[Haskell-cafe] Problems instancing a class

[Benjamin Franksen]

On Friday 17 February 2006 21:03, Juan Carlos Arevalo Baeza wrote:
>    What really got me is that if I don't use a type synonym:
>
> instance MyClass (HT.HashTable String String) where
>     htLookup h var =
>         do  result <- HT.lookup h var
>             case result of
>                 Nothing -> return ""
>                 Just s -> return s
>
>    it still complains:
>
> Illegal instance declaration for `MyClass (HashTable String String)'
>     (The instance type must be of form (T a b c)
>      where T is not a synonym, and a,b,c are distinct type variables)
> In the instance declaration for `MyClass (HashTable String String)'
>
>    even though the documentation in GHC states that HashTable is
> declared as "data".

(Guessing) Maybe because 'String' is not a type variable?

>    How exactly is type undecidable if this is allowed? I found here:
>
> http://hackage.haskell.org/trac/haskell-prime/wiki/FlexibleInstances
>
>    an example of a non-terminating instance:
>
> instance C b b => C (Maybe a) b
>
>    I don't see how/where/when this would be non-terminating. I
> mean... if you now define an instance "C Int Int", then you actually
> also have "C (Maybe Int) Int", C (Maybe (Maybe Int)) Int" and so on,
> I can see that. But how exactly is that a problem? Can you show me a
> use of C that cannot work with that instance?

What is (or better: might be) non-terminating is the process (algorithm) 
of /type inference/. This:

http://www.haskell.org//pipermail/haskell-prime/2006-February/000609.html

message contains a detailed explanation.

Ben