Type-Directed, Whitespace-Delimited Parsing for Embedded DSLs

Domain-specific languages improve ease-of-use, expressiveness and
verifiability, but defining and using different DSLs within a single
application remains difficult. We introduce an approach for embedded
DSLs where 1) whitespace delimits DSL-governed blocks, and
2) the parsing and type checking phases occur in tandem so that
the expected type of the block determines which domain-specific
parser governs that block. We argue that this approach occupies
a sweet spot, providing high expressiveness and ease-of-use while
maintaining safe composability. We introduce the design, provide
examples and describe an ongoing implementation of this strategy
in the Wyvern programming language. We also discuss how a more
conventional keyword-directed strategy for parsing of DSLs can
arise as a special case of this type-directed strategy.