Another Taste of Verse | Simon Peyton Jones | ZuriHac 2026

Simon Peyton Jones explores the design of Verse, an experimental functional logic programming language featuring choice, failure, and multi-valued expressions.

Simon Peyton Jones presents a deep exploration of Verse, the experimental programming language being developed at Epic Games, and explains how its design challenges many familiar assumptions about functional programming and type systems. Starting from seemingly conventional syntax, the talk gradually reveals a language built around choice, failure, finite-domain functions, and a highly unified view of computation. A central theme is that every expression in Verse can produce zero, one, or many values. Concepts such as choice, failure, conditionals, iteration, and array construction are all built on this foundation. Simon demonstrates how Verse integrates ideas from functional logic programming, allowing computations to behave like search spaces while maintaining a declarative programming model. The language treats arrays as finite-domain functions, enabling a uniform framework where indexing, function application, iteration, and narrowing all arise from the same underlying principles. The talk also examines Verse’s unusual type system. Types are represented as ordinary functions that can succeed or fail, making user-defined types, dependent constraints, and polymorphic type constructors natural language features. Through examples such as array types, pair types, and constrained numeric domains, Simon shows how familiar concepts from type theory and functional programming become executable components of the language. A particularly striking section explores the relationship between patterns and expressions. Rather than treating pattern matching as a separate mechanism, Verse uses a unified model where arbitrary expressions can appear in pattern positions. This leads to a novel interpretation of functions, matching, and verification, while drawing connections to dependent types, theorem proving, language design, compiler construction, and formal reasoning. The presentation concludes with reflections on the ongoing research behind Verse, highlighting its combination of declarative programming, functional logic programming, verification, effects, and semantics. Simon frames Verse as an ambitious attempt to rethink the foundations of programming languages and to explore a larger design space beyond traditional imperative and functional paradigms. Chapters 00:00:00 Introduction And Origins Of Verse 00:03:21 Declarative Programming And Language Overview 00:06:21 Choice, Failure, And Conditionals 00:10:09 Sequencing, Unification, And Search 00:12:33 Arrays, Tuples, And Iteration 00:19:59 Mapping And Transforming Arrays 00:24:12 Arrays As Finite-Domain Functions 00:27:07 Verification And Safe Function Calls 00:32:13 Narrowing And Functional Logic Programming 00:35:16 Building List-Like Operations 00:39:18 Variable Binding And Scope 00:41:08 Types As Partial Functions 00:46:08 Array Types And Type Constructors 00:49:55 Patterns And Expressions As One Concept 00:57:14 Matching Functions And Higher-Order Patterns 01:01:10 Conclusions And