type variable `b1' is ambiguous

How do I fix it or how do I rewrite it to get the intended result?

regards,
Zoran