A common approach to hiding implementation details is through the use of abstract types. In this paper, we present a simple theoretical account of abstract types that make use of a recently developed notion of conditional type equality. This is in contrast to most of the existing theoretical accounts of abstract types, which rely on existential types (or similar variants). In addition, we show that this new approach to abstract types opens a promising avenue to the design and implementation of module systems that can effectively support large-scale programming.
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