We present Parsimony, a programming-by-example development environment for synthesizing lexers and parsers by example. Parsimony provides a graphical interface in which the user presents examples simply by selecting and labeling sample text in a text editor. An underlying synthesis engine then constructs syntactic rules to solve the system of constraints induced by the supplied examples. Parsimony is more expressive and usable than prior programming-by-example systems for parsers in several ways: Parsimony can (1) synthesize lexer rules in addition to productions, (2) solve for much larger constraint systems over multiple examples, rather than handling examples one-at-a-time, and (3) infer much more complex sets of productions, such as entire algebraic expression grammars, by detecting instances of well-known grammar design patterns. The results of a controlled user study across 18 participants show that users are able to perform lexing and parsing tasks faster and with fewer mistakes when using Parsimony as compared to a traditional parsing workflow.
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