Of course that won't work because that type synonym is far from a monad. I think you meant to write `type Id a = a` or `type Id f a = f a` instead.

But I get your argument now. What you are saying is that we want some kind of a way to define functions that are either pure or return a monadic value. Currently that's not doable because these two have distinct types. It may be difficult to unify them via the type system. This is even more difficult for higher-order functions that take other functions.

> I think you meant to write `type Id a = a`

Indeed I did, sorry, unfortunately too late to edit. The following reproduces the error:

  {-# LANGUAGE TypeSynonymInstances #-}
  type Id a = a
  instance Monad Id where
    x >>= f = f x
    return = id

(The last two lines can be ommitted, since GHC aborts before checking the instance body. Edit: which is why I didn't notice I had the arguments to >>= backward; fixed?)

> It may be difficult to unify them via the type system. This is even more difficult for higher-order functions that take other functions.

IMO, the main difficulty in unifying `a ~ Monad m => m a` is inferring `m ~ \a1-> a1` in a way that allows `\a1-> a1` to inherit `Id`'s instances. It's a understandable deficiency, but it is a deficiency.