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Documents authored by Mezini, Mira


Document
Compiling with Arrays

Authors: David Richter, Timon Böhler, Pascal Weisenburger, and Mira Mezini

Published in: LIPIcs, Volume 313, 38th European Conference on Object-Oriented Programming (ECOOP 2024)


Abstract
Linear algebra computations are foundational for neural networks and machine learning, often handled through arrays. While many functional programming languages feature lists and recursion, arrays in linear algebra demand constant-time access and bulk operations. To bridge this gap, some languages represent arrays as (eager) functions instead of lists. In this paper, we connect this idea to a formal logical foundation by interpreting functions as the usual negative types from polarized type theory, and arrays as the corresponding dual positive version of the function type. Positive types are defined to have a single elimination form whose computational interpretation is pattern matching. Just like (positive) product types bind two variables during pattern matching, (positive) array types bind variables with multiplicity during pattern matching. We follow a similar approach for Booleans by introducing conditionally-defined variables. The positive formulation for the array type enables us to combine typed partial evaluation and common subexpression elimination into an elegant algorithm whose result enjoys a property we call maximal fission, which we argue can be beneficial for further optimizations. For this purpose, we present the novel intermediate representation indexed administrative normal form (A_{i}NF), which relies on the formal logical foundation of the positive formulation for the array type to facilitate maximal loop fission and subsequent optimizations. A_{i}NF is normal with regard to commuting conversion for both let-bindings and for-loops, leading to flat and maximally fissioned terms. We mechanize the translation and normalization from a simple surface language to A_{i}NF, establishing that the process terminates, preserves types, and produces maximally fissioned terms.

Cite as

David Richter, Timon Böhler, Pascal Weisenburger, and Mira Mezini. Compiling with Arrays. In 38th European Conference on Object-Oriented Programming (ECOOP 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 313, pp. 33:1-33:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)


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@InProceedings{richter_et_al:LIPIcs.ECOOP.2024.33,
  author =	{Richter, David and B\"{o}hler, Timon and Weisenburger, Pascal and Mezini, Mira},
  title =	{{Compiling with Arrays}},
  booktitle =	{38th European Conference on Object-Oriented Programming (ECOOP 2024)},
  pages =	{33:1--33:24},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-341-6},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{313},
  editor =	{Aldrich, Jonathan and Salvaneschi, Guido},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2024.33},
  URN =		{urn:nbn:de:0030-drops-208823},
  doi =		{10.4230/LIPIcs.ECOOP.2024.33},
  annote =	{Keywords: array languages, functional programming, domain-specific languages, normalization by evaluation, common subexpression elimination, polarity, positive function type, intrinsic types}
}
Document
Artifact
Compiling with Arrays (Artifact)

Authors: David Richter, Timon Böhler, Pascal Weisenburger, and Mira Mezini

Published in: DARTS, Volume 10, Issue 2, Special Issue of the 38th European Conference on Object-Oriented Programming (ECOOP 2024)


Abstract
Linear algebra computations are foundational for neural networks and machine learning, often handled through arrays. While many functional programming languages feature lists and recursion, arrays in linear algebra demand constant-time access and bulk operations. To bridge this gap, some languages represent arrays as (eager) functions instead of lists. In this paper, we connect this idea to a formal logical foundation by interpreting functions as the usual negative types from polarized type theory, and arrays as the corresponding dual positive version of the function type. Positive types are defined to have a single elimination form whose computational interpretation is pattern matching. Just like (positive) product types bind two variables during pattern matching, (positive) array types bind variables with multiplicity during pattern matching. We follow a similar approach for Booleans by introducing conditionally-defined variables. The positive formulation for the array type enables us to combine typed partial evaluation and common subexpression elimination into an elegant algorithm whose result enjoys a property we call maximal fission, which we argue can be beneficial for further optimizations. For this purpose, we present the novel intermediate representation indexed administrative normal form (A_{i}NF), which relies on the formal logical foundation of the positive formulation for the array type to facilitate maximal loop fission and subsequent optimizations. A_{i}NF is normal with regard to commuting conversion for both let-bindings and for-loops, leading to flat and maximally fissioned terms. We mechanize the translation and normalization from a simple surface language to A_{i}NF, establishing that the process terminates, preserves types, and produces maximally fissioned terms.

Cite as

David Richter, Timon Böhler, Pascal Weisenburger, and Mira Mezini. Compiling with Arrays (Artifact). In Special Issue of the 38th European Conference on Object-Oriented Programming (ECOOP 2024). Dagstuhl Artifacts Series (DARTS), Volume 10, Issue 2, pp. 18:1-18:7, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)


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@Article{richter_et_al:DARTS.10.2.18,
  author =	{Richter, David and B\"{o}hler, Timon and Weisenburger, Pascal and Mezini, Mira},
  title =	{{Compiling with Arrays (Artifact)}},
  pages =	{18:1--18:7},
  journal =	{Dagstuhl Artifacts Series},
  ISBN =	{978-3-95977-342-3},
  ISSN =	{2509-8195},
  year =	{2024},
  volume =	{10},
  number =	{2},
  editor =	{Richter, David and B\"{o}hler, Timon and Weisenburger, Pascal and Mezini, Mira},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DARTS.10.2.18},
  URN =		{urn:nbn:de:0030-drops-209168},
  doi =		{10.4230/DARTS.10.2.18},
  annote =	{Keywords: array languages, functional programming, domain-specific languages, normalization by evaluation, common subexpression elimination, polarity, positive function type, intrinsic types}
}
Document
LoRe: A Programming Model for Verifiably Safe Local-First Software (Extended Abstract)

