Live Pattern Matching with Typed Holes

Several modern programming systems, including GHC Haskell, Agda, Idris, and Hazel, support typed holes . Assigning static and, to varying degree, dynamic meaning to programs with holes allows program editors and other tools to offer meaningful feedback and assistance throughout editing, i.e. in a live manner. Prior work, however, has considered only holes appearing in expressions and types. This paper considers, from type theoretic and logical first principles, the problem of typed pattern holes. We confront two main difficulties, (1) statically reasoning about exhaustiveness and irredundancy when patterns are not fully known, and (2) live evaluation of expressions containing both pattern and expression holes. In both cases, this requires reasoning conservatively about all possible hole fillings. We develop a typed lambda calculus, Peanut, where reasoning about exhaustiveness and redundancy is mapped to the problem of deriving first order entailments. We equip Peanut with an operational semantics in the style of Hazelnut Live that allows us to evaluate around holes in both expressions and patterns. We mechanize the metatheory of Peanut in Agda and formalize a procedure capable of deciding the necessary entailments. Finally, we scale up and implement these mechanisms within Hazel, a programming environment for a dialect of Elm that automatically inserts holes during editing to provide static and dynamic feedback to the programmer in a maximally live manner, i.e. for every possible editor state. Hazel is the first maximally live environment for a general-purpose functional language.