Bidirectional Elaborators à la Carte

Abstract: Surface syntax in proof assistants like Rocq, Lean, Agda, and Idris is highly implicit, lacking many details that are needed for user-written code to denote precisely defined mathematical objects. Elaboration is an algorithm that accounts for these details by translating surface syntax to an explicit enough core syntax. The reliability and predictability of elaboration relies on several critical properties of the core type system, including decidability of judgemental equality and the injectivity of type constructors; these dependencies are witnessed in a concrete system by explicit calls to conversion checking and weak-head reduction subroutines. We introduce a dependently typed monadic domain specific language for the executable specification of correct-by-construction elaboration algorithms that is abstracted from any particular representation of normal forms or algorithm for conversion checking. In particular, we represent a bidirectionally typed surface language for Martin-Löf type theory by shallow embedding in this DSL so that the translation of surface terms into core terms amounts to elementary equational calculation. This translation is correct by construction in the sense that it cannot produce ill-typed terms, and is automatically stable under judgemental equality of core terms and even under substitution; from the latter property, we obtain a new denotational interpretation of the suspension of elaboration problems. Finally, a concrete elaboration algorithm is extracted by algebraic means from a presheaf model of the DSL built out of the bi-initial natural model of Martin-Löf type theory.

• The paper introduces a monadic DSL built on a partiality monad to construct elaborators that are correct-by-construction and support compositional reasoning.
• The paper demonstrates robust handling of variable scopes, substitution stability, and error suspension through presheaf semantics and modular combinators.
• The paper connects algebraic, categorical, and operational perspectives to enhance reliability and extensibility in dependent type theory-based proof assistants.

Executable Semantics for Modular Bidirectional Elaboration

Introduction

"Bidirectional Elaborators à la Carte" (2607.09564) develops a modular, semantic framework for specifying and reasoning about bidirectional elaboration algorithms in proof assistants based on dependent type theory. The authors present a functional, compositional semantics for elaboration, instead of a logic programming or rule-based operational approach. Their methodology leverages a monadic domain-specific language (DSL) built upon a partiality monad internal to a topos, allowing elaborators that are correct-by-construction, stable under substitution, and robust to the specific syntactic representation of core terms.

Background and Motivation

The gap between implicit, user-centric "surface" languages in proof assistants and the explicit, well-formed "core" languages required for machine reasoning necessitates a careful design of elaboration: the process that translates user code into precise mathematical objects. The authors highlight that the design and reliability of such elaborators depend critically on key metatheoretical properties—e.g., decidability of judgmental equality or injectivity of type constructors—of the target core type theory.

A principal motivation is to avoid ad hoc, folklore-driven implementation practices and instead provide a flexible, precise, and modular mathematical theory of elaboration. The approach generalizes the concept of parser combinators from functional programming to elaborator combinators, underpinned by the metatheory of dependent types in the context of categories with families (CwF) and natural models. This modularity and abstraction aim to enable component-wise reasoning about correctness, definedness, and structural properties of elaborate processes.

Semantic Model: Partiality Monad and Monad-Based DSL

The core technical contribution is a DSL for specifying elaborators within a partiality monad $\mathsf{PMC}$. This monad encapsulates fallibility (i.e., partiality or potential for failure/suspension) via a pair: a support (an open proposition) and a function defined on that support. Crucially, the support is not restricted to booleans but lies in a dominance—a universe of proof-irrelevant, possibly non-classical, open propositions in a topos.

The DSL provides semantic domains:

• $\mathsf{TypScript} = \mathsf{PMC}(\mathsf{Tp})$ for type scripts,
• $$\mathsf{SynScript} = \mathsf{PMC}(\widetilde{\mathsf{Tm}})$$ for synthesizing terms with their types,
• $$\mathsf{ChkScript} = (\alpha:\mathsf{Tp}) \to \mathsf{PMC}(\mathsf{Tm}(\alpha))$$ for checking-mode terms.

Binding and scope are handled via higher-arity lax monoidal structure operators, enabling compositional reasoning about variable scopes, hypothetical judgments, and the denotation of surface binders. Suspension of elaboration problems (as required for handling unsolved metavariables) is naturally interpreted using presheaf semantics, where conditions on supports are sieves stable under substitution.

