Treetop: A Context-Free Fluent API Generator for C#

Treetop is a proof-of-concept C# fluent API generator for context-free protocols. Treetop accepts a context-free grammar, specifying an API protocol or a domain-specific language, and embeds it in C# as a fluent API. The resulting API enforces the grammar at compile-time: A fluent API chain may compile if and only if it encodes a word derived from the grammar.

For example, consider the following context-free grammar deriving palindromes:

S ::= a S a
S ::= b S b
S ::= a
S ::= b
S ::=

(Note that the line S ::= specifies an S epsilon-production.) Treetop converts this grammar into a C# fluent API that accepts only palindromes at compile time:

// Compiles, "abbabba" is a palindrome
Start.a().b().b().a().b().b().a().Done<Palindrome>();
// Does not compile, "abbbaba" is not a palindrome
Start.a().b().b().b().a().b().a().Done<Palindrome>();

Project Layout

• treetop-core: The core logic of Treetop. This project also contains the Treetop source generator, CFProtocolGenerator.cs: Visual Studio does not support source generator projects with dependencies, so the generator had to be moved to the core project (see the following discussion). A designated generator project is planned to be added in the future.
• treetop-cli: The Treetop command line application. Depends on treetop-core.
• tests: Tests for treetop-core.
• sandbox: A sandbox project for testing the source generator within Visual Studio.