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• Abstract types:
  • Unknown type representation.
  • Packaged with operations that may be applied to type.
• Another example:
  • x: exists a. {const: a, op: a -> Int}
  • x.op(x.const)
• Restrict use of abstract types:
  • Simplify type checking.
  • value p: exists a. a x (a -> Int)
    = pack[a = Int in a x (a -> Int)](3, succ)
  • Value p is a package
  • Type a x (a -> Int) is the interface.
  • Binding a=Int is the type representation.
• General form:
  • pack [a = typerep in interface](contents)