Slim Lim: "Concrete syntax matters, actually"

Slim Lim argues that concrete syntax is a critical interface for programmers and directly affects how they reason about language behavior.

Topos Institute Colloquium, 4th of December 2025. ——— Too many programming languages researchers dismiss concrete syntax as an afterthought: arbitrary, superficial, or distracting from matters of “actual” (semantic) importance. This received view ignores a critical factor: the human at the computer. Concrete syntax defines the principal interface through which programmers interact with the vast majority of programming languages. Moreover, this interface is hardly decoupled from semantics; even trivial-seeming differences in keywords, sigils, and indentation can affect how programmers utilize and reason about language behavior. Using examples from asynchronous control flow, gradual subtyping, first-class functions, and more, I will make a case for the importance of concrete syntax, why language designers often overlook it, and what this implies for those of us who care about the usability of abstractions. Finally, I will describe some preliminary work evaluating the role of lexical ambiguity in programmer comprehension of type system features.