[Haskell-cafe] Why can't we make an instance declaration on a type synonym?

[Daniel Fischer]

Am Sonntag 03 Januar 2010 05:37:31 schrieb Jason Dusek:
>   Well, you can, with:
>
>     -XTypeSynonymInstances
>
>   though I'm not sure it addresses your specific need.

Doesn't help him here, he would need

instance Monad (State s) where ...

but that would be a partially applied type synonym. He would also need type level lambdas,

type State s = /\ a -> (s -> (a,s))

But type level lambdas and partially applied type synonyms make type inference undecidable 
if I remember correctly (if it wasn't that, they'd have other dire consequences).
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