Computation & Deduction
Spring 1997
Frank Pfenning

Lecture 28: Atomic Subtyping

We introduce a basic system of atomic subtyping and prove some of its central properties. In particular we show under which conditions principal types exist, and how subtyping and typability can be decided by effective procedures. We also illustrate how typing derivations can be compiled to simply-typed terms with explicit coercions. This raises the problem of coherence, which we address in the next lecture. We also show the implementation of these systems and proofs in Elf.
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