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International Symposium on Functional and Logic Programming

FLOPS 2020: Functional and Logic Programming pp 66–87Cite as

Session Types Without Sophistry

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Part of the book series: Lecture Notes in Computer Science ((LNTCS,volume 12073))

Abstract

Whereas ordinary types approximate the results, session types approximate communication among computations. As a form of typestate, they describe not only what is communicated now but also what is to be communicated next. Writing session-typed programs in an ordinary programming language such an OCaml requires inordinary cleverness to simulate type-level computations and linear typing – meaning the implementation and the error messages are very hard to understand. One is constantly reminded of template metaprogramming in C++.

We present a system exploring a very different approach to session typing: lowering type-level sophistry to ordinary programming, while maintaining the static assurances. Error messages are detailed and customizable, and one can use an ordinary debugger to investigate session-type problems. Our system is a binary-session–typed DSL for service-oriented programming in OCaml, supporting multiple communication channels, internal and external choices, recursion, and also channel delegation.

The key idea is staging: ordinary run-time checks in the generator play the role of “type-checks” from the point of view of the generated program. What is a fancy type to the latter is ordinary data to the generator.

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Notes

  1. 1.

    Binary session type systems like [14] and its successors, used in many libraries including ours, do not in general prevent deadlocks (see Sect. 5).

  2. 2.

    What we call an endpoint, Yoshida and Vasconcelos [51, §3] call a “polarized channel”, following Gay and Hole [12].

  3. 3.

    The right-associative infix operator @@ of low precedence is application: f @@ x + 1 is the same as f (x + 1) but avoids the parentheses. The operator is the analogue of in Haskell.

  4. 4.

    It should also be possible to supply a session type and check an expression against it, to verify its communication obeys the protocol stated in the type. After all, if we can infer a session type, we can check against it. However, we have not yet offered this facility in the public library interface.

  5. 5.

    To improve readability, we adjusted indentation and removed module references, while the rest is left as-is. Variables and are generated via let-insertion.

  6. 6.

    The code does not say that rendez-vous with itself, which is impossible. Communications on a are synchronous (i.e. and blocks until its counterpart becomes available) while those on session endpoints are asynchronous.

  7. 7.

    Ours and are called and in [51] (we changed the names to avoid association with exceptions).

  8. 8.

    The language is quite like the STATE language in [27, §7]: the imperative part of Reynolds’ Idealized Algol, as pointed out by Bob Atkey. Instead of we write .

  9. 9.

    That tutorial paper also compares canonical structures to related approaches, in particular, implicits and type classes.

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Acknowledgments

We thank anonymous reviewers for many, helpful comments and suggestions. This work was partially supported by JSPS KAKENHI Grant Number 18H03218 and 17K12662.

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Authors and Affiliations

  1. Tohoku University, Sendai, Japan

    Oleg Kiselyov

  2. Gifu University, Gifu, Japan

    Keigo Imai

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  1. Oleg Kiselyov
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Correspondence to Oleg Kiselyov or Keigo Imai .

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Editors and Affiliations

  1. Tohoku University, Sendai, Japan

    Keisuke Nakano

  2. Uppsala University, Uppsala, Sweden

    Konstantinos Sagonas

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Kiselyov, O., Imai, K. (2020). Session Types Without Sophistry. In: Nakano, K., Sagonas, K. (eds) Functional and Logic Programming. FLOPS 2020. Lecture Notes in Computer Science(), vol 12073. Springer, Cham. https://doi.org/10.1007/978-3-030-59025-3_5

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