Authors: Julian Haas, Ragnar Mogk, Elena Yanakieva, Annette Bieniusa, and Mira Mezini

Published in: LIPIcs, Volume 263, 37th European Conference on Object-Oriented Programming (ECOOP 2023)


Abstract
Local-first software manages and processes private data locally while still enabling collaboration between multiple parties connected via partially unreliable networks. Such software typically involves interactions with users and the execution environment (the outside world). The unpredictability of such interactions paired with their decentralized nature make reasoning about the correctness of local-first software a challenging endeavor. Yet, existing solutions to develop local-first software do not provide support for automated safety guarantees and instead expect developers to reason about concurrent interactions in an environment with unreliable network conditions. We propose LoRe, a programming model and compiler that automatically verifies developer-supplied safety properties for local-first applications. LoRe combines the declarative data flow of reactive programming with static analysis and verification techniques to precisely determine concurrent interactions that violate safety invariants and to selectively employ strong consistency through coordination where required. We propose a formalized proof principle and demonstrate how to automate the process in a prototype implementation that outputs verified executable code. Our evaluation shows that LoRe simplifies the development of safe local-first software when compared to state-of-the-art approaches and that verification times are acceptable.

Cite as

Julian Haas, Ragnar Mogk, Elena Yanakieva, Annette Bieniusa, and Mira Mezini. LoRe: A Programming Model for Verifiably Safe Local-First Software (Extended Abstract). In 37th European Conference on Object-Oriented Programming (ECOOP 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 263, pp. 12:1-12:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


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@InProceedings{haas_et_al:LIPIcs.ECOOP.2023.12,
  author =	{Haas, Julian and Mogk, Ragnar and Yanakieva, Elena and Bieniusa, Annette and Mezini, Mira},
  title =	{{LoRe: A Programming Model for Verifiably Safe Local-First Software (Extended Abstract)}},
  booktitle =	{37th European Conference on Object-Oriented Programming (ECOOP 2023)},
  pages =	{12:1--12:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-281-5},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{263},
  editor =	{Ali, Karim and Salvaneschi, Guido},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2023.12},
  URN =		{urn:nbn:de:0030-drops-182056},
  doi =		{10.4230/LIPIcs.ECOOP.2023.12},
  annote =	{Keywords: Local-First Software, Reactive Programming, Invariants, Consistency, Automated Verification}
}
Document
Algebraic Replicated Data Types: Programming Secure Local-First Software

Authors: Christian Kuessner, Ragnar Mogk, Anna-Katharina Wickert, and Mira Mezini

Published in: LIPIcs, Volume 263, 37th European Conference on Object-Oriented Programming (ECOOP 2023)


Abstract
This paper is about programming support for local-first applications that manage private data locally, but still synchronize data between multiple devices. Typical use cases are synchronizing settings and data, and collaboration between multiple users. Such applications must preserve the privacy and integrity of the user’s data without impeding or interrupting the user’s normal workflow - even when the device is offline or has a flaky network connection. From the programming perspective, availability along with privacy and security concerns pose significant challenges, for which developers have to learn and use specialized solutions such as conflict-free replicated data types (CRDTs) or APIs for centralized data stores. This work relieves developers from this complexity by enabling the direct and automatic use of algebraic data types - which developers already use to express the business logic of the application - for synchronization and collaboration. Moreover, we use this approach to provide end-to-end encryption and authentication between multiple replicas (using a shared secret), that is suitable for a coordination-free setting. Overall, our approach combines all the following advantages: it (1) allows developers to design custom data types, (2) provides data privacy and integrity when using untrusted intermediaries, (3) is coordination free, (4) guarantees eventual consistency by construction (i.e., independent of developer errors), (5) does not cause indefinite growth of metadata, (6) has sufficiently efficient implementations for the local-first setting.

Cite as

Christian Kuessner, Ragnar Mogk, Anna-Katharina Wickert, and Mira Mezini. Algebraic Replicated Data Types: Programming Secure Local-First Software. In 37th European Conference on Object-Oriented Programming (ECOOP 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 263, pp. 14:1-14:33, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


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@InProceedings{kuessner_et_al:LIPIcs.ECOOP.2023.14,
  author =	{Kuessner, Christian and Mogk, Ragnar and Wickert, Anna-Katharina and Mezini, Mira},
  title =	{{Algebraic Replicated Data Types: Programming Secure Local-First Software}},
  booktitle =	{37th European Conference on Object-Oriented Programming (ECOOP 2023)},
  pages =	{14:1--14:33},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-281-5},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{263},
  editor =	{Ali, Karim and Salvaneschi, Guido},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2023.14},
  URN =		{urn:nbn:de:0030-drops-182076},
  doi =		{10.4230/LIPIcs.ECOOP.2023.14},
  annote =	{Keywords: local-first, data privacy, coordination freedom, CRDTs, AEAD}
}
Document
A Direct-Style Effect Notation for Sequential and Parallel Programs

Authors: David Richter, Timon Böhler, Pascal Weisenburger, and Mira Mezini

Published in: LIPIcs, Volume 263, 37th European Conference on Object-Oriented Programming (ECOOP 2023)


Abstract
Modeling sequential and parallel composition of effectful computations has been investigated in a variety of languages for a long time. In particular, the popular do-notation provides a lightweight effect embedding for any instance of a monad. Idiom bracket notation, on the other hand, provides an embedding for applicatives. First, while monads force effects to be executed sequentially, ignoring potential for parallelism, applicatives do not support sequential effects. Composing sequential with parallel effects remains an open problem. This is even more of an issue as real programs consist of a combination of both sequential and parallel segments. Second, common notations do not support invoking effects in direct-style, instead forcing a rigid structure upon the code. In this paper, we propose a mixed applicative/monadic notation that retains parallelism where possible, but allows sequentiality where necessary. We leverage a direct-style notation where sequentiality or parallelism is derived from the structure of the code. We provide a mechanisation of our effectful language in Coq and prove that our compilation approach retains the parallelism of the source program.