Bidirectional Elaboration: Modular, Correct-by-Construction Combinators

The framework specifies universal combinators for all judgment forms in a dependently typed core theory: variable introduction, conversion, annotation, functions (lambda and application), dependent pairs, identity types, and so on. Each combinator is given as a semantic function—in the DSL monad—satisfying the following properties:

• Total correctness: Only well-typed terms can be constructed.
• Compatibility with substitution and judgmental equality: Reasoning is stable under core equivalence and substitutions.
• Compositionality: Properties of elaboration combinators (such as definedness) are wholly determined by their arguments (formalized via multilinearity and multistrictness).

Notably, the framework provides a formal mechanism for reasoning not just about "definedness" but also about the propagation and suspension of partial definitions. This regularizes error handling, asynchronous elaboration, and stable resumptions—crucial for features such as holes or delayed constraints found in interactive proof development.

Abstract Metatheoretic Layer and Connections to Modern Type Theory

The semantics is not tied to any specific syntax for types or terms but is parametrized abstractly over any algebraic (especially SOGAT-based) type theory. The framework thus exploits metatheorems such as normalization, strengthening, and decidability as external parameters, making the semantics broad and robust to syntactic redesigns.

By employing presheaf models over the category of contexts of the bi-initial model of type theory, the authors cleanly separate synthetic elaborator semantics from concrete, rule-based implementations. Moreover, they highlight that their semantic interpretation does not depend on any one construction of the syntactic model but is invariant across representations—enabling their framework's use for alternate presentations of a type theory (cf. [uemura:2021:thesis], [sterling:2025:grothendieck]).

Technical Results

The DSL and semantic calculus are shown to satisfy several strong properties:

• Multilinearity: All non-binding combinators, and bindings when strengthening holds, are multilinear as maps of algebras over the monad. Thus, the definedness and structure of any composite elaboration is strictly determined by those of its sub-terms.
• Multistrictness: If any subterm is undefined, the composite is undefined.
• Commutativity and Suspension: Since the partiality monad is commutative, the order of solving constraints and evaluating delayed elaborations is semantically irrelevant, modeling dynamic (partial) elaboration naturally.
• Subject Reduction Inequalities: An inequational theory over surface syntax (e.g., $\beta$- and $\eta$-reductions and erase-annotation reductions hold as inequalities in the partiality poset semantics). This reflects elaboration's correctness in the presence of refactorings that reduce annotation or syntactic bureaucracy.

These properties are established for both the standard Martin-Löf type theory and its extensions via general second-order algebraic theories.

Practical and Theoretical Implications

The semantics developed support modular, compositional construction of correct-by-construction elaborators that can be extracted, verified, and reasoned about independently of syntactic details. The approach naturally accommodates extension to new type-theoretic features (such as universe hierarchies or polymorphism), is robust against evolving implementations, and facilitates stable, asynchronous, and dynamic metaprogramming with "holes" (i.e., metavariables).

On a theoretical level, the approach clarifies the relationships between algebraic, categorical, and operational presentations of type theories and may inform the design of generic frameworks for parser/elaborator generators in the style of both semantic (combinator-based) and syntactic (rule-based) frameworks.

Moreover, the framework provides a vehicle for future work integrating advanced evaluation-order and polarity systems (cf. call-by-push-value, enriched effect calculi), denotational resource analysis (cf. [danner-licata-ramyaa:2015], [kavvos-morehouse-licata-danner:2019]), and integration with real proof assistants ([synthlean:cpp]).

Future Directions

The framework admits several clear directions:

• Extending to cumulative universe hierarchies, type class/implicit argument automation, and richer algebraic effects (notably in light of ongoing work on polynomial monads for metavariables).
• Mechanized formalization in existing proof assistants (Agda, Lean, Rocq) as semantic, modular specifications of elaboration, guiding or certifying low-level implementations.
• Further abstract investigation into the interaction between evaluation order, effectful metaprogramming, and higher-algebraic structure in elaboration/Eff calculi.

Conclusion

The work realizes a modular, highly structured, executably expressive semantics for bidirectional elaboration, achieving correctness-by-construction, stability, and compositionality at a level of generality apt for modern algebraic type theories. This closes a significant conceptual gap, elevating elaboration from folklore-based engineering to a subject of abstract mathematical investigation, supporting practical advances in the reliability and extensibility of dependent type theory-based proof assistants.

Reference:

"Bidirectional Elaborators à la Carte" (2607.09564)