A Functional Language For Generating Structured Text

This paper describes ST (StringTemplate), a domain-specific functional language for generating structured text from internal data structures that has the flavor of an output grammar. ST’s feature set is driven by solving real problems encountered in complicated systems such as ANTLR version 3’s retargetable code generator. Features include template group inheritance, template polymorphism, lazy evaluation, recursion, output auto-indentation, and the new notions of group interfaces and template regions. Experience shows that ST is easy to learn and satisfying to use. ST’s primary contribution is the clear identification and implementation of a tightly-bracketed solution to the problem of rendering data structures to text, as dictated by the nature of generation and the critical goal of strictly separating the generation logic from the output templates. First, the very nature of code generation suggests the use of a generational grammar. Next, enforcing separation of the generation logic from the output templates restricts the template language syntactically and semantically in such way that templates are easily shown to be equivalent to a grammar. Finally, the rules of separation, such as side-effect free expressions, coincide with the fundamentals of pure, functional programming.