Cite as

David Richter, Timon Böhler, Pascal Weisenburger, and Mira Mezini. A Direct-Style Effect Notation for Sequential and Parallel Programs. In 37th European Conference on Object-Oriented Programming (ECOOP 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 263, pp. 25:1-25:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


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@InProceedings{richter_et_al:LIPIcs.ECOOP.2023.25,
  author =	{Richter, David and B\"{o}hler, Timon and Weisenburger, Pascal and Mezini, Mira},
  title =	{{A Direct-Style Effect Notation for Sequential and Parallel Programs}},
  booktitle =	{37th European Conference on Object-Oriented Programming (ECOOP 2023)},
  pages =	{25:1--25:22},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-281-5},
  ISSN =	{1868-8969},
  year =	{2023},
  volume =	{263},
  editor =	{Ali, Karim and Salvaneschi, Guido},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2023.25},
  URN =		{urn:nbn:de:0030-drops-182181},
  doi =		{10.4230/LIPIcs.ECOOP.2023.25},
  annote =	{Keywords: do-notation, parallelism, concurrency, effects}
}
Document
Artifact
LoRe: A Programming Model for Verifiably Safe Local-First Software (Artifact)

Authors: Julian Haas, Ragnar Mogk, Elena Yanakieva, Annette Bieniusa, and Mira Mezini

Published in: DARTS, Volume 9, Issue 2, Special Issue of the 37th European Conference on Object-Oriented Programming (ECOOP 2023)


Abstract
Local-first software manages and processes private data locally while still enabling collaboration between multiple parties connected via partially unreliable networks. Such software typically involves interactions with users and the execution environment (the outside world). The unpredictability of such interactions paired with their decentralized nature make reasoning about the correctness of local-first software a challenging endeavor. Yet, existing solutions to develop local-first software do not provide support for automated safety guarantees and instead expect developers to reason about concurrent interactions in an environment with unreliable network conditions. We propose LoRe, a programming model and compiler that automatically verifies developer-supplied safety properties for local-first applications. LoRe combines the declarative data flow of reactive programming with static analysis and verification techniques to precisely determine concurrent interactions that violate safety invariants and to selectively employ strong consistency through coordination where required. We propose a formalized proof principle and demonstrate how to automate the process in a prototype implementation that outputs verified executable code. Our evaluation shows that LoRe simplifies the development of safe local-first software when compared to state-of-the-art approaches and that verification times are acceptable.

Cite as

Julian Haas, Ragnar Mogk, Elena Yanakieva, Annette Bieniusa, and Mira Mezini. LoRe: A Programming Model for Verifiably Safe Local-First Software (Artifact). In Special Issue of the 37th European Conference on Object-Oriented Programming (ECOOP 2023). Dagstuhl Artifacts Series (DARTS), Volume 9, Issue 2, pp. 11:1-11:2, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


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@Article{haas_et_al:DARTS.9.2.11,
  author =	{Haas, Julian and Mogk, Ragnar and Yanakieva, Elena and Bieniusa, Annette and Mezini, Mira},
  title =	{{LoRe: A Programming Model for Verifiably Safe Local-First Software (Artifact)}},
  pages =	{11:1--11:2},
  journal =	{Dagstuhl Artifacts Series},
  ISSN =	{2509-8195},
  year =	{2023},
  volume =	{9},
  number =	{2},
  editor =	{Haas, Julian and Mogk, Ragnar and Yanakieva, Elena and Bieniusa, Annette and Mezini, Mira},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DARTS.9.2.11},
  URN =		{urn:nbn:de:0030-drops-182510},
  doi =		{10.4230/DARTS.9.2.11},
  annote =	{Keywords: Local-First Software, Reactive Programming, Invariants, Consistency, Automated Verification}
}
Document
Artifact
A Direct-Style Effect Notation for Sequential and Parallel Programs (Artifact)

Authors: David Richter, Timon Böhler, Pascal Weisenburger, and Mira Mezini

Published in: DARTS, Volume 9, Issue 2, Special Issue of the 37th European Conference on Object-Oriented Programming (ECOOP 2023)


Abstract
Modeling sequential and parallel composition of effectful computations has been investigated in a variety of languages for a long time. In particular, the popular do-notation provides a lightweight effect embedding for any instance of a monad. Idiom bracket notation, on the other hand, provides an embedding for applicatives. First, while monads force effects to be executed sequentially, ignoring potential for parallelism, applicatives do not support sequential effects. Composing sequential with parallel effects remains an open problem. This is even more of an issue as real programs consist of a combination of both sequential and parallel segments. Second, common notations do not support invoking effects in direct-style, instead forcing a rigid structure upon the code. In this paper, we propose a mixed applicative/monadic notation that retains parallelism where possible, but allows sequentiality where necessary. We leverage a direct-style notation where sequentiality or parallelism is derived from the structure of the code. We provide a mechanisation of our effectful language in Coq and prove that our compilation approach retains the parallelism of the source program.

