Open Transactional Actions: Interacting with Non-transactional Resources in STM Haskell Jonathas Augusto de Oliveira Conceição Federal University of Pelotas Pelotas, Brazil jadoliveira@inf.ufpel.edu.br André Rauber Du Bois PPGC - Federal University of Pelotas Pelotas, Brazil dubois@inf.ufpel.edu.br Samuel da Silva Feitosa Federal University of Fronteira Sul Chapecó, Brazil samuel.feitosa@ufs.edu.br Gerson Geraldo Homrich Cavalheiro PPGC - Federal University of Pelotas Pelotas, Brazil gerson.cavalheiro@inf.ufpel.edu.br Rodrigo Geraldo Ribeiro PPGCC - Federal University of Ouro Preto Ouro Preto, Brazil rodrigo.ribeiro@ufop.edu.br Abstract This paper addresses the problem of accessing external re- sources from inside transactions in STM Haskell, and for that purpose introduces a new abstraction called Open Transac- tional Actions (OTAs) that provides a framework for wrap- ping non-transactional resources in a transactional layer. OTAs allow the programmer to access resources through IO actions, from inside transactions, and also to register commit and abort handlers: the former are used to make the accesses to resources visible to other transactions at commit time, and the latter to undo changes in the resource if the transaction has to roll back. OTAs, once started, are guaranteed to be executed completely before the hosting transaction can be aborted, guarantying that if a resource is accessed, its respec- tive commit and abort actions will be properly registered. We believe that OTAs could be used by expert programmers to implement useful system libraries and also to give a trans- actional semantics to fast linearizable data structures, i.e., transactional boosting. As a proof of concept, we present examples that use OTAs to implement transactional fle ac- cess and transactional boosted data types that are faster than pure STM Haskell in most cases. CCS Concepts: · Software and its engineering → Func- tional languages; Concurrent programming languages. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proft or commercial advantage and that copies bear this notice and the full citation on the frst page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specifc permission and/or a fee. Request permissions from permissions@acm.org. Haskell ’22, September 15–16, 2022, Ljubljana, Slovenia © 2022 Copyright held by the owner/author(s). Publication rights licensed to ACM. ACM ISBN 978-1-4503-9438-3/22/09. . . $15.00 htps://doi.org/10.1145/3546189.3549924 Keywords: transactional memory, functional programming, transactional boosting, Haskell ACM Reference Format: Jonathas Augusto de Oliveira Conceição, André Rauber Du Bois, Samuel da Silva Feitosa, Gerson Geraldo Homrich Cavalheiro, and Ro- drigo Geraldo Ribeiro. 2022. Open Transactional Actions: Interact- ing with Non-transactional Resources in STM Haskell. In Proceed- ings of the 15th ACM SIGPLAN International Haskell Symposium (Haskell ’22), September 15–16, 2022, Ljubljana, Slovenia. ACM, New York, NY, USA, 12 pages. htps://doi.org/10.1145/3546189.3549924 1 Introduction Transactional Memory (TM) is an abstraction that provides a higher-level abstraction for concurrent/parallel program- ming: critical sections of the code are accessed through trans- actions, similar to data-base transactions, that appear to the programmer to be executed atomically with respect to other concurrently executed transactions. The underlying trans- actional system must guarantee atomicity, consistency and isolation, when transactions concurrently access shared data. Many important aspects of software development are im- proved, such as composability, robustness, and correctness. STM Haskell is a mature and widely used extension of Haskell for transactional memory that was frst introduced in 2005 [18]. In STM Haskell, transactions operate on transac- tional variables (also called TVars) which are special shared memory variables that can only be accessed by transactions. The type system guarantees that the only operations allowed to occur inside transactions are the ones related to creating, writing and reading transactional variables, pure computa- tions, and also operations to compose transactions into new ones. Confict detection between transactions is done in a fne grain level, based on reads and writes to TVars. Such confict detection scheme, which is used in most TM sys- tems, has its disadvantages as it might limit concurrency for certain objects specially if they are the source of high- contention [20]. Furthermore, as the type system guarantees that only TVars operations are allowed in transactions the 54