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Solving the Expression Problem in C++, á la LMS

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Theoretical Aspects of Computing – ICTAC 2019 (ICTAC 2019)

Part of the book series: Lecture Notes in Computer Science ((LNTCS,volume 11884))

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Abstract

We give a C++ solution to the Expression Problem that takes a components-for-cases approach. Our solution is a C++ transliteration of how Lightweight Modular Staging solves the Expression Problem. It, furthermore, gives a C++ encoding to object algebras and object algebra interfaces. We use our latter encoding by tying its recursive knot as in Datatypes à la Carte.

This work is partially funded by the LightKone European H2020 Project under Grant Agreement No. 732505 and partially by the Belgian National Fund for Scientific Research (F.R.S.-FNRS).

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Notes

  1. 1.

    If the guarantee was for dynamically constructed terms too, we would have called it strong static type safety.

  2. 2.

    Although our codebase remains fully functional without that (using ordinary type parametrisation), we retain its usage here for enhanced readability.

  3. 3.

    Integration of a Decentralised Pattern Matching.

  4. 4.

    This is a paraphrase of the combinator taken from the online specification at the C++ Reference: https://en.cppreference.com/w/cpp/utility/variant/visit.

  5. 5.

    Posted online by James Iry on Wed, 22/10/2008 at http://lambda-the-ultimate.org/node/1134.

  6. 6.

    https://www.boost.org/doc/libs/1_67_0/doc/html/variant.html.

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Author information

Authors and Affiliations

  1. Université catholique de Louvain, Louvain-la-Neuve, Belgium

    Seyed Hossein Haeri

  2. University of the West of Scotland, Glasgow, UK

    Paul Keir

Authors
  1. Seyed Hossein Haeri
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  2. Paul Keir
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Correspondence to Seyed Hossein Haeri .

Editor information

Editors and Affiliations

  1. University of Sheffield, Sheffield, UK

    Robert Mark Hierons

  2. University of Bordeaux, Talence, France

    Mohamed Mosbah

A C++ Features Used

A C++ Features Used

A C++ (or ) can be type parameterised. The below, for example, takes two type parameters and :

figure ja

Likewise, C++ functions can take type parameters:

figure jb

From C++20 onward, certain type parameters need not to be mentioned explicitly. For example, the above function can be abbreviated as:

figure jd

A ( or non- ) can define nested type members. For example, the below defines to be :

figure jk

Nested types can themselves be type parameterised, like :

figure jm

C++17 added as a type-safe representation for unions. An instance of , at any given time, holds a value of one of its alternative types. That is, the static type of such an instance is that of the it is defined with; whilst, the dynamic type is one and only one of those alternative types. As such, a function that is to be applied on a needs to be applicable to its alternative types. Technically, a visitor is required for the alternative types. The function , takes a visitor in addition to a pack of arguments to be visited.

figure js

The variable above is bound to a \(\lambda \)-abstraction that, given an , returns its value times two. \(\lambda \)-abstractions can also capture unbound names. In such a case, the captured name needs to be mentioned in the opening square brackets before the list of parameters. For example, the \(\lambda \)-abstraction below captures the name :

figure jx

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Haeri, S.H., Keir, P. (2019). Solving the Expression Problem in C++, á la LMS. In: Hierons, R., Mosbah, M. (eds) Theoretical Aspects of Computing – ICTAC 2019. ICTAC 2019. Lecture Notes in Computer Science(), vol 11884. Springer, Cham. https://doi.org/10.1007/978-3-030-32505-3_20

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  • DOI: https://doi.org/10.1007/978-3-030-32505-3_20

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  • Online ISBN: 978-3-030-32505-3

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