TypeSynonymInstances with partial type

TypeSynonymInstances allow only full synonyms. With

{-# LANGUAGE TypeSynonymInstances #-}

type RTuple b a = (a, b)

instance Monad (RTuple b) where ...
-- or
instance Monad (RTuple b :: * -> *) where ...

I got next error:

test.hs:17:22:
    Type synonym `RevTuple' should have 2 arguments, but has been given 1
    In the instance declaration for `Monad (RTuple b :: * -> *)'

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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changed weight to 5

added Tbug Trac import labels

closed

Unfortunately, this is by design. Partial application of type synonyms is highly nontrivial; if done, it would require a large upheaval of type inference, and add a lot of complexity. For example, how GHC could know whether given return :: a -> (a,a), is Monad ((,) a) or Monad (RTuple a) required? The situation gets much more complicated with larger synonyms, and things get ugly. I suggest to ask this question on haskell-cafe or IRC and leave two links found by a google search: #785 (closed), http://stackoverflow.com/questions/4922560/.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedInvalid

added Pnormal label

mentioned in issue #785 (closed)