It has been claimed that logic programs are equivalent to or consequences of specifications. We argue this is generally not correct. Programs often make implicit assumptions about types, leading to the possibility of incorrect answers. If the assumptions are made explicit, so that the program is equivalent to the specification, the program is less efficient. We define when programs with type declarations are type correct and show all well typed answers returned by such programs are correct.
As well as making to relationship between programs and specifications clear, this type scheme can be used to detect certain programming errors. The semantics we define for type declarations can also be applied to other type schemes. This leads to a simple characterization of what errors are detected by these schemes, and a way to generalize these schemes to allow arbitrary type definitions. One such implementation is discussed.
KeywordsLogic Program Logic Programming Incorrect Answer Type Scheme Prolog Program
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