This paper addresses the problem of specifying and parsing the syntax of domain-specific languages (DSLs) in a modular, user-friendly way. That is, we want to enable the design of composable DSLs that combine the natural syntax of external DSLs with the easy implementation of internal DSLs. The challenge in parsing composable DSLs is that the composition of several (individually unambiguous) languages is likely to contain ambiguities. In this paper, we present the design of a system that uses a type-oriented variant of island parsing to efficiently parse the syntax of composable DSLs. In particular, we show how type-oriented island parsing is constant time with respect to the number of DSLs imported. We also show how to use our tool to implement DSLs on top of a host language such as Typed Racket.
Several systems use Ford's Parsing Expression Grammar (PEG) formalism @cite_0 . PEGs are stylistically similar to CFGs; however, PEGs avoid ambiguity by introducing a prioritized choice operator for rule alternatives and PEGs disallow left-recursive rules. We claim that these two restrictions are not appropriate for composing DSLs. The order in which DSLs are imported should not matter and DSL authors should be free to use left recursion if that is the most natural way to express their grammar.
@cite_0
2009706943
Parsing Expression Grammars (PEGs) are a formalism that can describe all deterministic context-free languages through a set of rules that specify a top-down parser for some language. PEGs are easy to use, and there are efficient implementations of PEG libraries in several programming languages.A frequently missed feature of PEGs is left recursion, which is commonly used in Context-Free Grammars (CFGs) to encode left-associative operations. We present a simple conservative extension to the semantics of PEGs that gives useful meaning to direct and indirect left-recursive rules, and show that our extensions make it easy to express left-recursive idioms from CFGs in PEGs, with similar results. We prove the conservativeness of these extensions, and also prove that they work with any left-recursive PEG.PEGs can also be compiled to programs in a low-level parsing machine. We present an extension to the semantics of the operations of this parsing machine that let it interpret left-recursive PEGs, and prove that this extension is correct with regard to our semantics for left-recursive PEGs. We present a semantics for left-recursive Parsing Expression Grammars.A small extension adds precedence associativity declarations to operator grammars.We give a semantics for compilation of left-recursive PEGs to a parsing machine.Our semantics are conservative: non-left-recursive PEGs are unaffected.