Code editors provide essential services that help developers understand, navigate, and modify programs. However, these services often fail in the presence of syntax errors. Existing syntax error recovery techniques, like panic mode and multi-option repairs, are either too coarse, e.g. in deleting large swathes of code, or lead to a proliferation of possible completions. This paper introduces tall tylr , an error-handling parser and editor generator that completes malformed code with syntactic obligations that abstract over many possible completions. These obligations generalize the familiar notion of holes in structure editors to cover missing operands, operators, delimiters, and sort transitions. tall tylr is backed by a novel theory of tile-based parsing, conceptually organized around a molder that turns tokens into tiles and a melder that completes and parses tiles into terms using an error-handling generalization of operator-precedence parsing. We formalize melding as a parsing calculus, meldr, that completes input tiles with additional obligations such that it can be parsed into a well-formed term, with success guaranteed over all inputs. We further describe how tall tylr implements molding and completionranking using the principle of minimizing obligations . Obligations offer a useful way to scaffold internal program representations, but in tall tylr we go further to investigate the potential of materializing these obligations visually to the programmer. We conduct a user study to evaluate the extent to which an editor like tall tylr that materializes syntactic obligations might be usable and useful, finding both points of positivity and interesting new avenues for future work.

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.