Cite as

David Richter, Timon Böhler, Pascal Weisenburger, and Mira Mezini. A Direct-Style Effect Notation for Sequential and Parallel Programs (Artifact). In Special Issue of the 37th European Conference on Object-Oriented Programming (ECOOP 2023). Dagstuhl Artifacts Series (DARTS), Volume 9, Issue 2, pp. 17:1-17:3, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


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@Article{richter_et_al:DARTS.9.2.17,
  author =	{Richter, David and B\"{o}hler, Timon and Weisenburger, Pascal and Mezini, Mira},
  title =	{{A Direct-Style Effect Notation for Sequential and Parallel Programs (Artifact)}},
  pages =	{17:1--17:3},
  journal =	{Dagstuhl Artifacts Series},
  ISSN =	{2509-8195},
  year =	{2023},
  volume =	{9},
  number =	{2},
  editor =	{Richter, David and B\"{o}hler, Timon and Weisenburger, Pascal and Mezini, Mira},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DARTS.9.2.17},
  URN =		{urn:nbn:de:0030-drops-182573},
  doi =		{10.4230/DARTS.9.2.17},
  annote =	{Keywords: do-notation, parallelism, concurrency, effects}
}
Document
Artifact
Algebraic Replicated Data Types: Programming Secure Local-First Software (Artifact)

Authors: Christian Kuessner, Ragnar Mogk, Anna-Katharina Wickert, and Mira Mezini

Published in: DARTS, Volume 9, Issue 2, Special Issue of the 37th European Conference on Object-Oriented Programming (ECOOP 2023)


Abstract
This work is about programming support for local-first applications that manage private data locally, but still synchronize data between multiple devices. Typical use cases are synchronizing settings and data, and collaboration between multiple users. Such applications must preserve the privacy and integrity of the user’s data without impeding or interrupting the user’s normal workflow - even when the device is offline or has a flaky network connection. From the programming perspective, availability along with privacy and security concerns pose significant challenges, for which developers have to learn and use specialized solutions such as conflict-free replicated data types (CRDTs) or APIs for centralized data stores. This work relieves developers from this complexity by enabling the direct and automatic use of algebraic data types - which developers already use to express the business logic of the application - for synchronization and collaboration. Moreover, we use this approach to provide end-to-end encryption and authentication between multiple replicas (using a shared secret) that is suitable for a coordination-free setting. This artifact demonstrates the approach in the context of a realistic case study. It shows that an implementation of the approach can handle realistic workloads, that the size of the data types does not grow indefinitely, and that it is feasible to always enable encryption for the intended scenario.

Cite as

Christian Kuessner, Ragnar Mogk, Anna-Katharina Wickert, and Mira Mezini. Algebraic Replicated Data Types: Programming Secure Local-First Software (Artifact). In Special Issue of the 37th European Conference on Object-Oriented Programming (ECOOP 2023). Dagstuhl Artifacts Series (DARTS), Volume 9, Issue 2, pp. 26:1-26:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


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@Article{kuessner_et_al:DARTS.9.2.26,
  author =	{Kuessner, Christian and Mogk, Ragnar and Wickert, Anna-Katharina and Mezini, Mira},
  title =	{{Algebraic Replicated Data Types: Programming Secure Local-First Software (Artifact)}},
  pages =	{26:1--26:4},
  journal =	{Dagstuhl Artifacts Series},
  ISSN =	{2509-8195},
  year =	{2023},
  volume =	{9},
  number =	{2},
  editor =	{Kuessner, Christian and Mogk, Ragnar and Wickert, Anna-Katharina and Mezini, Mira},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DARTS.9.2.26},
  URN =		{urn:nbn:de:0030-drops-182668},
  doi =		{10.4230/DARTS.9.2.26},
  annote =	{Keywords: local-first, data privacy, coordination freedom, CRDTs, AEAD}
}
Document
Artifact
Prisma: A Tierless Language for Enforcing Contract-Client Protocols in Decentralized Applications (Artifact)

Authors: David Richter, David Kretzler, Pascal Weisenburger, Guido Salvaneschi, Sebastian Faust, and Mira Mezini

Published in: DARTS, Volume 8, Issue 2, Special Issue of the 36th European Conference on Object-Oriented Programming (ECOOP 2022)


Abstract
Decentralized applications (dApps) consist of smart contracts that run on blockchains and clients that model collaborating parties. dApps are used to model financial and legal business functionality. Today, contracts and clients are written as separate programs - in different programming languages - communicating via send and receive operations. This makes distributed program flow awkward to express and reason about, increasing the potential for mismatches in the client-contract interface, which can be exploited by malicious clients, potentially leading to huge financial losses. In this paper, we present Prisma, a language for tierless decentralized applications, where the contract and its clients are defined in one unit. Pairs of send and receive actions that “belong together” are encapsulated into a single direct-style operation, which is executed differently by sending and receiving parties. This enables expressing distributed program flow via standard control flow and renders mismatching communication impossible. We prove formally that our compiler preserves program behavior in presence of an attacker controlling the client code. We systematically compare Prisma with mainstream and advanced programming models for dApps and provide empirical evidence for its expressiveness and performance.

