78 research outputs found
Introducing a Calculus of Effects and Handlers for Natural Language Semantics
In compositional model-theoretic semantics, researchers assemble
truth-conditions or other kinds of denotations using the lambda calculus. It
was previously observed that the lambda terms and/or the denotations studied
tend to follow the same pattern: they are instances of a monad. In this paper,
we present an extension of the simply-typed lambda calculus that exploits this
uniformity using the recently discovered technique of effect handlers. We prove
that our calculus exhibits some of the key formal properties of the lambda
calculus and we use it to construct a modular semantics for a small fragment
that involves multiple distinct semantic phenomena
No value restriction is needed for algebraic effects and handlers
We present a straightforward, sound Hindley-Milner polymorphic type system
for algebraic effects and handlers in a call-by-value calculus, which allows
type variable generalisation of arbitrary computations, not just values. This
result is surprising. On the one hand, the soundness of unrestricted
call-by-value Hindley-Milner polymorphism is known to fail in the presence of
computational effects such as reference cells and continuations. On the other
hand, many programming examples can be recast to use effect handlers instead of
these effects. Analysing the expressive power of effect handlers with respect
to state effects, we claim handlers cannot express reference cells, and show
they can simulate dynamically scoped state
An Effect System for Algebraic Effects and Handlers
We present an effect system for core Eff, a simplified variant of Eff, which
is an ML-style programming language with first-class algebraic effects and
handlers. We define an expressive effect system and prove safety of operational
semantics with respect to it. Then we give a domain-theoretic denotational
semantics of core Eff, using Pitts's theory of minimal invariant relations, and
prove it adequate. We use this fact to develop tools for finding useful
contextual equivalences, including an induction principle. To demonstrate their
usefulness, we use these tools to derive the usual equations for mutable state,
including a general commutativity law for computations using non-interfering
references. We have formalized the effect system, the operational semantics,
and the safety theorem in Twelf
Koka: Programming with Row Polymorphic Effect Types
We propose a programming model where effects are treated in a disciplined
way, and where the potential side-effects of a function are apparent in its
type signature. The type and effect of expressions can also be inferred
automatically, and we describe a polymorphic type inference system based on
Hindley-Milner style inference. A novel feature is that we support polymorphic
effects through row-polymorphism using duplicate labels. Moreover, we show that
our effects are not just syntactic labels but have a deep semantic connection
to the program. For example, if an expression can be typed without an exn
effect, then it will never throw an unhandled exception. Similar to Haskell's
`runST` we show how we can safely encapsulate stateful operations. Through the
state effect, we can also safely combine state with let-polymorphism without
needing either imperative type variables or a syntactic value restriction.
Finally, our system is implemented fully in a new language called Koka and has
been used successfully on various small to medium-sized sample programs ranging
from a Markdown processor to a tier-splitted chat application. You can try out
Koka live at www.rise4fun.com/koka/tutorial.Comment: In Proceedings MSFP 2014, arXiv:1406.153
Inferring Algebraic Effects
We present a complete polymorphic effect inference algorithm for an ML-style
language with handlers of not only exceptions, but of any other algebraic
effect such as input & output, mutable references and many others. Our main aim
is to offer the programmer a useful insight into the effectful behaviour of
programs. Handlers help here by cutting down possible effects and the resulting
lengthy output that often plagues precise effect systems. Additionally, we
present a set of methods that further simplify the displayed types, some even
by deliberately hiding inferred information from the programmer
Type driven development of concurrent communicating systems
This work was kindly supported by SICSA (the Scottish Informatics and Computer Science Alliance) and EPSRC grant EP/N024222/1 (Type-driven Verification of Communicating Systems).Modern software systems rely on communication, for example mobile applications communicating with a central server, distributed systems coordinating a telecommunications network, or concurrent systems handling events and processes in a desktop application. However, reasoning about concurrent programs is hard, since we must reason about each process and the order in which communication might happen between processes. In this paper, I describe a type-driven approach to implementing communicating concurrent programs, using the dependently typed programming language Idris. I show how the type system can be used to describe resource access protocols (such as controlling access to a file handle) and verify that programs correctly follow those protocols. Finally, I show how to use the type system to reason about the order of communication between concurrent processes, ensuring that each end of a communication channel follows a defined protocol.Publisher PDFPeer reviewe
Typed Equivalence of Effect Handlers and Delimited Control
It is folklore that effect handlers and delimited control operators are closely related: recently, this relationship has been proved in an untyped setting for deep handlers and the shift_0 delimited control operator. We positively resolve the conjecture that in an appropriately polymorphic type system this relationship can be extended to the level of types, by identifying the necessary forms of polymorphism, thus extending the definability result to the typed context. In the process, we identify a novel and potentially interesting type system feature for delimited control operators. Moreover, we extend these results to substantiate the folklore connection between shallow handlers and control_0 flavour of delimited control, both in an untyped and typed settings
A Functional Abstraction of Typed Invocation Contexts
In their paper "A Functional Abstraction of Typed Contexts", Danvy and
Filinski show how to derive a monomorphic type system of the shift and reset
operators from a CPS semantics. In this paper, we show how this method scales
to Felleisen's control and prompt operators. Compared to shift and reset,
control and prompt exhibit a more dynamic behavior, in that they can manipulate
a trail of contexts surrounding the invocation of previously captured
continuations. Our key observation is that, by adopting a functional
representation of trails in the CPS semantics, we can derive a type system that
encodes all and only constraints imposed by the CPS semantics
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