The Hugs 98 User's Guide
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# 6.2. Restricted type synonyms

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.

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