Cite as

David Richter, David Kretzler, Pascal Weisenburger, Guido Salvaneschi, Sebastian Faust, and Mira Mezini. Prisma: A Tierless Language for Enforcing Contract-Client Protocols in Decentralized Applications (Artifact). In Special Issue of the 36th European Conference on Object-Oriented Programming (ECOOP 2022). Dagstuhl Artifacts Series (DARTS), Volume 8, Issue 2, pp. 16:1-16:3, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@Article{richter_et_al:DARTS.8.2.16,
  author =	{Richter, David and Kretzler, David and Weisenburger, Pascal and Salvaneschi, Guido and Faust, Sebastian and Mezini, Mira},
  title =	{{Prisma: A Tierless Language for Enforcing Contract-Client Protocols in Decentralized Applications (Artifact)}},
  pages =	{16:1--16:3},
  journal =	{Dagstuhl Artifacts Series},
  ISSN =	{2509-8195},
  year =	{2022},
  volume =	{8},
  number =	{2},
  editor =	{Richter, David and Kretzler, David and Weisenburger, Pascal and Salvaneschi, Guido and Faust, Sebastian and Mezini, Mira},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DARTS.8.2.16},
  URN =		{urn:nbn:de:0030-drops-162149},
  doi =		{10.4230/DARTS.8.2.16},
  annote =	{Keywords: Domain Specific Languages, Smart Contracts, Scala}
}
Document
Extended Abstract
Prisma: A Tierless Language for Enforcing Contract-Client Protocols in Decentralized Applications (Extended Abstract)

Authors: David Richter, David Kretzler, Pascal Weisenburger, Guido Salvaneschi, Sebastian Faust, and Mira Mezini

Published in: LIPIcs, Volume 222, 36th European Conference on Object-Oriented Programming (ECOOP 2022)


Abstract
Decentralized applications (dApps) consist of smart contracts that run on blockchains and clients that model collaborating parties. dApps are used to model financial and legal business functionality. Today, contracts and clients are written as separate programs - in different programming languages - communicating via send and receive operations. This makes distributed program flow awkward to express and reason about, increasing the potential for mismatches in the client-contract interface, which can be exploited by malicious clients, potentially leading to huge financial losses. In this paper, we present Prisma, a language for tierless decentralized applications, where the contract and its clients are defined in one unit. Pairs of send and receive actions that "belong together" are encapsulated into a single direct-style operation, which is executed differently by sending and receiving parties. This enables expressing distributed program flow via standard control flow and renders mismatching communication impossible. We prove formally that our compiler preserves program behavior in presence of an attacker controlling the client code. We systematically compare Prisma with mainstream and advanced programming models for dApps and provide empirical evidence for its expressiveness and performance. The design space of dApp programming and other multi-party languages depends on one major choice: a local model versus a global model. In a local model, parties are defined in separate programs and their interactions are encoded via send and receive effects. In a global language, parties are defined within one shared program and interactions are encoded via combined send-and-receive operations with no effects visible to the outside world. The global model is followed by tierless [Christian Queinnec, 2000; Cooper et al., 2007; Choi and Chang, 2019; Fowler et al., 2019; Serrano et al., 2006; Serrano and Prunet, 2016; Radanne et al., 2016; Weisenburger et al., 2018] and choreographic [Kohei Honda et al., 2011; Fabrizio Montesi et al., 2014; Saverio Giallorenzo et al., 2020] languages. However, known approaches to dApp programming follow the local model, thus rely on explicitly specifying the client-contract interaction protocol. Moreover, the contract and clients are implemented in different languages, hence, developers have to master two technology stacks. The dominating approach in industry is Solidity [Mix, 2019] for the contract and JavaScript for clients. Solidity relies on expressing the protocol using assertions in the contract code, which are checked at run time [Solidity documentation - common patterns, 2020]. Failing to insert the correct assertions may give parties illegal access to monetary values to the detriment of others [Nikolić et al., 2018; Luu et al., 2016]. In research, contract languages [Ankush Das et al., 2019; Michael J. Coblenz, 2017; Franklin Schrans et al., 2018; Franklin Schrans et al., 2019; Michael J. Coblenz et al., 2019; Michael J. Coblenz et al., 2019; Reed Oei et al., 2020; Sam Blackshear et al., 2019] have been proposed that rely on advanced type systems such as session types, type states, and linear types. The global model has not been explored for dApp programming. This is unfortunate given the potential to get by with a standard typing discipline and to avoid intricacies and potential mismatches of a two-language stack. Our work fills this gap by proposing Prisma - the first language that features a global programming model for Ethereum dApps. While we focus on the Ethereum blockchain, we believe our techniques to be applicable to other smart contract platforms. Prisma enables interleaving contract and client logic within the same program and adopts a direct style (DS) notation for encoding send-and-receive operations (with our awaitCl language construct) akin to languages with async/await [Gavin M. Bierman et al., 2012; Scala async rfc]. DS addresses shortcomings with the currently dominant encoding of the protocol’s finite state machines (FSM) [Mix, 2019; Michael J. Coblenz, 2017; Franklin Schrans et al., 2018; Franklin Schrans et al., 2019; Michael J. Coblenz et al., 2019; Michael J. Coblenz et al., 2019]. We argue writing FSM style corresponds to a control-flow graph of basic blocks, which is low-level and more suited to be written by a compiler than by a human. With FSM style, the contract is a passive entity whose execution is driven by clients. whereas the DS encoding allows the contract to actively ask clients for input, fitting dApp execution where a dominant contract controls execution and diverts control to other parties when their input is needed. In the following Prisma snippet, the payout function is a function invoked by the contract when it is time to pay money to a client. In Prisma, variables, methods and classes are separated into two namespaces, one for the contract and one for the clients. The payout method is located on the contract via the annotation @co. The body of the method diverts the control to the client using awaitCl(...) { ... }, hence the contained readLine call is executed on the client. Note that no explicit send/receive operations are needed but the communication protocol is expressed through the program control flow. Only after the check client == toBePayed that the correct client replied, the current contact balance balance() is transferred to the client via transfer. @co def payout(toBePayed: Arr[Address]): Unit = { awaitCl(client => client == toBePayed) { readLine("Press enter for payout") } toBePayed.transfer(balance()) } Overall, Prisma relieves the developer from the responsibility of correctly managing distributed, asynchronous program flows and the heterogeneous technology stack. Instead, the burden is put on the compiler, which distributes the program flow by means of selective continuation-passing-style (CPS) translation and defunctionalisation and inserts guards against malicious client interactions. We needed to develop a CPS translation for the code that runs on the Ethereum Virtual Machine (EVM) since the EVM has no built-in support for concurrency primitives which could be used for asynchronous communication. While CPS translations are well-known, we cannot use them out-of-the-box because the control flow is interwoven with distribution in our case. A CPS translation that does not take distribution into account would allow malicious clients to force the contract to deviate from the intended control flow by sending a spoofed continuation. Thus, it was imperative to prove correctness of our distributed CPS translation to ensure control-flow integrity of the contract.

