infix type constructors

[Okasaki, C. DR EECS]

I'm not sure how long this has been implemented in GHC,
but I just noticed that infix type constructors are allowed,
as in

  data a :- b = ...

The syntactic asymmetry between type constructors and
data contructors has bothered me for a while, so this
is a welcome change!  However, this syntax seems to
be supported for "data" and "newtype" declarations,
but not for "type" declarations.  For example,

  type a :- b = ...

does not seem to be allowed.  Is there a reason for this?
Or was it just an oversight?

-- Chris