ARCHITECTURE.md

Architecture of PRQL compiler

Compiler works in the following stages:

1. Lexing & parsing - split PRQL text into tokens, build parse tree and convert into our AST (Abstract Syntax Tree, see ast module). Parsing is done using PEST parser (prql.pest), AST is constructed in parser.rs.

2. Semantic analysis - resolves names (identifiers), extracts declarations, determines frames (columns of the table in each step). It declares Context that contains current scope (list of accessible names) and declarations (values of identifiers). When an AST is resolved:

   • function declarations and other definitions are extracted into Context::declarations,
   • identifiers and function calls produce a lookup in Context::scope that finds associated declaration and saves the reference in Node::declared_at,
   • function calls to transforms (from, derive, filter) are converted from FuncCall into Transform, which is more convenient for later processing.

3. SQL Translator - converts resolved AST into SQL. It extracts all tables and pipeline and splits them into "atomic pipeline"s, which can be expressed with a single SELECT statement. Each atomic pipeline is then:

   • materialized (where needed, identifiers are converted into their declarations),
   • translated into SQL AST,
   • concatenated into one large SQL AST.