Cite as

David Richter, David Kretzler, Pascal Weisenburger, Guido Salvaneschi, Sebastian Faust, and Mira Mezini. Prisma: A Tierless Language for Enforcing Contract-Client Protocols in Decentralized Applications (Extended Abstract). In 36th European Conference on Object-Oriented Programming (ECOOP 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 222, pp. 35:1-35:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


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@InProceedings{richter_et_al:LIPIcs.ECOOP.2022.35,
  author =	{Richter, David and Kretzler, David and Weisenburger, Pascal and Salvaneschi, Guido and Faust, Sebastian and Mezini, Mira},
  title =	{{Prisma: A Tierless Language for Enforcing Contract-Client Protocols in Decentralized Applications}},
  booktitle =	{36th European Conference on Object-Oriented Programming (ECOOP 2022)},
  pages =	{35:1--35:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-225-9},
  ISSN =	{1868-8969},
  year =	{2022},
  volume =	{222},
  editor =	{Ali, Karim and Vitek, Jan},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2022.35},
  URN =		{urn:nbn:de:0030-drops-162632},
  doi =		{10.4230/LIPIcs.ECOOP.2022.35},
  annote =	{Keywords: Domain Specific Languages, Smart Contracts, Scala}
}
Document
Dealing with Variability in API Misuse Specification

Authors: Rodrigo Bonifácio, Stefan Krüger, Krishna Narasimhan, Eric Bodden, and Mira Mezini

Published in: LIPIcs, Volume 194, 35th European Conference on Object-Oriented Programming (ECOOP 2021)


Abstract
APIs are the primary mechanism for developers to gain access to externally defined services and tools. However, previous research has revealed API misuses that violate the contract of APIs to be prevalent. Such misuses can have harmful consequences, especially in the context of cryptographic libraries. Various API-misuse detectors have been proposed to address this issue - including CogniCrypt, one of the most versatile of such detectors and that uses a language (CrySL) to specify cryptographic API usage contracts. Nonetheless, existing approaches to detect API misuse had not been designed for systematic reuse, ignoring the fact that different versions of a library, different versions of a platform, and different recommendations/guidelines might introduce variability in the correct usage of an API. Yet, little is known about how such variability impacts the specification of the correct API usage. This paper investigates this question by analyzing the impact of various sources of variability on widely used Java cryptographic libraries (including JCA/JCE, Bouncy Castle, and Google Tink). The results of our investigation show that sources of variability like new versions of the API and security standards significantly impact the specifications. We then use the insights gained from our investigation to motivate an extension to the CrySL language (named MetaCrySL), which builds on meta-programming concepts. We evaluate MetaCrySL by specifying usage rules for a family of Android versions and illustrate that MetaCrySL can model all forms of variability we identified and drastically reduce the size of a family of specifications for the correct usage of cryptographic APIs.

Cite as

Rodrigo Bonifácio, Stefan Krüger, Krishna Narasimhan, Eric Bodden, and Mira Mezini. Dealing with Variability in API Misuse Specification. In 35th European Conference on Object-Oriented Programming (ECOOP 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 194, pp. 19:1-19:27, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021)


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@InProceedings{bonifacio_et_al:LIPIcs.ECOOP.2021.19,
  author =	{Bonif\'{a}cio, Rodrigo and Kr\"{u}ger, Stefan and Narasimhan, Krishna and Bodden, Eric and Mezini, Mira},
  title =	{{Dealing with Variability in API Misuse Specification}},
  booktitle =	{35th European Conference on Object-Oriented Programming (ECOOP 2021)},
  pages =	{19:1--19:27},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-190-0},
  ISSN =	{1868-8969},
  year =	{2021},
  volume =	{194},
  editor =	{M{\o}ller, Anders and Sridharan, Manu},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2021.19},
  URN =		{urn:nbn:de:0030-drops-140621},
  doi =		{10.4230/LIPIcs.ECOOP.2021.19},
  annote =	{Keywords: API misuse, cryptographic API misuse detection, code generation, domain engineering, cryptographic standards}
}
Document
How to Avoid Making a Billion-Dollar Mistake: Type-Safe Data Plane Programming with SafeP4

Authors: Matthias Eichholz, Eric Campbell, Nate Foster, Guido Salvaneschi, and Mira Mezini

Published in: LIPIcs, Volume 134, 33rd European Conference on Object-Oriented Programming (ECOOP 2019)


