Many programming languages allow programmers to regulate accessibility; i.e., annotating a declaration with keywords such as export and private to indicate where it can be accessed. Despite the importance of name accessibility for, e.g., compilers, editor auto-completion and tooling, and automated refactorings, few existing type systems provide a formal account of name accessibility. We present a declarative, executable, and language-parametric model for name accessibility, which provides a formal specification of name accessibility in Java, C#, C++, Rust, and Eiffel. We achieve this by defining name accessibility as a predicate on resolution paths through scope graphs. Since scope graphs are a language-independent model of name resolution, our model provides a uniform approach to defining different accessibility policies for different languages. Our model is implemented in Statix, a logic language for executable type system specification using scope graphs. We evaluate its correctness on a test suite that compares it with the C#, Java, and Rust compilers, and show we can synthesize access modifiers in programs with holes accurately.@en

Static name binding (i.e., associating references with appropriate declarations) is an essential aspect of programming languages. However, it is usually treated in an unprincipled manner, often leaving a gap between formalization and implementation. The scope graph formalism mitigates these deficiencies by providing a well-defined, first-class, language-parametric representation of name binding. Scope graphs serve as a foundation for deriving type checkers from declarative type system specifications, reasoning about type soundness, and implementing editor services and refactorings. In this paper we present an overview of scope graphs, and, using examples, show how the ideas and notation of the formalism have evolved. We also briefly discuss follow-up research beyond type checking, and evaluate the formalism.

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An important aspect of type checking is name resolution — i.e., determining the types of names by resolving them to a matching declaration. For most languages, we can give typing rules that define name resolution in a way that abstracts from what order different units of code should be checked in. However, implementations of type checkers in practice typically use multiple phases to ensure that declarations of resolvable names are available before names are resolved. This gives rise to a gap between typing rules that abstract from order of type checking and multi-phased type checkers that rely on explicit ordering.

This paper introduces techniques that reduce this gap. First, we introduce a monadic interface for phased name resolution which detects and rejects type checking runs with name resolution phasing errors where names were wrongly resolved because some declarations were not available when they were supposed to be. Second, building on recent work by Gibbons et al., we use applicative functors to compositionally map abstract syntax trees onto (phased) monadic computations that represent typing constraints. These techniques reduce the gap between type checker implementations and typing rules in the sense that (1) both are given by compositional mappings over abstract syntax trees, and (2) type checker cases consist of computations that roughly correspond to typing rule premises, except these are composed using monadic combinators. We demonstrate our approach by implementing type checkers for Mini-ML with Damas-Hindley-Milner type inference, and LM, a toy module language with a challenging import resolution policy.
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Static type systems can greatly enhance the quality of programs, but implementing a type checker that is both expressive and user-friendly is challenging and error-prone. The Statix meta-language (part of the Spoofax language workbench) aims to make this task easier by automatically deriving a type checker from a declarative specification of a type system. However, so far Statix has not been used to implement dependent types, which is a class of type systems which require evaluation of terms during type checking. In this paper, we present an implementation of a simple dependently typed language in Statix, and discuss how to extend it with several common features such as inductive data types, universes, and inference of implicit arguments. While we encountered some challenges in the implementation, our conclusion is that Statix is already usable as a tool for implementing dependent types.

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Fast analysis response times in IDEs are essential for a good editor experience. Incremental type-checking can provide that in a scalable fashion. However, existing techniques are not reusable between languages. Moreover, mutual and dynamic dependencies preclude traditional approaches to incrementality. This makes finding automatic approaches to incremental type-checking a challenging but important open question.
In this paper, we present a technique that automatically derives incremental type-checkers from type system specifications written in the Statix meta-DSL. We use name resolution queries in scope graphs (a generic model of name binding embedded in Statix) to derive dependencies between compilation units. A novel query confirmation algorithm finds queries for which the answer changed due to an edit in the program. Only units with such queries require reanalysis. The effectiveness of this algorithm is improved by (1) splitting the type-checking task into a context-free and a context-sensitive part, and (2) reusing a generic mechanism to resolve mutual dependencies. This automatically yields incremental type-checkers for any Statix specification.
Compared to non-incremental parallel execution, we achieve speedups up to 147x on synthetic benchmarks, and up to 21x on real-world projects, with initial overheads below 10%. This suggests that our framework can provide efficient incremental type-checking to the wide range of languages supported by Statix.@en

To warrant programmer productivity, type checker results should be correct and available quickly. Correctness can be provided when a type checker implementation corresponds to a declarative type system specification. Statix is a type system specification language which achieves this by automatically deriving type checker implementations from declarative typing rules. A key feature of Statix is that it uses scope graphs for declarative specification of name resolution. However, compared to hand-written type checkers, type checkers derived from Statix specifications have sub-optimal run time performance.

In this paper, we identify and resolve a performance bottleneck in the Statix solver, namely part of the name resolution algorithm, using partial evaluation. To this end, we introduce a tailored procedural intermediate query resolution language, and provide a specializer that translates declarative queries to this language.

Evaluating this specializer by comparing type checking run time performance on three benchmarks (Apache Commons CSV, IO, and Lang3), shows that our specializer improves query resolution time up to 7.7x, which reduces the total type checking run time by 38 - 48%.
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