Restricted type synonyms are a mechanism for defining abstract datatypes. You can achieve similar effects, and more portably, using the Haskell 98 module system.
The idea is that you can say that a type synonym is transparent in the definitions of certain functions (the operations on the type), but opaque elsewhere, by writing
type Table a b = [(a,b)] in
empty :: Table a b,
isEmpty :: Table a b -> Bool,
add :: a -> b -> Table a b -> Table a b,
search :: a -> Table a b -> Maybe b
empty = []
isEmpty = null
add a b t = (a,b):t
search = lookup
or equivalently
type Table a b = [(a,b)] in empty, isEmpty, add, search empty :: Table a b empty = [] ...See Section 7.3.5 of the Hugs 98 User Manual for details.