Abstract
The P4 programming language offers high-level, declarative abstractions that bring the flexibility of software to the domain of networking. Unfortunately, the main abstraction used to represent packet data in P4, namely header types, lacks basic safety guarantees. Over the last few years, experience with an increasing number of programs has shown the risks of the unsafe approach, which often leads to subtle software bugs. This paper proposes SafeP4, a domain-specific language for programmable data planes in which all packet data is guaranteed to have a well-defined meaning and satisfy essential safety guarantees. We equip SafeP4 with a formal semantics and a static type system that statically guarantees header validity - a common source of safety bugs according to our analysis of real-world P4 programs. Statically ensuring header validity is challenging because the set of valid headers can be modified at runtime, making it a dynamic program property. Our type system achieves static safety by using a form of path-sensitive reasoning that tracks dynamic information from conditional statements, routing tables, and the control plane. Our evaluation shows that SafeP4’s type system can effectively eliminate common failures in many real-world programs.

Cite as

Matthias Eichholz, Eric Campbell, Nate Foster, Guido Salvaneschi, and Mira Mezini. How to Avoid Making a Billion-Dollar Mistake: Type-Safe Data Plane Programming with SafeP4. In 33rd European Conference on Object-Oriented Programming (ECOOP 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 134, pp. 12:1-12:28, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)


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@InProceedings{eichholz_et_al:LIPIcs.ECOOP.2019.12,
  author =	{Eichholz, Matthias and Campbell, Eric and Foster, Nate and Salvaneschi, Guido and Mezini, Mira},
  title =	{{How to Avoid Making a Billion-Dollar Mistake: Type-Safe Data Plane Programming with SafeP4}},
  booktitle =	{33rd European Conference on Object-Oriented Programming (ECOOP 2019)},
  pages =	{12:1--12:28},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-111-5},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{134},
  editor =	{Donaldson, Alastair F.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2019.12},
  URN =		{urn:nbn:de:0030-drops-108041},
  doi =		{10.4230/LIPIcs.ECOOP.2019.12},
  annote =	{Keywords: P4, data plane programming, type systems}
}
Document
Fault-tolerant Distributed Reactive Programming

Authors: Ragnar Mogk, Lars Baumgärtner, Guido Salvaneschi, Bernd Freisleben, and Mira Mezini

Published in: LIPIcs, Volume 109, 32nd European Conference on Object-Oriented Programming (ECOOP 2018)


Abstract
In this paper, we present a holistic approach to provide fault tolerance for distributed reactive programming. Our solution automatically stores and recovers program state to handle crashes, automatically updates and shares distributed parts of the state to provide eventual consistency, and handles errors in a fine-grained manner to allow precise manual control when necessary. By making use of the reactive programming paradigm, we provide these mechanisms without changing the behavior of existing programs and with reasonable performance, as indicated by our experimental evaluation.

Cite as

Ragnar Mogk, Lars Baumgärtner, Guido Salvaneschi, Bernd Freisleben, and Mira Mezini. Fault-tolerant Distributed Reactive Programming. In 32nd European Conference on Object-Oriented Programming (ECOOP 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 109, pp. 1:1-1:26, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)


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@InProceedings{mogk_et_al:LIPIcs.ECOOP.2018.1,
  author =	{Mogk, Ragnar and Baumg\"{a}rtner, Lars and Salvaneschi, Guido and Freisleben, Bernd and Mezini, Mira},
  title =	{{Fault-tolerant Distributed Reactive Programming}},
  booktitle =	{32nd European Conference on Object-Oriented Programming (ECOOP 2018)},
  pages =	{1:1--1:26},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-079-8},
  ISSN =	{1868-8969},
  year =	{2018},
  volume =	{109},
  editor =	{Millstein, Todd},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2018.1},
  URN =		{urn:nbn:de:0030-drops-92064},
  doi =		{10.4230/LIPIcs.ECOOP.2018.1},
  annote =	{Keywords: reactive programming, distributed systems, CRDTs, snapshots, restoration, error handling, fault tolerance}
}
Document
CrySL: An Extensible Approach to Validating the Correct Usage of Cryptographic APIs

Authors: Stefan Krüger, Johannes Späth, Karim Ali, Eric Bodden, and Mira Mezini

Published in: LIPIcs, Volume 109, 32nd European Conference on Object-Oriented Programming (ECOOP 2018)


Abstract
Various studies have empirically shown that the majority of Java and Android apps misuse cryptographic libraries, causing devastating breaches of data security. It is crucial to detect such misuses early in the development process. To detect cryptography misuses, one must first define secure uses, a process mastered primarily by cryptography experts, and not by developers. In this paper, we present CrySL, a definition language for bridging the cognitive gap between cryptography experts and developers. CrySL enables cryptography experts to specify the secure usage of the cryptographic libraries that they provide. We have implemented a compiler that translates such CrySL specification into a context-sensitive and flow-sensitive demand-driven static analysis. The analysis then helps developers by automatically checking a given Java or Android app for compliance with the CrySL-encoded rules. We have designed an extensive CrySL rule set for the Java Cryptography Architecture (JCA), and empirically evaluated it by analyzing 10,000 current Android apps. Our results show that misuse of cryptographic APIs is still widespread, with 95% of apps containing at least one misuse. Our easily extensible CrySL rule set covers more violations than previous special-purpose tools with hard-coded rules, with our tooling offering a more precise analysis.

