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| Content Provider | ACM Digital Library |
|---|---|
| Author | Wells, J. B. Turbak, Franklyn A. Mairson, Harry G. Kfoury, Assaf J. |
| Abstract | We investigate finite-rank intersection type systems, analyzing the complexity of their type inference problems and their relation to the problem of recognizing semantically equivalent terms. Intersection types allow something of type τ1 Λ τ2 to be used in some places at type τ1 and in other places at type τ2. A finite-rank intersection type system bounds how deeply the Λ can appear in type expressions. Such type systems enjoy strong normalization, subject reduction, and computable type inference, and they support a pragmatics for implementing parametric polymorphism. As a consequence, they provide a conceptually simple and tractable alternative to the impredicative polymorphism of System F and its extensions, while typing many more programs than the Hindley-Milner type system found in ML and Haskell.While type inference is computable at every rank, we show that its complexity grows exponentially as rank increases. Let K(0, n) = n and K(t + 1, n) = $2^{K(t,n)};$ we prove that recognizing the pure λ-terms of size n that are typable at rank k is complete for DTIME[K(k−1, n)]. We then consider the problem of deciding whether two λ-terms typable at rank k have the same normal form, generalizing a well-known result of Statman from simple types to finite-rank intersection types. We show that the equivalence problem is DTIME[K(K(k − 1, n), 2)]-complete. This relationship between the complexity of typability and expressiveness is identical in wellknown decidable type systems such as simple types and Hindley-Milner types, but seems to fail for System F and its generalizations. The correspondence gives rise to a conjecture that if Τ is a predicative type system where typability has complexity t(n) and expressiveness has complexity e(n), then t(n) = Ω(log* e(n)). |
| Starting Page | 90 |
| Ending Page | 101 |
| Page Count | 12 |
| File Format | |
| ISBN | 1581131119 |
| DOI | 10.1145/317636.317788 |
| Language | English |
| Publisher | Association for Computing Machinery (ACM) |
| Publisher Date | 1999-09-01 |
| Publisher Place | New York |
| Access Restriction | Subscribed |
| Content Type | Text |
| Resource Type | Article |
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