Cite as

Stefan Krüger, Johannes Späth, Karim Ali, Eric Bodden, and Mira Mezini. CrySL: An Extensible Approach to Validating the Correct Usage of Cryptographic APIs. In 32nd European Conference on Object-Oriented Programming (ECOOP 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 109, pp. 10:1-10:27, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)


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@InProceedings{kruger_et_al:LIPIcs.ECOOP.2018.10,
  author =	{Kr\"{u}ger, Stefan and Sp\"{a}th, Johannes and Ali, Karim and Bodden, Eric and Mezini, Mira},
  title =	{{CrySL: An Extensible Approach to Validating the Correct Usage of Cryptographic APIs}},
  booktitle =	{32nd European Conference on Object-Oriented Programming (ECOOP 2018)},
  pages =	{10:1--10:27},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-079-8},
  ISSN =	{1868-8969},
  year =	{2018},
  volume =	{109},
  editor =	{Millstein, Todd},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2018.10},
  URN =		{urn:nbn:de:0030-drops-92151},
  doi =		{10.4230/LIPIcs.ECOOP.2018.10},
  annote =	{Keywords: cryptography, domain-specific language, static analysis}
}
Document
CrySL: An Extensible Approach to Validating the Correct Usage of Cryptographic APIs (Artifact)

Authors: Stefan Krüger, Johannes Späth, Karim Ali, Eric Bodden, and Mira Mezini

Published in: DARTS, Volume 4, Issue 3, Special Issue of the 32nd European Conference on Object-Oriented Programming (ECOOP 2018)


Abstract
In this artefact, we present CrySL, an extensible approach to validating the correct usage of cryptographic APIs. The artefact contains executables for CogniCrypt_{SAST}, the analysis CrySL-based analysis, along with the CrySL rules we used in in the original paper's experiments. We also provide scripts to re-run the experiments. We finally include a tutorial to showcase the CogniCrypt_{SAST} on a small Java target program.

Cite as

Stefan Krüger, Johannes Späth, Karim Ali, Eric Bodden, and Mira Mezini. CrySL: An Extensible Approach to Validating the Correct Usage of Cryptographic APIs (Artifact). In Special Issue of the 32nd European Conference on Object-Oriented Programming (ECOOP 2018). Dagstuhl Artifacts Series (DARTS), Volume 4, Issue 3, pp. 6:1-6:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)


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@Article{kruger_et_al:DARTS.4.3.6,
  author =	{Kr\"{u}ger, Stefan and Sp\"{a}th, Johannes and Ali, Karim and Bodden, Eric and Mezini, Mira},
  title =	{{CrySL: An Extensible Approach to Validating the Correct Usage of Cryptographic APIs (Artifact)}},
  pages =	{6:1--6:4},
  journal =	{Dagstuhl Artifacts Series},
  ISSN =	{2509-8195},
  year =	{2018},
  volume =	{4},
  number =	{3},
  editor =	{Kr\"{u}ger, Stefan and Sp\"{a}th, Johannes and Ali, Karim and Bodden, Eric and Mezini, Mira},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DARTS.4.3.6},
  URN =		{urn:nbn:de:0030-drops-92371},
  doi =		{10.4230/DARTS.4.3.6},
  annote =	{Keywords: cryptography, domain-specific language, static analysis}
}
Document
A Co-contextual Type Checker for Featherweight Java

Authors: Edlira Kuci, Sebastian Erdweg, Oliver Bracevac, Andi Bejleri, and Mira Mezini

Published in: LIPIcs, Volume 74, 31st European Conference on Object-Oriented Programming (ECOOP 2017)


Abstract
This paper addresses compositional and incremental type checking for object-oriented programming languages. Recent work achieved incremental type checking for structurally typed functional languages through co-contextual typing rules, a constraint-based formulation that removes any context dependency for expression typings. However, that work does not cover key features of object-oriented languages: Subtype polymorphism, nominal typing, and implementation inheritance. Type checkers encode these features in the form of class tables, an additional form of typing context inhibiting incrementalization. In the present work, we demonstrate that an appropriate co-contextual notion to class tables exists, paving the way to efficient incremental type checkers for object-oriented languages. This yields a novel formulation of Igarashi et al.'s Featherweight Java (FJ) type system, where we replace class tables by the dual concept of class table requirements and class table operations by dual operations on class table requirements. We prove the equivalence of FJ's type system and our co-contextual formulation. Based on our formulation, we implemented an incremental FJ type checker and compared its performance against javac on a number of realistic example programs.

Cite as

Edlira Kuci, Sebastian Erdweg, Oliver Bracevac, Andi Bejleri, and Mira Mezini. A Co-contextual Type Checker for Featherweight Java. In 31st European Conference on Object-Oriented Programming (ECOOP 2017). Leibniz International Proceedings in Informatics (LIPIcs), Volume 74, pp. 18:1-18:26, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2017)


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@InProceedings{kuci_et_al:LIPIcs.ECOOP.2017.18,
  author =	{Kuci, Edlira and Erdweg, Sebastian and Bracevac, Oliver and Bejleri, Andi and Mezini, Mira},
  title =	{{A Co-contextual Type Checker for Featherweight Java}},
  booktitle =	{31st European Conference on Object-Oriented Programming (ECOOP 2017)},
  pages =	{18:1--18:26},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-035-4},
  ISSN =	{1868-8969},
  year =	{2017},
  volume =	{74},
  editor =	{M\"{u}ller, Peter},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2017.18},
  URN =		{urn:nbn:de:0030-drops-72628},
  doi =		{10.4230/LIPIcs.ECOOP.2017.18},
  annote =	{Keywords: type checking, co-contextual, constraints, class table, Featherweight Java}
}
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